Framing for 12/12 Pitch Over 30′ Span
What kind of framing would you use for a 12/12 pitch roof over a 30’ span? The plans I’ve been given are very unspecific, and an engineer’s approval is not required in my area.
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I have a customer that wants a 12/12 pitch roof on a 30’ span. (The rise and run are 15’.) There’s an 18†overhang projection, which brings the span to 33’. Using a 1 ½â€ ridge board would bring the span to 31 ½â€, creating a rafter length of 21’-11 ¼â€. (Thicker ridge material will change the rafter length only slightly.) It runs along 50 ft long walls (each 10’ high) with an overhang also 18†on both gable ends, a total length of 53’.
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There are two 4’ high 2×6 knee walls (running 4’ in from each exterior wall) supporting the entire length of the roof on each side. Each gable end however, has a 4’w x 7’h window centered in it, with a 4’w x 4’h window on each side, making it impossible to run a continuous post from sole plate to ridge beam. There are also no posts running along the interior of the roof up to the ridge beam, the customer wants to use it for living space.
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The rafters will probably be secured to a 2×6 top plate, attached directly to the floor (trusses decked with ¾â€ T & G plywood). Rafters will of course also be spaced to land over trusses. The truss spacing is undetermined as of yet.
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RAFTER SPACING
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Will a 24†spacing suffice, or is a 16†spacing a must?
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RAFTER SIZE
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What is the minimum rafter size for this span? Will 2×8’s work? Would the rafters have to be SYP (southern yellow pine), or could they be SPF (spruce pine fir)? These are basically the only choices in my area.
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RIDGE MATERIAL
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What kind of ridge board would I have to use? Can I get away with 2x stock, or do I need to use engineered lumber?
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POST REQUIREMENTS
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Can I get away with no posts at all, or will I need to build an elaborate header around the windows in each gable end, and then run a post up to it, and then from it to the ridge. Will the plans work with no posts along the inside of the roof?
–T
Edited 3/21/2006 11:31 am ET by JourneymanCarpenterT
Replies
I'd use 2x12 24" oc . If I went 16" oc then I'd use 2x10. Go here for some spanning questions http://cwc.ca/design/tools/calcs/SpanCalc_2002/
Make sure you enter the loading correctly and the species, etc. Also if you are using US species make sure you enter that too.
If. that room is vaulted you need a ridge BEAM. Your lumberyard can size that for you, but 50' is a long long span for that beam. Will there be any intermediate supports? Your header in the gables will be determined off of that.
Also, if you do post the ridge in between the gables, you'll need to trace that load path down to the crawl. We pour thickened piers in the crawl to hold that load. Last one I set up and poured was 30" x 30" x 10" with #4 bar 6"oc both ways.
I should add the caveat that I'd need to see the plans and I'm not an engineer.
several comments
It doesn't look like any ridge beam is needed, With the heels attached at that plate, trhe floor joissats can act as rafter ties to prevent spread and sag, IF you have the rafter secured with adequate fasteners and/or hardware.
The span is 15' You only calc the span on the legth of the lumber on each side
Overhang is not a part of the SPAN, it only applies to calculating how long you need to order the lumber
The knee walls do not or should not support the rafters unless they have a load path provided below them. For instance, if they transfer roof load to the mid-span oif the floor joists below and those floor joists are not sized to ahndle the roof loads transfeered to them, the floor will then sag, unless there are walls, beams, opr posts designed to handle that load transfer to foundation. Always think in terms of continuous vertical load path.
Finally, the answer will depend a LOT on where you are - what sort of roof live loads you experience there. In some places 2x6 is enough. The load determines rafter size and layout.
Foir most places the 2x10 is plenty at 16" oC. I would use 2x12 @16" OC if for no other reason than it provides more space for insulation. The 16" layout is better for the roof sheathing and sheetrock, but there are other ways around that too.
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Piffin,
It's been a few years, but I remember asking the insulator about 2x12s and he said its cheaper to use 2x10 and high density. I've never researched it, but that's what he said.
high density whatCells, foam, batts???
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batts
I thought so, but by time you etget airflow venting and the insulation in a 9" space, you will not have the needed r-value. Only way I would do it that way is to plan on using foam panels to thermal break running across the face of the rafters and add r-value
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
"The span is 15' You only calc the span on the legth of the lumber on each side Overhang is not a part of the SPAN, it only applies to calculating how long you need to order the lumber"<!----><!----><!----><!----><!---->
Thank you for the clarification. In the past, I've mostly worked on large commercial projects were rafter calculations were rarely used. A lot of guys probably would've ripped into me for that one (it's always the real mistakes these guys seem to miss), so I appreciate your tact.<!----><!----><!----><!---->
"The knee walls do not or should not support the rafters unless they have a load path provided below them. For instance, if they transfer roof load to the mid-span oif the floor joists below and those floor joists are not sized to ahndle the roof loads transfeered to them, the floor will then sag, unless there are walls, beams, opr posts designed to handle that load transfer to foundation. Always think in terms of continuous vertical load path."<!----><!----><!----><!---->
Thanks for this clarification as well. One of the guys at JLC suggested I calculate the span from knee wall to knee wall. One of the reasons I always like to get more than one opinion. (It's always the real ones they don't catch.)<!----><!----><!----><!---->
"Finally, the answer will depend a LOT on where you are - what sort of roof live loads you experience there. In some places 2x6 is enough. The load determines rafter size and layout."<!----><!----><!----><!---->
<!----><!----><!----><!---->Midwest<!----><!---->. A rural area near a lake, and kind of windy. A winter here might see nearly as much snow as the <!----><!----><!----><!---->Chicago<!----><!----><!----><!----> land area.<!----><!----><!----><!---->
"Foir most places the 2x10 is plenty at 16" oC. I would use 2x12 @16" OC if for no other reason than it provides more space for insulation. The 16" layout is better for the roof sheathing and sheetrock, but there are other ways around that too."<!----><!----><!----><!---->
I just met with the customer today, and it turns out he was planning to go with 2x12. Thanks for the suggestions.<!----><!---->
-T
Edited 3/21/2006 11:28 am ET by JourneymanCarpenterT
Two things -
Is this a building that's 30' wide, or is the rafter length 30' ??
It bothers me when you say: "Rafters will of course also be spaced to land over trusses."
I assume you mean wood webbed floor trusses? If so, are they designed for the concentrated loads of the kneewalls?
.
To me, this situation sounds like a good one for attic trusses or I-joist rafters.
"Is this a building that's 30' wide, or is the rafter length 30' ??"<!----><!----><!---->
I can see why you would be confused; it's the width of the building. Piffin caught it in his reply.<!----><!---->
""It bothers me when you say: "Rafters will of course also be spaced to land over trusses." <!----><!---->
I don't see why. If the spacing is 16" O.C., the trusses will land on every stud. If the spacing is 24" O.C., the trusses will land on a stud at every 4'. My point was simply: "Stacked framing."<!----><!---->
I assume you mean wood webbed floor trusses? If so, are they designed for the concentrated loads of the kneewalls?<!----><!---->
The customer hasn’t given me that information yet, but I would figure the same thing. As for whether they're "designed for the concentrated loads of the knee walls," I don't know, but do you think that would be possible?<!----><!---->
To me, this situation sounds like a good one for attic trusses or I-joist rafters.<!----><!---->
I appreciate the suggestion.<!----><!---->-T
JC
You speak of 10' high walls.
What ties the tops of these walls together?
Also you mentioned 4' high knee walls.
Are these walls in a loft area?
I did 36' span with 12' loft and put collar ties 1/3 the way up,
and floor joists in loft area.
something has to keep your walls from spreading.
Just a thought. Good luck!
T C
<What ties the tops of these walls together?>Original poster The rafters will probably be secured to a 2x6 top plate, attached directly to the floor (trusses decked with ¾” T & G plywood). Rafters will of course also be spaced to land over trusses. The truss spacing is undetermined as of yet.
" did 36' span with 12' loft and put collar ties 1/3 the way up,"If the ties are in the lower third, they are rafter ties and will keep rafters from spreading.A collar tie is the member palced in the upper one third of the rafters and has a different purposeThat's for youFor him, the purchase on the floor means the floor system completes the lower third of the triangle that you completed with your rafter ties.
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>>"The rafters will probably be secured to a 2x6 top plate, attached directly to the floor (trusses decked with ¾” T & G plywood). Rafters will of course also be spaced to land over trusses"
Your post has an inconsistency that might be causing some head scratching (made me scratch my head). If you have a non-bearing ridge, you have to connect the rafters to the trusses to transfer the force that wants to puch the rafters apart to the trusses. You talk about attaching rafters to a top plate, but over each truss. Your trusses are on the top plate and so should be the birdsmouth of each rafter. I assume you mean you space the rafters to land over the studs of the wall below.
I originally had a different answer for you but when I did some "back of the envelope" calculations, I changed my mind. Since I did the calculations already, here they are. You can ignore them.
Non-bearing ridge "back of the envelope" calculation: 30' total width times 10 psf dead load plus 40 psf live load divided by two walls as load paths = 750 plf at each plate. 12/12 pitch = 750 plf in tension (equal force vectors, down and out. This is where my memory is vague but I'm pretty sure that's how it works). 16" OC rafter spacing = 1,000 tension at each rafter to truss connection, with that force being in shear at the connection. Will nails into the trusses handle 1,000# shear force at each rafter? Probably. Can each truss handle a total of 2,000# tension? Probably. (Call the truss manufacturer and I bet their engineers will fax you an approved connection for free).
Bearing ridge "back of the envelope" calculation: 15' times 10 psf dead plus 40 psf live = 750 plf / 2 ends of each rafter = 375 plf x two halves of the roof = 750 plf at the ridge. 750 plf times 50' span = 37,500# total load on the ridge. Ain't no ordinary piece of wood for the ridge beam that will hold that. You would need an engineered beam or steel.
"A job well done is its own reward. Now would you prefer to make the final payment by cash, check or Master Card?"
Edited 3/11/2006 3:47 pm ET by philarenewal
U assume the live load at 40# without knowing where he is
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Piffin:
You're right, but make it a 20 psf live load and it's still a good bit of weight to be farting around with. Say it's 20,000# on the ridge instead of 37,500. Say it's 650# of shear on the nails instead of 1,000#. Still enough force to consider for a guy that to his credit is smart enough to ask but is still learning how to size rafters. I figure if he calls the truss folks, they will give him an approved connection and he's off to the races on that issue.
I started a long post going into the need to connect the rafters to the trusses and a question about whether the trusses could handle the tension, but then realized there isn't really all that much tension on the trusses from the 15' run, so I dropped that and since I already did some quick (likely full of mistakes) calculations, I just posted them and if the OP says, "yeah, I get all that, what I want to know is . . ." then great.
If the OP says to himself, "hey, that really is a boatload of weight, maybe I should get some hands on help from that old guy I used to work with for the first time out of the gate" then maybe I save the guy some big problems. I'm the stubborn type so if a bunch of guys jump on me and say "you're crazy, etc." usually just makes me that much more stubborn so I try not to do that to other guys. You've given the guy some good advice, so you must look at it the same way. A guy asks a question -- try to give some help. Just saying "no, no, no" probably doesn't help.
OP's also talking about (I can't really tell and the more I think about it, the more I scratch my head), laying a plate on the deck and framing a non-bearing ridge roof on top of that, and he calls the plate on the deck a top plate. (still not sure what he means and I read it differently every time I look at it).
As long as I got you, I was wondering about that put a plate on the deck and put the rafters on that (if that's what this guys is saying). I've never seen that done but what do you think of it? Way I've always done it is to put the rafters along side the joists on the wall top plate and fasten them together. If it's a non-bearing ridge, I don't see how he's going to get a strong enough connection between the deck and the rafters and he's obviously not putting collar ties down low.
Anyway, all this will prevent alzheimers for me. Really makes the old noggin work at it.
Edit -- just saw your recent post -- make that rafter ties down low ;-) Still, you think he'll get a strong enough connection between the rafter and the deck? Just asking.
"A job well done is its own reward. Now would you prefer to make the final payment by cash, check or Master Card?"
Edited 3/11/2006 3:25 pm ET by philarenewal
I've done the plate on deck thing a buch I got it from severalof the old houses here that have stoof forever or a hundred years anyway.That joint where the styresses and load transfer from the rafter to the floor assembly is the critical point in all this. When it is not direct connection as typical in new cosntruction, therre is more chang=ce for error and failure. I use a 2x6 plate there instead of 2x4 so there is a good seat and more nailing for the connectors. I toe nail and then add A-35s. The 2x6 is glued and nailed to the sheathing below.That is how I do it anyway. I didn't get specific in the beginning and just used the word adequate I think to imply that he needs to pay attention there. I hope he asked himself the question, "What is adequate?" as you have done here. you answered my Q about live laod by making an example of only 20# but I was thinking 70 - having worked in so many snow locations. Tim U is in this thread too and I think he works around 120# loads sometimes.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
70 to 120# snow load? I'm moving to Florida!
Too old for this snow crup anymore anyway.
"A job well done is its own reward. Now would you prefer to make the final payment by cash, check or Master Card?"
"As long as I got you, I was wondering about that put a plate on the deck and put the rafters on that (if that's what this guys is saying). I've never seen that done but what do you think of it?"
This is very common around here and done all the time. Sometimes it has to be done to raise the soffit up for tall circular or transom windows and doors. Or certain roof lines and with different pitches and roof trusses have to blend in.This house I framed last summer the rafters sat on top of plates above the I-joists and decking to accommodate the two main trusses that went in the main section of the house. The trusses had a different pitch but sat on a higher wall but the fascia lines and overhangs had to match along with several steps of freeze board and trim that we installed.When you sit them on a top plate you have to install hurricane ties or you have to install angled 2x4's nailed along side the rafters and into the joists.Here's a couple pictures. One picture with the forklift you can see where the rafters sit on top. One picture you can see how high the double freeze board is. The last picture if you look in the background you under the trusses you can see all the plumbcuts of the rafters sitting on the top plate.Joe Carola
View Image
Edited 3/12/2006 6:54 pm ET by dustinf
I get it. The proverbial picture worth a thousand words.
Nice work. Style look like kind of a nod to greek revival. I really like the exterior trim. Beautiful.
"A job well done is its own reward. Now would you prefer to make the final payment by cash, check or Master Card?"
Joe,
You got any other pics of that house? As always, you do perfect work. Also did you get that drawing I sent you of the engineered roof? I sent it to your hotmail, but don't know if you use that email or not.
I hope little Joey is feeling better. As soon as he's 18, I'm going to ask him to come out west and frame with us :-)
"Non-bearing ridge "back of the envelope" calculation: 30' total width times 10 psf dead load plus 40 psf live load divided by two walls as load paths = 750 plf at each plate. 12/12 pitch = 750 plf in tension (equal force vectors, down and out. This is where my memory is vague but I'm pretty sure that's how it works). 16" OC rafter spacing = 1,000 tension at each rafter to truss connection, with that force being in shear at the connection. Will nails into the trusses handle 1,000# shear force at each rafter? Probably. Can each truss handle a total of 2,000# tension? Probably. (Call the truss manufacturer and I bet their engineers will fax you an approved connection for free).”<!----><!----><!---->
“Bearing ridge "back of the envelope" calculation: 15' times 10 psf dead plus 40 psf live = 750 plf / 2 ends of each rafter = 375 plf x two halves of the roof = 750 plf at the ridge. 750 plf times 50' span = 37,500# total load on the ridge. Ain't no ordinary piece of wood fo r the ridge beam that will hold that. You would need an engineered beam or steel."<!----><!---->
Very much appreciated. I would also appreciate knowing where you come up with these numbers so I can figure it out for myself next time.<!----><!---->-T
>>"I would also appreciate knowing where you come up with these numbers so I can figure it out for myself next time."<!----><!---->
Hello:
I came up with the numbers from some residential wood frame engineering I learned long ago. Pick up any decent text on residential frame engineering and it will get you started.
Big (BIG) caution, though. As I said, these are "back of the envelope" calculations -- just sort of sanity checks that are well worth knowing how to do (like if you calculate that a nailed connection needs to resist, say, 4 tons, something has gone very, very wrong ;-). BUT if you get to a point in what you're building that you have to make these types of engineering calculations count, hire an engineer -- that's what they're for. It's really easy to make a simple math mistake, and a simple mistake can kill people.
Another wothwhile resource is the code book. Where I am, applicable code is the IRC 2003. You can probably pick it up in any bookstore (I got mine at a Barnes & Noble). Very helpful with span tables and connection methods that are pre-engineered and so tried and true, no extra engineering required. You can pretty much build almost anything residential and not need much custom engineering. Most of it has been worked out long ago and it's incorporated in the code.
If you can find one, best resource is an old timer willing to teach you. Never laid out a roof before? -- no big deal -- find a friend who has done it and ask for some hands on help. It ain't rocket science, but working with someone who has done it before a million times will save you a lot of time and a lot of problems.
Best of luck with it.
"A job well done is its own reward. Now would you prefer to make the final payment by cash, check or Master Card?"
I'm usually pretty placid but man, you don't know what size rafters, what spacing to use, what kind of ridge, and you're going to build this for someone? If you have that many questions about it why in the world are you even thinking about it? Maybe drawings aren't required in your area but you sure need someone else to lay it all out and that guy isn't you by the sound of the questions.
Wasn't last week when you were a drywaller?
That being said, here's my 2 cents. You're new at this, and its obvious, and thats ok, because we've all been there. Some days even the old farts feel that way. What they have a leg up on is knowing when the proposed project is enough out of the comfort zone that it isn't something they need to be bidding on. Whatever your forte is, keep the wagons circled close around that. Don't go taking on things that are so far out that you have to go looking up answers to more than a couple of minor detail type questions. The word you're missing here is liability. Lets say you frame this roof, and you're wrong on something. What happens then. What happens to the structure, the inhabitants, you?
"A bore is a man who, when you ask him how he is, tells you." -Bert Taylor
I apologize. You were just getting on my last nerve. When I speak to people, I generally treat them with respect (unless their rude to me), and I expect the same professional courtesy.-T
I'd follow BossHog's lead I joists are strong light and true.
I built with 12" nom I-joists on a 16 / 12 pitch. Ridge beam was 2 pcs of 5X18 timberstrand. Ridge was centered over a 26 foot wide room, Length 55' with one intermediate post. Rafter tails sitting on a 2x6 wall. You need metal strapping on the tails and saddles at the peak. (I guess that is standard now) Exterior overhang was about 3 foot. 24 inches on center. Archy spec'd for northern Wisconsin snow load.
At that point I needed collar ties at the midpoint 1 x 8 pine or fir. I chose 2- 1x4's in red oak one on either side of the I-Joist. I then filled the bottom 1-3/4 inch gap between the collar ties with an oak strip. After routing the edges with a 3/8 roundover bit, it looks like solid beam from 20 foot below. Covered with T&G Cedar it makes a beautiful cathedral ceiling.
Consider the insulation. I had to find R38 10"x24 cathedral ceiling insulation for the roof. See what you need for your climate. Possibly a 14" I-joist would let you use 12 inch deep insulation. Your climate your call.
Consider putting in a ceiling fan(S) if this is a cathedral. It's tough to wire but really helps destratify your heat.
Oh yea. If it is 16 OC or 24 OC you are only talking about 250 bucks worth of lumber. Small potatos in the 100's of thousands total investment. More important in floor joists but rafters as well.
T, I read the entire thread of this at JLC, and I only got to post 20 here before I lost total respect for you, not only as a tradesman, but as a poster.
I could give you my take on what I'd use in my area for your particular situation, but it's useless to you, 1) Because you're in snow country; I'm in a high-wind zone, and 2) You're simply not experienced or qualified enough to do a job like this.
You've pizzed several good people off, both here and at JLC; people who were honestly giving you worthwhile advice. You take personal offense when several posters have told you, "Hey man, maybe you should rethink doing this yourself."
Bro, that ain't gonna fly with anyone, whether here on on your jobsite.
Do whatever, but understand your words and attitude are only burning your bridges with those that are honestly trying to help.
Jason Pharez Construction
Framing & Exterior Remodeling
jason... i sure would be tempted to go over to JLC to watch... but i spend to much time here as it is...
a double -addiction would be fatalMike Smith Rhode Island : Design / Build / Repair / Restore
>>"i sure would be tempted to go over to JLC to watch"
Can anyone find out where this guy works so we can go to the jobsite to watch? Probably better than an Osborn's episode. ;-)
(and yeah, if he actually does this on his own and does something that could hurt somebody bad, someone could drop the dime on 'em -- don't mean to make too light of it).
"A job well done is its own reward. Now would you prefer to make the final payment by cash, check or Master Card?"
Phil & Mike, the guys at JLC really blasted him, especially after all the other "how much?" questions like he's posted here.
He's worse than most DIY'ers IMO, because at least when you tell them "Hey, you don't need to be doing this" they usually listen!Jason Pharez Construction
Framing Contractor
Yo, what ended up happening with your project? I saw you got an engineer on board. Who else on that ship?
Inquiring minds want to know.
"A job well done is its own reward. Now would you prefer to make the final payment by cash, check or Master Card?"
I met with him Saturday. I'm supposed to e-mail him a proposal, along with some references. I've been so busy estimating other projects, and taking care of other business, that I haven’t written it up yet. I better get on it. There's not much point of doing all the work of tracking down a job if you never sign a contract.<!----><!----><!---->-T
Post 20 deleted?? Man I hate that! No wonder the thread made little sense at one point...
One poster mentioned it, and I am REALLY surprised the OP doesn't have it, and hasn't really consulted it, because the answers are so often in it...the CODE book!
Heck, you really don't even have to do much math.....it's all spelled out in tables. and there is even a table for nailing schedules.
But I am glad that the questions were asked, theres always a wealth of good info here, but a drawing would have made it easier.
Great looking project, Joe.Jake Gulick
[email protected]
CarriageHouse Design
Black Rock, CT
Yes, I have to get myself a code book. I'm surprised I don't have one yet myself. I've long known they're available from Craftsman-Book Company.<!----><!----><!---->
I stopped in at Barnes & Noble, but they told me a new IRC (International Residential Code) is scheduled to be released this month. I ordered that one. I would've looked into the one they had, but it was still in the plastic wrap. I didn't want to be the vandal that broke it open.<!----><!---->
While I was in the "ENGINEERING" section (where they keep the code books) however, I picked up a book called Carpenter's Calculations Manual, published by McGraw-Hill. Their trade tip boxes are pretty obvious, but it's packed with useful figures and tables. It was priced at $29.95. A little more than I like to pay for a book its size, but after flipping through it, I decided it was worth it.<!----><!---->
On the back cover, they recommend their website; http://www.books.mcgraw-hill.com. I never searched for books there before, but I think I just might in the future.<!----><!---->
In chapter 7 on “RAFTERS,” every kind of 2"x12" exceeded a 15' span at 16" O.C. This is what had me confused, the span. Since I've never had to size a rafter before, the span I was thinking of was the span of the building. I was familiar with online calculators, such as the one by the American Wood Council, as well as the Canadian Wood Council. When you type in a 30' span instead of 15' though, it obviously tells you it won't work. I feel like a real OP not realizing that the rafter span referred to half the span of the building. If I had, I probably would've never created this thread.
-T
Edited 3/16/2006 3:40 pm ET by JourneymanCarpenterT
journey ( hey , i had their old CD on today " Raised on Radio )..
anytime i find a book title , the first thing i do is check on Amazon to see if i can get it used... often i'll find $30 books for $1.57 and $5 for shipping
also.. the code... i like to go see the building department , introduce myself... and ask them what the best way to get the current local code is...
in a lot of jurisdictions, a state agency is charged with getting them in the hands of the people who need them..
failing that.. you can find out EXACTLY which code and which version your jurisdiction uses.. most do NOT use the latest edition.. every state has commissions that review the code, and modify it...
if you google on your state... ( for example : RI 1 & 2 Family Dwelling Code ) you'll find the reference to the current code and the amendments
irregardless ( i've been dying to use that word for two days ) you can often down load in printable form any page of hte code for free
Mike Smith Rhode Island : Design / Build / Repair / Restore
irregardless ( i've been dying to use that word for two days ) you can often down load in printable form any page of hte code for free
Free? Maybe I'll take a peek into a code book before I retire. Its been over thirty years and I've never layed eyes on one! I'm a sad sack carpenter aint I?
blue
yeah , you is..... and furder more... a cornfused one at that...
heck you don't even know if you're a contrarian... or a libertarian
as to downloading the building codes......'course , the free ones might only be in the commie blue states like Rhode IslandMike Smith Rhode Island : Design / Build / Repair / Restore
>>"Its been over thirty years and I've never layed eyes on one!"
Sure you're real name isn't luckydevil?
If I had your years of experience, I wouldn't need a copy on my desk. But I don't, so I do. ;-)
"A job well done is its own reward. Now would you prefer to make the final payment by cash, check or Master Card?"
Woops, how did that happen? That was for you.-T
How did WHAT happen? You mean post #20 being deleted???
That was for me???
How can that be??? I hadn't responded in this thread until well after post 20.
Do we need to add "smileys" to this board? And the ability to "quote"??Jake Gulick
[email protected]
CarriageHouse Design
Black Rock, CT
“How did WHAT happen? You mean post #20 being deleted???<!----><!----><!---->
That was for me???<!----><!---->
How can that be??? I hadn't responded in this thread until well after post 20.<!----><!---->
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Do we need to add "smileys" to this board? And the ability to "quote"??”<!----><!---->
Ha ha ha. Calm down my friend. Post #37 was for you. Somehow I ended up sending it to daycon. You will see that daycon hasn’t posted on this thread at all. That’s fine though, I could always use a good chuckle. : )
(For the record, I don’t think post #20 should’ve been viewed as being so controversial. I simply pointed out the flaws in RW’s reasoning in post #10. I pointed out that layout wasn’t my question, that everyone who’s hung a sheet of rock isn’t strictly a drywaller, and I questioned who had more experience, seeing as how he hadn’t filled out his occupation. . . I might have also pointed out that his post wasn’t characterized by knowledge, and compared him to a kid making a ruckus at a violin concert . . . maybe. As I told him, he was getting on my last nerve. I’ve been getting a lot of condescending posts lately. I don’t like know-it-alls, not that he is one, but that was my impression from post #10.
The only reason I even deleted it was as a courtesy to RW, and because JasonPharez, and perhaps some other posters found it offensive.
Maybe I do need more :) in my posts. Some :), some :-), and maybe some ;). Some people have taken things that I’ve said way too seriously, when I’m often only intending to respond in kind. Or, perhaps they can give, but they can’t take. . . lol:)
No hard feelings, I hope. : )
-T
Edited 3/18/2006 10:15 pm ET by JourneymanCarpenterT
Edited 3/18/2006 10:25 pm ET by JourneymanCarpenterT
Because I cut and pasted, the same mathematical error I posted at JLC, appeared here. I therefore thought it appropriate to post my reply to these comments from JLC here as well:
“JourneymanCarpenterT wrote: I have a customer that wants a 12/12 pitch roof on a 30’ span. (The rise and run are 15’.) There’s an 18” overhang projection, which brings the span to 33’. Using a 1 ½” ridge board would bring the span to 31 ½”, creating a rafter length of 21’-11 ¼”.
That's a awful thick ridge board your using. Your actual rafter length is 23'-0.83", that's including the deduction for a 2x sub-fascia. From your recent questions how could you possibly call yourself a journeyman carpenter?"
Ed Michnick:
Ah yes, I see what I did. 33’ (span) - 1 ½” (thickness of standard ridge board) = 32’ – 10 ½”. For that post I used The Rafter Book: A Modern Handbook for Roof Framers in conjunction with a basic calculator. The Rafter Book is available at most any Union Carpenters Training Center. It’s also available at the JLC Bookstore. Apparently I subtracted 1 ½ ft instead of 1 ½ inches. (33 - .5 = 32.5) The Rafter Book, in the tables beginning on page 28, under the heading “12 in 12 – 45º” states: A common rafter length for a span of 31’ is 21’ – 11 1/16”. Adding the ⅜” of length for the .5” (Apparently I was still confusing feet with inches) = 21’ – 11 7/16. To make matters worse, I apparently was looking at the inches for the ¼” span listed just above it, which is 3/16”. 21’ – 11 1/16” + 3/16” = 21’ – 11 ¼”. Haste truly does make waste. I stand corrected.
I’m not quite sure I see what you did though:
After some minor adjustments, the customer and I are now satisfied with the contract. I’m going to have to make up a materials list now. I guess that includes figuring out how long the rafters will actually be. It’s probably going to be pretty tough, seeing as how some people think I’m just an apprentice. I guess I’ll give it the old ‘trade school try.’ ; )
The Rafter Book
A 33’ span (2 x 18” overhang = 3’. 3’ + 30’ = 33’) - 1 ½” (thickness of standard ridge board) = 32’ – 10 ½”. According to The Rafter Book, in the tables beginning on page 28, and under the heading “12 in 12 – 45º”: A common rafter length for a span of 32’ is 22’ – 7 ½”. Adding the given length for the additional 10 ½” (7 7/16”) = 23’ – 2 15/16” (22’ – 7 ½” + 7 7/16” = 23’ – 2 15/16”).
Furthermore, a span of 32’ – 10 ½” - 6” (1 ½” fascia + 1 ½” sub fascia x 2) = 32’ – 4 ½”, with a rafter length of 22’ – 10 11/16”. That translates to 22’ – 10.6875”. (To find the decimal equivalent of a fraction, divide the numerator by the denominator.)
Construction Master
Let me double check that with my Construction Master to be sure:
Keystroke:
[On/C] [On/C]
12 [Inch] [Pitch]
32 [Feet] 6 [Inch] [÷] 2 [=]
[Run]
[Diag]
Display:
0.
PITCH 12 INCH
16 FEET – 3 INNCH
RUN 16 FEET – 3 INCH
DIAG 22 FEET – 11 INCH ¾
That’s slightly different, but only 1/16” stronger than what I had with The Rafter Book. Converting the fractions into decimal equivalents would appear this way: 22’ – 11.75”.
Pythagorean Theorem
Now I’ll check my work with an age old method, the Pythagorean Theorem (A² + B² = C²): 32’ – 10 ½” ÷ 2 = 16’ – 5 ¼”, or 16.4375’ (To change inches to decimals of a foot, divide by 12. 5.25 ÷ 12 = .4375). Theoretically, the rise and run should be the same for a 12/12 pitch (45º angle). A² (270.1914) + B² (270.1914) = C² (540.3828). The square root (√) of C = 23.24614, or 23’ – 2 11/16” (To change decimals of a foot to inches, multiply by 12. To change decimals of an inch to 16ths multiply by 16).
Here’s what I’ll have after subtracting 6” from the span for fascia and sub fascia: 32’ – 4 ½” ÷ 2 = 16’ 2 ¼”, or 16.1875. A² (262.0352) + B² (262.0352) = C² (524.0703). The square root (√) of C² = 22.89258, or 22’ – 10 11/16”.
1.414
Because a 12/12 pitch is a square, I can also find the common rafter length by multiplying the run (or the rise) by 1.414: 16.1875 x 1.414 = 22.889125. .889125 x 12 = 10.6695. .6695 x 16 = 10.712, or real close to 11. 22’ + 10” + 11/16” = 22’ – 10 11/16”.
Framing Square
What if I was at the job site instead of sitting at a desk? Here’s how I’d do it with a framing square: According to Swanson’s framing square model #TA 127, the common rafter length per foot of run for a 12/12 pitch is 16.97”. 16.97” x 16.4375’ (see the first paragraph under Pythagorean Theorem) = 278.9444” ÷ 12 = 23.24536’, or 23’ – 2 15/16”.
16.97 x 16.1875 (span after subtracting for fascia and sub fascia) = 274.7019” ÷ 12 = 22.89182’, or 22’ – 10 11/16”.
Online Roof Calculator
It all checks out so far. Wait . . . maybe I just think I know basic mathematics! : ) I better check it with The Roof Calculator: Oh no! No! No! It’s actually: 22’ – 10 23/32”. I’m a whole 32nd off. Blast those 32nd’s of an inch! ; )
If that doesn’t work, I’m sure I’ll figure it out when I test the first rafter I cut. That is, if I can think to do it without straining my brain too much. : )
Ah, yes. I see what you did. You subtracted for sub fascia only: 33’ (span) - 1 ½” (ridge board) – 3” (fascia on each side) = 32’ – 7 ½”. 32’ – 7 ½” span = 23’ – 0 13/16”, or 23’ – 0.8125 / 23’ – 0.83. Why? The way I understand it, framing with sub fascia is an optional framing method (used to secure soffit boards), which is always used in conjunction with fascia. The very prefix “sub,” implies that something will be added to it. Personally, I think you’ll get a better product by running the rafters a little long and snapping a line, but that’s just my opinion.
I very well could’ve made a mistake. After all, I created this thread because-never having worked for myself before, and having mostly worked on large commercial projects-I didn’t realize that the span on rafter span tables was only half of the building span. If you can explain what I’m doing wrong now, it would be much appreciated if you can inform me . . .
PS:
1 ½” is a standard thickness for a ridge board. ; )
-T
Reason for editing: Quotation Boarders
Edited 3/20/2006 4:54 pm ET by JourneymanCarpenterT
Edited 3/20/2006 4:55 pm ET by JourneymanCarpenterT
Edited 3/20/2006 4:57 pm ET by JourneymanCarpenterT
If you can explain what I’m doing wrong now, it would be much appreciated if you can inform me . . .
You seem to have been doing quite a bit of homework. Before I answer your question, can you explain to me the difference between a bearing and non-bearing ridge and why it makes a difference in what you are building? When you can do that, I'll answer your question.
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"A job well done is its own reward. Now would you prefer to make the final payment by cash, check or Master Card?"
"You seem to have been doing quite a bit of homework. Before I answer your question, can you explain to me the difference between a bearing and non-bearing ridge and why it makes a difference in what you are building? When you can do that, I'll answer your question."<!----><!----><!---->
The homework was done years ago, I just decided to show it now because everyone seems to want me to prove myself.<!----><!---->
As to whether it makes a difference: It doesn’t. I realized that when Piffen posted: "The span is 15' You only calc the span on the legth of the lumber on each side" in post #3. See also my comments in post #37. You can answer my question if you’d like, but it was posed more or less as a rhetorical one.
To prove myself again : ), and to show that I take time to answer people’s questions without ridiculing them (I’m not implying that you have done that, you haven’t.), the following is a copy of one of my recent posts in response to a question at JLC. To save you some time, I’ll also include a copy of an architects answer to my question on the same thread.<!----><!---->
Structural vs. non-structural ridge<!----><!---->
CREATED BY TIMBERLAND: Got a question that's been driving me crazy for some time now. I've seen the terminology floated around here and at work from time to time and I have my ideas but I'm not too sure if I am correct or not and hope you guys can send me in the right direction. What exactly is a structural ridge in typical applications as opposed to a non-structural ridge? I am thinking that it's a ridge that is under the rafters and not flush to them? How do you determine if/when you need a structural ridge?<!----><!---->
Posted By Timberland: Ok, Dan, I think that may have helped my understanding a bit but I still am thinking that the walls would want to 'kick' out w/out any ceiling joists or collar ties, structural ridge or not, especially with longer spans...but like you said, it's all in the mechanical connections.<!----><!---->
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Posted by JourneymanCarpenterT: Timberland: Excuse me if this is redundant information. Here’s how I understand a “structural” verses “non-structural ridge:” Imagine a rectangle made up of 4 exterior walls. Say the shorter gable ends are each 30’ long, and the longer walls are 50’ long. (30’ x 50’) Have the outline of this 3D box firmly fixed in your mind. Now imagine the outline of triangles equally spaced (perhaps 16” O.C.) along the top. Connect the peak of each triangle with a continuous line. (The ridge) Finally, picture a big boulder placed on atop the ridge of those triangles, right in the middle of the two gable ends. Let the boulder be just heavy enough to create great stress, yet not bust the roof. Can you see the stress from the boulder on the ridge, and then being transferred through the rafters? What prevents the rafters from pushing the two long wall’s top sides out? It’s the base of the triangles, the joists. This is a non-structural ridge. The ridge doesn’t support the weight on the roof structure, the triangle formed by the rafters and joists do. Now remove the base of each triangle. (A cathedral ceiling) If the same size boulder is there creating the same amount of stress, what just happened to the two walls along the eave? They were pushed top side out. The ridge caved under the boulder, and all the rafters followed. Without the joists, the ridge must now be strong enough to support the structure. How can this be done? In my opinion, the basic difference between a “structural,” as opposed to a “non-structural” ridge, is simply: A structural ridge transfers the load through the gable end walls. Take that same scenario without the joists, but now beef up the ridge by making it some sort of beam instead of a board. Even if that bolder can’t bust the beam, it will still bust the structure, and the beam will come crashing to the bottom. If however, that beam is resting atop two beefy posts in each gable end (say 3 2x uprights fastened together), the ridge won’t budge. The ridge now supports the structure in lieu of the missing joists. Hence, it is now a structural ridge. I noticed you also brought up “longer spans.” This was one of my concerns recently. Having always worked for someone ells in the past, I never had to size a rafter. The only span for roofs I ever used was the span of a building. While I was familiar with various online calculators, they will all obviously tell you that no rafter will span 30’ at 16” O.C. The span for rafters on rafter tables however, is just that, the span of the rafter. A rafter only spans half the span of the building. According to standard rafter tables, any 2”x12” will span that. What if the span of the rafters was 30’, or even more? For such longer spans, the rafters and joists are no longer able to support themselves. Does this mean now you need a structural ridge? That was a question I posed on a recent thread. I would think a structural ridge would definitely increase the span capabilities of a rafter. One poster at “breaktime” suggested I-joist rafters. I had read about them before, but I overlooked the possibility at first because they’re not readily available in my area. I-joist rafters are kind of to 2x rafters what a ridge beam is to a ridge board, in that they’re stronger and able to span greater distances. In regards to the earlier question as to when should the rafters be placed to meet above a structural ridge, as opposed to meeting the ridge itself, here’s my take. I think there are two reasons. 1: It’s simply the look you’re going for. 2: The ridge beam you’ll be using is so wide that it will extend below the rafters anyway. Thus, you place it completely below the rafters, making it look like that’s the look you’re going for. As I said in the beginning, this is simply “how I understand it.” Of course, I’m no engineer. If any of the above seems incorrect to those who are engineers online, I’m sure they could clarify things for you.<!----><!---->
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Posted by Sweep8: JourneymaCarpenterT: The sizing of a rafter for a given span is not affected by the nature of its supports assuming they are simple, non-moment-resisting connections.<!----><!---->-T
Good man. You get the prize.
Your rafter calculations look OK by me (haven't looked at the numbers closely but your methods and cross checking are impressive enough). If I was doing it for a first time (or even a second or third or fifth), I might be inclined to cut a single rafter or two and make sure they fit properly. Once you are sure you have a good pattern, cut the rest using the original as the pattern.
Best of luck with it.
"A job well done is its own reward. Now would you prefer to make the final payment by cash, check or Master Card?"