I have a building that’s 12′ 4.5″ wide, and 15′ 4.25″ long.
I’d like to space all the rafters evenly, but I can’t figure out, how to figure out where to put the first jack rafters, so that my layout each way is even. The rafters don’t have to be at 16″ or 2′ centers .. just evenly spaced all around.
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Replies
My main concern would be how do you plan on sheathing the roof? If your rafters fall at some weird length that is not compatible with your OSB/plywood sheathing you will be screwed. 16 and 24 work because they fit into your sheathing widths and lengths (48 and 96). I suppose you could do some other multiples--12", but what a pain. See what I'm saying?
or 19.2
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming
WOW!!! What a Ride!
If your rafters fall at some weird length that is not compatible with your OSB/plywood sheathing you will be screwed. 16 and 24 work because they fit into your sheathing widths and lengths (48 and 96).
That doesn't matter in this case. Both dimensions are less than 16', so he can break the sheet on a rafter near the middle and cut the hips.
Sorry, but on this roof that doesn't matter because I'm not using sheet sheathing. You can think of this as a skip-sheathed roof, so rafter OC layout doesn't matter, except that I don't want to go much over 24" OC.
The main deal here is that I want all the rafters evenly spaced.
Measure total width, subtract the thickness of a rafter... Divide by the nunber of bays you want or accept... that is your OC measure...
Find center of lay out and lay out to the left and right of center...
Shift a half of a measure of your OC measure, left or right of the CL, for odd number of bays
Adjust the number of bays to your liking....
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming
WOW!!! What a Ride!
Edited 10/8/2004 6:15 pm ET by IMERC
Imerc, that works for one side, or the other side, but doesn't give me equal spacing all around the perimeter for all 4 walls.
Greencu, does that 18" work for my building ? Did you figure it out ? How'd you do it ?
How far from the corner do all 8 jacks start then ?
The difference between the two dimensions is 35.75". Divide by 2 and you get approx. 18". You'll have a 3' ridge on the long side. Frame that first and back down the hips. If you want a true hip with different pitches, you got me. Hummmmm, might still work now that I think about it.
I invented the 17d nail.
greencu thanks for beating me to that.
joewood don't worry about that short side, it is so short that after you make the first run of p/w the centers won't matter. I thought the goal here was to get all of the hip jacks to all be the same length.
Do you know how to use the scale on the side of your framing square to find the difference in each length for a given pitch?
20.45"OC is very close......
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming
WOW!!! What a Ride!
Easy, start your lay-out in the middle and go both ways.
Rootburl, that's only good if all 4 of the walls are the same length. This is a hip roof, with the length longer than the width.
Imerc, how did you arrive at that, and how far from each corner would the first jacks go ? Did you figure that out to my roof specifically ?
Seems to me that I've seen plenty of hips roofs with all the rafters spaced evenly. Maybe it was just because they'd laid out for 16 or 24" centers.
I did the OC spacing..
12'4.5"= 148.5"-1.5= 147" divided by the # of desired bays
7, 8 or 9 apply here....
Take quoient from this and divide it into yur long side... That will give you the bays for the long side across the full length...
This is where a little seat of the pants figuring comes in... Regardless of what you do you'll have a little left over on one side as where the side will break...
7 bays give you 20.4"OC to the short side...
20.4"OC gives you 8.97 bays on the long side... Go for 9.... Do the math for 9 equal bays...
The difference in bay size is .07"...
Adjust each bay for .035"...
The wider ones get reduced by .035" and the smaller ones get increased by .035"....
Bummer ... just caught it... Make that 20.365"OC's....
decide yur pitch and set the ridge to accomodate...
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming
WOW!!! What a Ride!
You said, "This is where a little seat of the pants figuring comes in... Regardless of what you do you'll have a little left over on one side as where the side will break..."
Are you saying that on one side you'll have a different space left over that the other side?
You said that your O.C spacing will be 20.365". Are your jacks being cut the same size and butting each other?
The way I'm doing it your O.C. spacing is with in 1/8" - 1/4" and all your jack are the same size and the bays are the same size measuring from the hip where it crosses the plate.
You can view in in plan view as if the hips where they cross the plate at 45° was the inside of two 4x4 posts for a deck railing and all the common and jack rafters are the balusters and the distance of the first set of jacks coming in from the hip would be same distance between all the commons and jacks.
Joe Carola
Joe,
I know you said or I think you said somewhere that you wanted the same amount of rafters but would you mind if you had this?
148.5/8 = 18.56" (Gives you 7 rafters @ 18.56" O.C.)
184.25/10 = 18.42" (Gives you 9 Rafters @ 18.42" O.C.)
18.56 - 18.42 = .14" or 1/8" Difference.
So one side is 18-9/16" the other is 18-7/16". If you lay the jacks out to butt each other at 18-1/2" you'll never see the difference.
Is it that important to have the exact same amount of rafters on both sides? I assume this roof is all open.
Joe Carola
Edited 10/8/2004 8:49 pm ET by Framer
No Joe, not the same # of rafters on each side, but the same OC for all the rafters.
I think Imerc has it, i'm just digesting everything he wrote now.
you said excact...
Done this in one form ar another...
I hope you can understand the thought process and sortta what I wrote...
GAWD I HATE TYPING!!!!!!!!!
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming
WOW!!! What a Ride!
Joe,
Imerc said in his post:
7 bays give you 20.4"OC to the short side...
20.4"OC gives you 8.97 bays on the long side... Go for 9.... Do the math for 9 equal bays...
The difference in bay size is .07"...
Adjust each bay for .035"...
The wider ones get reduced by .035" and the smaller ones get increased by .035"....
Bummer ... just caught it... Make that 20.365"OC's....
decide yur pitch and set the ridge to accomodate...
Joe, That's what Imerc said above. Using your dimensions.
7 x 20.365 = 142.55" where the short side is 148.5" - 1.5" = 147" as Imerc stated. 147-142.55 = 4.45" left over. Doesn't work.
9x20.365 = 183.28" where the long side is 184.25". It's close for that side but Imerc didn't say anything about deducting for the rafter.
Am I missing something?
I said this to you in my last post.
148.5/8 = 18.56" (Gives you 7 rafters @ 18.56" O.C.)
184.25/10 = 18.42" (Gives you 9 Rafters @ 18.42" O.C.)
18.56 - 18.42 = .14" or 1/8" Difference.
So one side is 18-9/16" the other is 18-7/16". If you lay the jacks out to butt each other at 18-1/2" you'll never see the difference.
What's wrong with those centers?
If you want exact spacing for the bays I could give you that.
You would subtract half the thickness of the hip at 45° coming in from each corner it would be 1-1/16" each plus the amount of rafters times their thickness from the length of each side.
Short side.
148.5 - 2.12 = 146.38
7(Rafters) x 1.5" = 10.5"
146.38 - 10.5 = 135.88/8(Bays) = 16.98" (Between each Rafter)
Long Side.
184.25 - 2.12 = 182.13
9(Rafters) x 1.5" = 13.5"
182.13 - 13.5 = 168.63/10(Bays) = 16.86" (Between each Rafter)
16.98 - 16.86 = .12" or 1/8" Difference.
Joe Carola
http://forums.taunton.com/tp-breaktime/messages?msg=48515.7
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming
WOW!!! What a Ride!
short side = 148.5"
long side = 184.25"
the ratio of the long side to the short side is 184.25/148.5=1.240741
I round this out to 1.25, or a ten to eight ratio
This is the ratio of the number of equally spaced bays on the long side to the number of equally spaced bays on the short side
if we divide the short side by 8, we get 148.5/8=18.5625
then divide the long side by 10 to get 184.25/10=18.425
These are within about 1/8", and for practical purposes the same
I'll call it 18.5" centersLes Barrett Quality Construction
Less, your numbers are exactly what I said in my first post and my other posts.
http://forums.taunton.com/tp-breaktime/messages?msg=48515.16
Joe Carola
Joe, you have always had the right solution in your posts. I just tried to provide a different angle on obtaining the results. The algebraic solution was so he could substitute different wall lengths into the equations and be able to come up with any solution he was looking for. I didn't know why he wanted equal spacing or exactly what he meant by that, especially at the corners.Les Barrett Quality Construction
Joe Wood,
All your pretty pictures show a square pavilion. The dimensions you gave make it rectangular. So your pics cannot be applied. The other posters are trying to find a common denominator for the rectangle, and that’s fine, but they won’t work with out including the ridge length as a factor. It is the main factor as I stated in my previous post.
I have played around with your project as an academic exercise, just for fun. If you don’t believe what I have to say or do not take my advice it is no skin loss to me. It’s your pavilion. The odd dimensions are throwing it into new uncharted waters though. Sort of.
I maintain my assertion that the only rafter spacing you can use correctly is 1’ 5-7/8” o.c. (half the ridge’s length) or a multiple of it. (2x) or 2’ 11-3/4” (full ridge length). That’s it. No other spacings are correct for equality with the given dimensions.
This spacing is based on the difference between the length and width of your building. The difference is the unadjusted length (2’ 11-3/4”) of the ridge, real or imaginary, depending on how you configure the design of your roof. Full hip, French hip, Dutch hip, Whatever hip.
I did a few plan view layouts,
pic 2 is for a full hip
pic 3 is a Dutch hip with the spacing offset by one half
pic 4 is a Dutch hip with no offset on the spacing.
If I were framing this pavilion, pics 2 and 4 are my favorites because the last jack has a more pleasing placement. All pics show an 18” overhang.
Like I said, just for fun.
Joe,
If you want equal spacing with nothing left over on each side and all your jack rafters to be cut the same length and butt eachother you just play with the dimensions for any rectangle.
Short Side - 13'
Long Side - 19'
13'/9 = 1-5-5/16"
19'/13 = 1'5-9/16"....... 1/4" difference.
Short Side - 17'3-3/4"
Long Side - 32'8-1/2"
17'3-3/4"/12 = 1'5-5/16"
32'8-1/2"/23 = 1'5-1/16"........ 1/4" difference.
Short Side - 16'
Long Side - 28'
16'/12 = 1'4"
28'/21 = 1'4"
Your Dimensions.
Short Side - 12'4-1/2"
Long Side - 15'4-1/4"
12'4-1/2"/8 = 1'6-9/16"
15'4-1/4"/10 = 1'6-7/16"... 1/8" difference.
They all come up with 1/8" - 1/4" difference and one came up even.
Joe Carola
How did I know the rafter god was going to reply! ;0)You would never see that 1/8 inch.Hope your doin well Joe.Take care,Erik___________________________________________
Common sense is a gift from God that cannot be taught.
If you used 18" centers, the jack rafters will work out, but you will have small bays at the bottom of each hip.
I invented hip.
Generally, your hip comes in the same distance as your common rafters or 1/2 the width of the building. Subtract 12' 4.5" from 15' 4.25" = 2' 11.75". That's for 6' 2.25" in on each end. This is the length of your ridge, 2' 11.75". If you place a common rafter at each end of the ridge and one in the center, talking long side, you will have a space between the rafters of 15.75" if your rafters are 1.5". Add 3/4" for 1/2 the rafter thickness and your OC is 16.5".
If you build your hip with a common rafter coming right off the end grain of the ridge to the short side (width) you will want to start your layout from the center. I would definitely layout from the center for this one. Just remember those two rafters (one on each end) will be 1/2 the thickness of the ridge longer than the six other common rafters you'll need on that short ridge. This is because you subtract 1/2 the thickness of a ridge from your common rafters. Clear as mud, right.
You want all of your hip jack rafters to be the same so you can cut eight all alike for each length. Unfortunately you won't be able to use the hip scale to figure how much shorter to make each set of jacks but it will be pretty close to the 16" OC figures.
Beat it to fit / Paint it to match
Joe, it is possible to evenly space all the rafters all the way around. The formula is quite simple.
add the four dimensions (I'm assuming that you aredealing with a rectangle) and divide by any number...the answer is your evenly spaced rafters.
Here's my figures: 12 +12+ 15+15= 54'..........4.5+4.5 +4.25+4.25=17.5"
add em all up and you get a total of 55.475'
divide that by say.....28 rafters and you will have an oc spacing of 1.98125' which equals 23 3/4" oc.
technically, its 23.775" oc but I think we can drop the .005" for this project.
enjoy
blue
ps...don't expect the jacks to shake hands.
Joe,
I believe this is what you want.
Subtract the width of the building from the length to find the Ridge length. Divide the ridge by the number of spaces that you think will look nice. The result is the o.c. spacing for your rafter layout.
Divide the span by 2 for the centers of the King Commons.
15’4 ¼” – 12’ 4 ½” = 2, 11 ¾” (Ridge length)
2’ 11 ¾” / 2 (2 spaces at ridge) = 1’ 5 7/8” (Rafter spacing, o.c.)
The cut length of the ridge will be the ridge length plus the thickness of one rafter.
2’ 11 ¾” + 1 ½” = 3’ 1 ¼”
Or the length of your building minus twice the effective run.
15’ 4 ¼” - (2 x 6’ 1 ½”” = 12’ 3”) = 3’ 1 ¼” (SAA)
The effective run to calculate the rafter’s length will be the span minus the thickness of the ridge divided by 2. 1 ½” is typically the thickness of the entire roof’s framing components. (12’ 4 ½” – 1 ½” = 12’ 3”) / 2 = 6’ 1 1/2” (effective run)
Do not worry about the placement of the smallest jacks as they will shake hands perfectly at the hip and look just fine because they are evenly spaced with all the rafters. They do not need to be equally spaced from the corners of your building as the over hang will affect the visual aesthetics of the spacing too.
If you were to provide the Pitch of this roof I will post a cut list for you. Please include the level overhang dimension too if you want tail lengths.
Well, you can see (first 3 pics of an 8:12 roof) that equally spacing them isn't what I'm after .. to much space between the last jack and the hip.
The last 2 pics are of a 4:12 roof that Joe helped me with last week .. my first Dutch Hip .. thanks again Joe, you really helped me with the basics in understanding this roof style !
What I did with that one was to place my two outer Main Commons 6' back from the corner, then fill in everything between. Guess what ? All my rafters were spaced the same, 2' 6 13/16, on both sides ! Is this begginners Luck or what ? I guess this is what happens when you have a square bldg, but I wonder if the spacing would stay equal if I started those 2 main commons say at 5'6 ?
You see how the last jacks are over towards the hip ? That's what I'd like with all my hips, is have the last jacks always over there. I was hoping there was a Golden Rule about where you always place your first jack, so that first space wasn't over-span, and then, how to make all the rafter spaces all the same, on both sides, maybe by varying where those first jacks go.
I guess I understand now that you just have to Play with it .. try some different combinations, in order to get equal spaces. I'll keep messin with it.
Joe, Uncl Dunc is right. You are setting up too many hard and fast rules. Mathematically speaking, it's impossible.
After opening picture #1, I finally realized that you simply didn't want to end up with a wide span at the smallest jack. The solution for Pic #1 is to simply add one more set of jacks. The rafter itself won't bear on the plate, but it will attach to the hip and to the fascia that is shown on pics #3 and #4. Please understand that the smallest jack will have very little weight carrying capacity and be little more than a sheathing stiffener and/or decoration.
For purposes of many roof projects, using the plates as your span makes sense. But for decorative applications, if spacing is important, then is is best to use the overall roof dimensions at the fascia line.
In any event, if you are anal and want an exact match you would have to find the lowest common denominator. If your two spans don't have a reasonable common denominator, you might end up with rafters spaced something like 2.375 inches on center.....
At least they'd have "the exact spacing"!
No ones going to know if there are several spans with a plus or minus of 1/2 to 1".
blueIf you want to read a fancy personal signature... go read someone else's post.
Yep, you're entirely right Blue Eye, and that's probably exactly what I'll do with this model. I am going to have stringers around the topside, just like in the last few images, so structually it'll be just fine, and will look alot better !
You will by now have seen that I sent the method I would use to Framer.
You can just add another 18.5" for the overhang.
If you have some sort of zen reason for being even more equal than the 18.5" will give you, and assuming that your framing dimensions are written in stone, you could adjust the overhang to get an equal distance. This can be done algebraically with a couple of equations.
let d be the equal distance between the center lines of the rafters on both sides
let x be the number of bays on the short side
let y be the number of bays on the long side
for your current scenario,
xd = 148.5
yd = 184.25
therefore d = 148.5 / x and d = 184.25 / y
substituting, we get 148.5 / x = 184.25 / y
cross multiply to get 148.5y = 184.25x
divide both sides by 184.25
to get x = .80597y, in other words, x is about 81% of y
This is the source of the eight to ten ratio in my other post, from an algebraic perspective
Now you can vary the span from outside to outside of the walls to get closer to a perfect eight to ten ratio
If the zen of this structure does not require overhangs to be perfectly equal, it will be easy to move the exterior lines a skosh for the desired result
But then, what is width of a sawblade?
Les Barrett Quality Construction
You've set yourself an impossible task. You're assuming that there is some rafter spacing that is evenly divisible into both of your wall lengths, and for these two lengths, there just is no such spacing, at least none between 1 and 4 feet.
It's kind of like calculating the rise for stairs. You'd like to use 7" everywhere, but that won't always work, because you have to have a whole number of risers. In your case, you can start with either wall and divide its length by a whole number of rafter bays to get a spacing, but when you divide the other length by that spacing, you never get a whole number of rafter bays. This is not a matter of knowing how to lay out roofs, just an immutable mathematical reality.
In engineering terms, you've specified (at least) one too many constraints.
Short Side 148.5"
Long Side 184.25"
148.5/8 = 18.56" (Gives you 7 rafters @ 18.56" O.C.)
184.25/10 = 18.42" (Gives you 9 Rafters @ 18.42" O.C.)
18.56 - 18.42 = .14" or 1/8" Difference.
So one side is 18-9/16" the other is 18-7/16". If you lay the jacks out to butt each other at 18-1/2" you'll never see the difference.
Why won't that work?
Joe Carola
Cumulative errors
No, that's exactly what I did Joe, but I sure don't like that big space at the hip overhang. What I'm going to do is put the first jacks closer in to the hip, then start playing with the spacing.
Sure is nice having this forum hey ? Bounce questions off of everyone. Wish I would have had it back when I was starting out ! Thanks everyone, really appreciate the help.
Joe,
You said, "but I sure don't like that big space at the hip overhang. What I'm going to do is put the first jacks closer in to the hip, then start playing with the spacing."
Your right about that big space because the overhang was not included in my first post with these dimensions
Short Side 148.5"
Long Side 184.25"
148.5/8 = 18.56" (Gives you 7 rafters @ 18.56" O.C.)
184.25/10 = 18.42" (Gives you 9 Rafters @ 18.42" O.C.)
18.56 - 18.42 = .14" or 1/8" Difference.
So now all you simply have to do is add the overhang on both ends of the short side and the long side. We'll use 12" Overhang.
Short Side 148.5" + 24" = 172.5"
Long Side 184.25" + 24" = 212.25"
172.5/10 = 17.25" (Gives you 9 Rafters @ 17.25" O.C.)
208.25/12 = 17.35" (Gives you 11 Rafters @ 17.35" O.C.)
17.35" - 17.25" = .1" OR 1/8" Difference.
So now your first jack will be the same difference off the end of the hip overhang as the rest of the jacks.
Just figure your overhangs in first for both the short side and the long side and this method will work.
Joe Carola
Edited 10/10/2004 6:41 pm ET by Framer
I thought my message was getting too long, so I left out the paragraph that said you and several other guys offered solutions that are probably close enough for all practical purposes. I didn't mean to imply that there was anything wrong with your solution. My aim was just to point out that the reason the question is hard is that for these particular dimensions, there is no exact solution. Close enough is the very best we can do in this case.
As Sam said, there is still the question of accumulated errors. I haven't tried to add up the differences. I'm willing to accept your assurance that the difference won't be visible, especially since I'm not building the thing. :)