I’m laying out some timber rafters but for clarity sake let’s say they’re 2X. Let’s also say that at the peak they overlap and are cut off at the pitch off the opposing slope. I’m wondering how I can quickly layout the angle at the end of the rafter. In the past I have used an adjustable Starrett square which I could with confidence set to the correct angle, I guess I could also draw the angle and use a bevel square or set some guides on my framing square to keep it, but is their another way to do it perhaps with a framing square, or something else?
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Replies
You can get an inexpensive set of stair gauges for your framing square, or even use a couple of clamps and a straight piece of anything clamped to your framing square as a reference.
Dustin
If both roof pitches on either side of the ridge are the same....
90 minus twice the pitch in degrees= the angle in degrees of the cut you need to make.
Scribe it once on a piece of stock using a protractor or a speed square and then use the stair guage buttons that Dustin referred to to set up a framing square to this scribed angle.
It is the same pitch on both sides, I do know the angle and I do have stair guides which I use for all my rafter layout, I guess what I really was wondering was whether anyone knew a way to figure out this angle with their framing square. Rather than drawing a line with a protractor and then setting the framing square to match the line.
What pitch is your roof? I'll give you the angle in relation to 12 so you can set your square up. Do you own a CM calculator? If you do much roof framing, I find it's a nice addition to the belt.
Thanks for your replies. My pitch is 5/12. I do own a CM calculator, I'm not sure if it was such a good thing or not to get, on one hand it has made my life so much easier, on the other I used to do the trig. to verify my angles and it would be nice to remember the math once and awhile. With my pitch at 5/12 the associated angle is 22.62 d, and therefore the angle in question is 45.23 d. I guess I could basically set my square to 12/12. This isn't going to give me the level of precision I'm looking for, for the exposed timber frame joint. Basically I was laying these rafters out the other night, and it's been awhile, and I was dissapointed with myself because I couldn't find a precise way to lay these rafters out with the tools I had on hand.
44.76º
Jon Blakemore RappahannockINC.com Fredericksburg, VA
It's actually 11 7/8 in 12 for a 5 pitch
5 pitch + 22.62 degrees
22.62 X 2 = 45.24
90 - 45.24 = 44.76
44.76 degrees = 11 7/8" pitch.
I know what you mean about the Construction Master. But you can look at it this way..... you didn't know how to get the answer with math anyway, but here it is with the calculator.
Better to have the answer on hand when you need it and then go back and try to learn what you don't know with trig. At least that's the way I look at it. The CM gets me out of situations I would probably have to booger or spend a gross amount of time on onsite. And then I can go back in the comfort of my home and try to work through the math and learn something new. That takes the mystery out of it and is a way to continue to build my foundation of roof framing skills.
The key is to not get lazy. The calculator is a tool... just like your saw. Sure you could cut those rafters by hand.... but there's a better way out there. One that takes some of the human error out of things. But the calculator is meaningless and useless if you don't know what it is your plugging in. The calculator lets me frame roofs with relative efficiency and accuracy that I wouldn't be able to duplicate if I had to rely soley on my math skills. As long as the goal is to try to learn as much as the calculator already knows I don't see the harm.
Or you could stand at the sawhorses and scratch your head some more. ;)
OK, I've had dinner now, ready to start?;)
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Everything Brian told you was write. The best way is to use a speedsquare for that mark because it's easier to mark the 44.76° angle and that's even hard to get on the speedsquare.There's a few ways to do it with the speedsquare but the most accurate way and exact way to do it is to use a pattern. You can't go wrong that way because your scribing your exact plumbcut angle on the top.Scribe 5/12 plumbcut first with a stock pattern that has the 5/12 plumbcut and then flip it over holding it on the plumbcut mark and scribe the top and that's your angle.Joe Carola
Edited 12/6/2005 11:10 am ET by Framer
thanks.
erick,
just run em long and cut em off with a chainsaw.
Mr. T.
"I YAM WHAT I YAM AND THATS ALL THAT I YAM"
-U.S. Sweet Potato Council
just run em long and cut em off with a chainsaw.
You and me see the world through the same colored glasses I think :-) Gotta love the chainsaw
Hi Framer,
Based on your drawing and the fact we are using a structural ridge beam, should we put a birds mouth at the bottom of that plumb cut ? My thinking would have the rafter resting on the ridge beam and tied to the adjacent rafter??
Or should I quit thinking like a watch maker <G>
I've never framed a roof, but always found it interesting.
Bill
Thats a great formulae as it cuts out one step from the method I would have used to figure that angle. Thanks bro!
Glad I could help.
But I'd like to know, what method would you have used? Reason I'm asking, is that the method I shared is the only one I know. And to be honest I'm not really sure why it works but I'd like to figure it out. I just stumbled across it one day. Maybe your method will fill in the blanks for me.
First tell me you have a structural ridge beam holding this up.
if not, the fasteners pinning the two of these togeether is all there is to resist the loading of half the roof. That is a recipe for disaster.
Welcome to the
Taunton University of Knowledge FHB Campus at Breaktime.
where ...
Excellence is its own reward!
No structural ridge, what do you need one of those for... just kidding, they are actually 4X timber rafters joining at the peak with a mortise and tenon connection and supported with lower (what I call) purlin plates. I just thought it would be easier to describe the location in question the way in which I did.
Whew!Say the pitch and I will race Brian to get your answer.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
This is like pulling teeth.
Erick, it can be done with a framing square. create a plumbline, then mark the angle of the opposing roof, using that plumbline. Or, create a level line, them mark the angle of the opposing roof.
It's very simple but since I don't do it very much, it takes me two or three stabs at it with the square to remember exactly how to do it.
blue
"Based on your drawing and the fact we are using a structural ridge beam, should we put a birds mouth at the bottom of that plumb cut ?"Every house or addition I've ever framed the rafters had a birdsmouth and are run next to each other and the birdsmouth is nailed to the top of the beam and along side of the opposing rafter and that's all on the plans and figured out by the Architect. Don't you need an Architect for this? Around here the homeowner can draw their own plans. Is this your own house?You also said this, "they are actually 4X timber rafters joining at the peak with a mortise and tenon connection and supported with lower (what I call) purlin plates."Is this a Timber Framed house because it sounds like your rafters will butt each other at the top? So you wouldn't need to overlap them and have that overlap cut.Joe Carola
Ok, so I'm a bit confused by that previous message from 43BillH but so be it. Yes, the rafters I originally asked about are for my own house, which I also designed, since that's part of what I do for a living. So all the birdsmouth/plumcut stuff aside, the house is a timberframe with a portion of the roof framed with 4X rafters and lower 'purlin plates'. The rafters will be joined to one another at the peak but not butt plumb. Not that that wouldn't work okay but the detail I prefer has the cheek cuts of the shorter stick flush out with the underside of the longer stick with a tenon on the shorter stick joined into a mortise on the longer one. If I could figure out how to attach a drawing showing what I'm talking about I would.
Are you talking about a tongue and fork type joint? that would be normal for rafters like you describe. Purlin plates?. Use the math like was written and you can't go wrong . One thing to remmeber large timbers will not be as forgiving as 2x stock ,so accuracy counts. do the trig, or you will be chasing air out of joints for hours. Nothing looks as bad as bad timber joiney
Erick,Your talking about the rafters running along side each other like I said I always do like in this drawing, right?Joe Carola
That's how I initially desribed it because I thought it was quicker and easier to explain the angle I was looking for. In fact though, it will be as the picture in the message following yours shows and nothing does look as bad as bad timberframe joint, especially like in my case where I won't have green timber to blame.
Kind of like this, right?
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Piffin,I think you got it.This is a picture of the trusses I put in and the section with the blue circle is what your talking about. The second picture I just cleaned it up so you can see the joint better and you can also see the peg.Joe Carola
nice work joe
Sorry about that Olle but I didn't make those. They were Trusses from a job that I framed and I installed them. They were made in New York.Joe Carola
Now I can sleep happy. I learned something - a new stopped joint let in. That was a good looking joint!
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Nice joint, I won't have the shoulder though.
I think that is what he is talking about. So the purlins go above the rafters or are they housed into the rafters
I was sure to mention that 'purlin plate' is just what we've always called it. It is quite the misnomer however. As I am using it, it is simply a beam somewhere in let's say the top third of the span of the rafter which the rafter will essentially cantilever over and then join at the peak with the other rafter which will also be supported somewhere along its top third. Aside from providing a nice aesthetic, I like doing this because particulary in situations of long interior spans, instead of an enormous (solid sawn/timberframe) structural ridge, carrying lets say half of the tributary area of the roof you can get away with two smaller beams sharing the load and not looking so clunky.
erick, this thread was well under way by the time I found it and I didn't think I had anything interesting or different to contribute ... until I read your post #6.
"what I really was wondering was whether anyone knew a way to figure out this angle with their framing square"
You may find this Difference of Pitches calculation interesting. The rise/run for the framing square is calced directly in terms of the pitches and can often be done without a calculator.
Joe Bartok
Edited 12/9/2005 12:16 pm ET by JoeBartok
I should have explained the usage of that formula.
With your rafter configuation one pitch is "negative" with respect to the other. Substituting your 5/12 pitches the solution for the angle at the rafter overlap is:
[5/12 – (-5/12)] ÷ [1 + 5/12 × (-5/12)]
= [5/12 + 5/12] ÷ [1 – 5/12 × 5/12]
= [10/12] ÷ [1 – 25/144]
= [10/12] ÷ [119/144]
= 120/119 = 1.00840 = 123/32" over 12
Expressed as an angle:
arctan (1.00840) = 45.23973°
Joe Bartok
Edited 12/9/2005 2:13 pm ET by JoeBartok
Thank you for that...
I'm no expert, this question is simply out of curiosity.
Why would a structural ridge be needed provided there were enough fasteners? At the heel end of the rafter on most roofs, the only thing holding them from kicking out under the weight of the roof is the fasteners when it is through nailed to the ceiling joists.
When you use joist hangers, the only thing resisting the load there is the nails.
Check out Page #37 of this TF Guild pdf.
Joe Bartok
Interesting read.
Thanks
If anyone is interested here's some more "Framing Square Math":
Although it's a bit tough to relate the rafter layout to the diagram Sum of Pitches is the formula I should have posted in the first place.
This is a handy method of Bisecting an Angle using known values.
Joe Bartok
If there is a rafter tioe, or some other engineering done to restrain the walls from giving way under the outthrust of the forces of the roof, that idea is fine, but it is almost impossible to restrain or oppose those forces only with fasteners.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Maybe put them up with wild ends, then trim them all in place with a recip? Might or might not be quicker and easier, depends on how hard it is to work up there.
-- J.S.
Here's a way you can do it with the framing square and the speedsquare.
The Framing Square:
Draw your 5/12 plumbcut and set the framing square at 2-1/2" (1/2 of 5) at the top of the plumbcut and then mark the 6" mark (1/2 of 12) and scribe the blue line all the way threw to the bottom of the rafter and that's your overcut angle. If your rafter stock is big enough you can set the square at 5 and 12 because it will fit on bigger stock
The Speedsquare:
Draw a 5/12 plumbcut and hold your speedsquare with the pivot point at the top starting with the common pitches from left to right and slide the square to the left until it hits the 5 on the common mark and then scribe the right side of the square and that's your overcut angle.Joe Carola
Edited 12/7/2005 8:48 pm ET by Framer
I've got one of those big 12" metal speed squares. What's nice about it is that you can put a single stair guage button on it for times when you're working repetitively in degrees and not pitch. I'd put one button on it at the appropriate degree marking and it's long enough to mark most boards at one pass of the pencil. The "pivot point" acts as the other "button".
Just another option.
Who you kidding. You got that idea from Blueyedevil................;-)Joe Carola
Yeah I was hoping he'd join in on the Breaktime Christmas swap...... I've got just the right gift picked out. {G}
"Yeah I was hoping he'd join in on the Breaktime Christmas swap...... I've got just the right gift picked out."I think I beat you to it.Joe Carola
dude!!
you must REALLY know your rafter angles!!!
that sqar dont have no numbers on it!!
You Da Man!!!
well sort of...
Mr. T.
"I YAM WHAT I YAM AND THATS ALL THAT I YAM"
-U.S. Sweet Potato Council
I'm not that big on xmas and gift swapping. All of that sounds like a big pain. I'd probbly not get around to shipping anything till July.
blue
"I'm not that big on xmas and gift swapping. All of that sounds like a big pain. I'd probbly not get around to shipping anything till July."I guess that means your not shipping my present for my Birthday on Monday. Now I'm going to keep your speedsquare.........;-)Joe Carola
Edited 12/8/2005 6:59 am ET by Framer
I can't take credit for that one Joe. I haven't seen my stair gauges in 29 years. I know I own a set but I never use them. I probably used them twice before I decided that they were too bulky for what I do. I need my framing square to be able to lie flat without messing with the guages.
blue