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ICanCU · Oct 6, 2007, 07:24 PM

I want to know how long my rafters should be if my overall span is 141" (11' 9")

The pitch I would like to use on the roof is a 6 / 12 pitch

Also how do I determine the birds mouth and plumb cuts?

I would also like to use a 8 inch overhang.

Thanks in advance!

Keith

MOWERMAN2468 · Oct 7, 2007, 07:19 AM

You need to purchase a "swanson, or quick square". Some will come with a "blue book" that will have the formulas, and the instructions on how to use the square and make the cuts. As far as the plumb cuts put on all the rafters, use a chalk line and on the top of the rafter on each end make a mark, then "pop" the line from one end to the other. Then you will have a reference point on each rafter. Then use these reference points and a two foot level, at each mark, put the level against the rafter, get the level plumb, make the mark, and make the cut on the mark, that is how you get the plumb cuts. I have a book for the square out in the shop or a work truck, but you are going to need the square anyway. The square I am referring to is not a "L" shape, but is triangular shaped. There will be a hollowed out groove and the center will be hollowed out for reading degrees with the square. Good luck. "hollowed out" may not be the best term to have used here.

MOWERMAN2468 · Oct 7, 2007, 07:22 AM

Also, if you know how to figure the hypotneuse of a triangle, that would give you the length of the rafter, I would recommend adding about a foot to each rafter on the tail end to make sure you have enough length to cut your plumb mark. Also, an 8" overhang is not very much. i would reccommend at the very least a 12" overhang, more common is the 16" overhang.

MOWERMAN2468 · Oct 7, 2007, 07:48 AM

Originally Posted by ICanCU

I want to know how long my rafters should be if my overall span is 141" (11' 9")

The pitch I would like to use on the roof is a 6 / 12 pitch

Also how do I determine the birds mouth and plumb cuts?

I would also like to use a 8 inch overhang.

Thanks in advance!

Keith

141" / 2 = 70 1/2"
70.5 / 12 = 5.875'
5.875' x 6"/' = 35.25"
so
c= sq root of a squared + b squared
c = 35.25 x 35.25 = 1242.5625" + 70.5 x 70.5 = 4970.25"
c = sq root of 1242.5625 + sq root of 4970.25 = 78.82139621" (this is the length) BUT ADD for the length of the cut at the top, and add the length of the overhang plus 12" umm, roughly an 8 foot 2 x 4 should be long enough. But I would still recommend the use of a larger overhang. Say at least 12", then i would use a 10' 2 x 4 .
if you draw out a right triangle, and label the longest top side ( hypotneuse ) "c",
the upright leg labeled "a", the bottom base side "b", then use the formula of
"c" = square root of "a" squared + "b" squared.
then you will have the length of the rafter. But it will have to allow room for the cuts on both ends.

glavine · Oct 8, 2007, 02:29 PM

I want take time to explain how to do this, but you need to go to Lowe's and get a carpenters square, also buy a swanson speed square with the book that come with it, inside it will give you pic details that you need to lay this out.
Also a big help to you will be to get a framing calculator, there $50 and worth every penny there also at Lowe's with the squares.

mcmxix · Oct 12, 2007, 04:05 PM

Originally Posted by ICanCU

I want to know how long my rafters should be if my overall span is 141" (11' 9")

The pitch I would like to use on the roof is a 6 / 12 pitch

Also how do I determine the birds mouth and plumb cuts?

I would also like to use a 8 inch overhang.

Thanks in advance!

Keith

Sorry but basic maths on the length. Point to note current UK build regs say that shouldn't make any cuts along a rafter use metal joints and nails - no birds mouths over purlins

mcmullanryan · Apr 9, 2009, 01:35 PM

Originally Posted by mcmxix

Sorry but basic maths on the length. Point to note current UK build regs say that shouldn't make any cuts along a rafter use metal joints and nails - no birds mouths over purlins

Correct no birds mouth over purlin though he is talking about the wall plate! And it's a cut roof not truss

creahands · Apr 9, 2009, 07:12 PM

Hi icancu

On a framing square there is table that will give length of rafter per foot. If u hold 6 on short leg and 12 on long leg, this will give u the plumb cut. Measure from top of plumb cut using the info from square. Deduct 1/2 the thickness of ridge board.

The length of rafter will be 5' 101/2" times 13.42" minus 1/2 thickness of ridge board.

Good luck

Chuck

21boat · Apr 17, 2009, 09:37 AM

Now lets assume the 2' square was left at the shop or lost, and you want a template that's usable to trace off.

A 6/12 pitch is 6" inch rise for every one foot of horizontal length. Rough cut a 2by 14" or a piece of ply the width of the rafter, but make sure its square at ends.

Lay on a board and pencil top, bottom, and end on another board. Measure back 12" back from the end cap line on the bottom line traced. You now have a 12" mark and the end cap penciled is the control here for the pitch/rise. Measure up from there 6".

Now lay the scab back down and slide to the right a couple of inches keeping the bottom left of the scab tail end at the 12" mark and the opposite bottom end at/on the 6" rise mark. Now transfer old line ( end cap) and mark the scab bottom and top keeping it intersecting at the 12" mark and the 6" rise mark.

You know have a template tracer for the angle to cut end rafter to ridge board ( top of ridge peak. You can measure down from top of ridge board towards bottom where the rafter will end on the ridge board and measure from there to the bottom of the outside plate 1/2" down from top of double plate.

OR
Lay up the already cut rafter against the ridge board and rest it on the outside top plate and mark the inside and out side width for the birds mouth. From those plumbed lines measure up 1 1/2" for the seat depth. There will be a slight change in the angle or width but it is very small. Re tweak after that and adjust it on second rafter when first is the tracer.

The over hang I like to let run wild until all of the rafters are set. Reason for this is because the outside plates has deflections and will kick things a tad

After rafters are set then I level out for the overhang. You wanted 8" there you will have to decide if you are going to use a 1by or 2 by to cap the overhang for the fascia.

ballengerb1 · Apr 17, 2009, 01:32 PM

Lets all keep in mind the OP put this question out there 2 years ago. I sure hopoe he is done by now. LOL

mcmullanryan · May 20, 2009, 01:31 AM

Originally Posted by mcmullanryan

Correct no birds mouth over purlin though he is talking about the wall plate! And its a cut roof not truss

Actually there is a cicumstance which would call for the rafter to have a birdmouth over the purlin, when the purlin in fitted plumb this is required.

rartis9 · Mar 7, 2012, 11:03 PM

How do I figure out the length and seat cut of rafters if my building is 12ft.wide a 6/12 pitch

hkstroud · Mar 8, 2012, 09:52 AM

The location of the bird's mouth along the rafter should be the square root of the length of the run to the exterior edge of the wall (72") minus 1/2 the thickness of the ridge board squared, plus the height or rise squared.

Assuming 2 X material for ridge board.

72" - 0.75" = 71.25= run

71.25 X 71.25 = 5076.5 = square of run

36" = rise = the top edge of the ridge board

36" X 36" = 1295 = square of rise

5076.5 + 1295 = 6371.5 =square of run + square of rise

Square root of 6371.5 = 79.82

Measure along top edge of rafter 79.82" and mark.

Place framing square on the mark. Align the 6" mark of one leg of the square and the 12" mark of the other leg with the top edge of the rafter. Just like you did when cutting the ends of the rafter. Draw a line across the rafter.

Place your square on that line. Slide the square up or down that line to obtain the desired depth of cut for the plumb line and desired length of the seat cut. Draw a perpendicular line. The length of the seat is a trade off with how much material is left to support the over hang. Assuming a 2x4 rafter you probably want a plumb cut about 1 to 1 1/2". That should give you a seat cut of a little less than 3".

If 12' is the exterior dimension of the framing, you add the thickness of the siding to the length of the run. Otherwise you will be notching the siding to fit around the rafters.

When using you framing square be consistent. If you use the inside measurements on one leg, use the inside measurements on the other leg. If you use the inside markings on one leg and the outside markings on the other you won't like it. Yes, I have done that and it doesn't work..

When cutting bird's mouth, make all your cuts inside the bird's mouth. In other words when you make the plumb cut it is easy to make the cut because you see the line. When you go to make the seat cut, if you can see the line you will be cutting outside the bird's mouth and you bird's mouth will be off by the thickness of the saw blade. However, that's all right if you do them all that way.

hkstroud · Mar 10, 2012, 10:26 AM

However, you can avoid all the measuring and calculating.

creahands · Mar 10, 2012, 04:30 PM

I like HK's method.

When I have the ridge board in place, I nail the rafter to end of ridge board and end of wall with rafter set flush to top of ridge and at location of birds mouth. Mark it and make my cuts. Leave rafter run long and cut over hang after all rafts are set.

Chuck