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-   -   Hip, common rafter birdsmouth depth? (http://www.diychatroom.com/f19/hip-common-rafter-birdsmouth-depth-68144/)

johnd393 04-02-2010 02:42 AM

Hip, common rafter birdsmouth depth?
 
I'm building a 24' x 28' garage with a hip roof 6 in 12 slope.
The rafters are 2x6's and hips 2x8's. Walls are 2x4's.
If I layout birdsmouth on the 6x6 common rafters, To keep the top of the 2x8 hip at the same hight as the other rafters, the birdsmouth on the hip rafter seems too deep with the heel hanging over the inside of the wall. To have the hip fully bearing on the wall I can only have like a 5/16x5/8 inch birdsmouth on the common rafters. I read somewhere that the seat cut should be at least 1.5 inch. According to what I worked out drawing it with a CAD program. If I cut a birdsmouth with 1.5 inch horizontal cut on the common rafters, the horizontal cut of the hip hangs out a little over an inch inside the wall.
John

jlhaslip 04-02-2010 07:47 AM

2 Attachment(s)
Quote:

Originally Posted by johnd393 (Post 423014)
I'm building a 24' x 28' garage with a hip roof 6 in 12 slope.
The rafters are 2x6's and hips 2x8's. Walls are 2x4's.
If I layout birdsmouth on the 6x6 common rafters, To keep the top of the 2x8 hip at the same hight as the other rafters, the birdsmouth on the hip rafter seems too deep with the heel hanging over the inside of the wall. To have the hip fully bearing on the wall I can only have like a 5/16x5/8 inch birdsmouth on the common rafters. I read somewhere that the seat cut should be at least 1.5 inch. According to what I worked out drawing it with a CAD program. If I cut a birdsmouth with 1.5 inch horizontal cut on the common rafters, the horizontal cut of the hip hangs out a little over an inch inside the wall.
John

I assume that you are using 6/17 to calculate the hip rafter layout??? Are you 'stepping off' for the hip length? or working from calculated lengths?

The deeper 2 x 8 hip rafter will intrude into the attic space and unless you plan on a cathedral ceiling, should not be an issue. Fir out the underside is easier than back cutting on a cathedral, but I digress...

Cut the common rafters so the level cut of the birdsmouth sits exactly on the top plate. see image 1 below.

Cut the hip rafter using the same 'height above plate at the birdsmouth.

The 'drop' of the hip rafter for a 6/12 is 1/4" which means the level cut needs to be 'raised' up on the rafter to allow the sheathing to 'plane out' on the top surface of the roof.
see image 2 below. The red lines are for the dropped hip.

Best source of info for roof cutting that I have found is "The Roof Framers's Bible" by Barry D Mussel available from Amazon.

johnd393 04-02-2010 12:22 PM

It's not a cathedral ceiling. there's 2x10 ceiling joists.
Your excellent drawing illustrates my, perceived, problem.
Only about the end 5-11/16 inches of of that 10-27/64 inch is supported.
Is this something I don't need to worry about?

The walls are up. I just have to sheath the front and the top 32 in of the sides & back, than I start the roof. The walls are about 10 1/2 feet high. Doing all the framing myself, so far.


What did you use to draw that?

jogr 04-02-2010 01:10 PM

Thats ok. A 2x4 wall is only going to allow 3 1/2" of bearing for any common rafter and only about 5" at the corner for the hip rafter.

jlhaslip 04-02-2010 02:19 PM

nothing to worry about. That's exactly how it should be.

deltacad 2-D drafting software package.

Gary in WA 04-02-2010 10:31 PM

"Only about the end 5-11/16 inches of of that 10-27/64 inch is supported.
Is this something I don't need to worry about?" ---- wrong, worry! Use 1-1/2" of bearing for the commons. The heel of the rafter needs to bear as much as possible on the wall, especially the hip for strength needed:

Figure #4: http://www.housingzone.com/proremode...CA462902.html#


Be safe, Gary

Joe Carola 04-02-2010 10:37 PM

Quote:

Originally Posted by johnd393 (Post 423131)
It's not a cathedral ceiling. there's 2x10 ceiling joists.
Your excellent drawing illustrates my, perceived, problem.
Only about the end 5-11/16 inches of of that 10-27/64 inch is supported.
Is this something I don't need to worry about?

Nothing to worry about at all. This is done every day on thousands and thousands of houses with no problems at all.

johnd393 04-03-2010 03:48 AM

Thank for all the replies. I'm gonna do the 1.5 inch bearing on the commons which feels like the best compromise. About 1 1/8" of the heel of the hip will be over air but it's only about 3/8" of the width of the 2x8.
John




Quote:

Originally Posted by GBR in WA (Post 423382)
"Only about the end 5-11/16 inches of of that 10-27/64 inch is supported.
Is this something I don't need to worry about?" ---- wrong, worry! Use 1-1/2" of bearing for the commons. The heel of the rafter needs to bear as much as possible on the wall, especially the hip for strength needed:

Figure #4: http://www.housingzone.com/proremode...CA462902.html#


Be safe, Gary


johnd393 04-23-2010 12:47 AM

Update: I ended up with 2" bearing on the commons cus I cut the 1st one wrong. I laid everything out to the 24' x 28' frame dimensions, than bumped the seat plumb cut out 1/2 inch so it would sit over the sheathing. For some reason the hip seat plumb cut wants to hang out 3/4" from the corner. I double checked the book and I can't see where I made any error. The only wierd thing I did is I shortened the rafters at the bottom plumb line instead of the top. The commons are shortened 3/4" perp to the plumb line and the hips 1- 1/16". Before shortening commons are 161", if I remember right, and hips 216". (6/12 slope) I cut the double cheek plumb cut at the top of the hips than measured down 216" along the top edge from the point, than made the lower plumb line. Than shortened 1 1/16" perp to the lower plumb line. I used some clamps to pull the hip seats against the building corner which made the hips bow a little. Tomarrow I'll see if some jacks will straighten the hips.

mrgins 04-23-2010 11:33 AM

The hip rafter does not support anything. You could even use a 2x6. The hip jacks support the hip rafter, rather like the commons support a non-loadbearing ridge.

tpolk 04-23-2010 06:39 PM

shortening your cut for the ridge thickness at the wall cut is incorrect and will change your vertical elevation at the wall as it relates to the hip. you can also rip a two sided peak on your hip and ridge so you dont have to guess where the rafters/jacks hit

johnd393 04-23-2010 07:16 PM

If you take a board and put a plumb cut at both ends and than you cut an inch off either end, you get the same thing, don't you?

tpolk 04-23-2010 07:33 PM

sorry thought you shortened at the birdsmouth at the wall.

johnd393 04-24-2010 11:41 AM

I did.
I decided to pull the nails out at the wall end of the hips and trim the seat so they the don't sit to high.

tpolk 04-24-2010 04:23 PM

so you lowered not shortened. why?


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