# Hip, common rafter birdsmouth depth?



## jlhaslip (Dec 31, 2009)

johnd393 said:


> 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.


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## johnd393 (May 12, 2007)

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?


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## jogr (Jul 24, 2007)

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.


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## jlhaslip (Dec 31, 2009)

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

deltacad 2-D drafting software package.


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## Gary in WA (Mar 11, 2009)

"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/proremodeler/article/CA462902.html#


Be safe, Gary


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## Joe Carola (Apr 14, 2006)

johnd393 said:


> 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.


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## johnd393 (May 12, 2007)

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






GBR in WA said:


> "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/proremodeler/article/CA462902.html#
> ...


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## johnd393 (May 12, 2007)

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.


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## mrgins (Jan 19, 2009)

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.


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## tpolk (Nov 7, 2009)

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


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## johnd393 (May 12, 2007)

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?


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## tpolk (Nov 7, 2009)

sorry thought you shortened at the birdsmouth at the wall.


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## johnd393 (May 12, 2007)

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.


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## tpolk (Nov 7, 2009)

so you lowered not shortened. why?


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## Joe Carola (Apr 14, 2006)

johnd393 said:


> 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.


John,

Read this post I made at Contractor Talk and check out the picture of the hip I marked and the drawing I made. This might clear up how you have to mark the hip HAP cut. Hope this helps.

http://www.contractortalk.com/f14/hips-valleys-61506/#post720163


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## Gary in WA (Mar 11, 2009)

Yep. And simpler if you are older on page #33: http://books.google.com/books?id=Z8...r=3#v=onepage&q=dropping a hip rafter&f=false

Be safe, Gary


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## johnd393 (May 12, 2007)

Joe Carola said:


> John,
> 
> Read this post I made at Contractor Talk and check out the picture of the hip I marked and the drawing I made. This might clear up how you have to mark the hip HAP cut. Hope this helps.
> 
> http://www.contractortalk.com/f14/hips-valleys-61506/#post720163



That's pretty much what I did except the rafter was not shortened at the top. 

This might be more than you want to read so feel free to skip it.

The garage is 24X28 with a 6/12 slope.
I used a speed square for all the plumb lines.
Starting with the commons, I made the plumb cut at the top. 
I measured down along the top edge, 161" and made the wall plumb line. 
I moved the wall plumb line to shorten the rafter half the ridge or 3/4" 90deg to the plumb line. Than positioned the square to mark a 2" seat cut. 
Than to fit over the sheathing I extended the seat cut 1/2" beyond the plumb line. Than cut it. 
The ridge board is 49 1/2" or 1 rafter thickness longer then the lenght - width of the building.
I built a scaffold in the center so I could work at the ridge. I didn't do anything to set the height of the ridge except nailing 2 commons on one side and raising it till it was plumb with a small level.
The commons and ridge all fit together nicely.
For the hips I made the double cheek cut at the top.
Measured along the top edge from the point of the cheek cut, 216" and made the wall plumb line.
Moved the wall plumb line 1 1/16" to shorten 1/2 the 45 degree ridge thickness. 
Marked the HAP along a line 3/4" in from the shortened plumb line.
Marked the seat at this HAP point and cut the seat to the shortened plumb line.
I trimmed the sheathing at the corners so I did not extend the hip seat cut to clear the sheathing like I did the commons.
The hips are or fit as if, they are too long.


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## Joe Carola (Apr 14, 2006)

johnd393 said:


> That's pretty much what I did except the rafter was not shortened at the top.
> 
> This might be more than you want to read so feel free to skip it.
> 
> ...


When pre-cutting the hips like that, you can run into this problem because if one wall isn't perfect the hips won't fit. You have to cut the hips at the top now in order for them to fit right because cutting the bottom at the HAP line would not be correct


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## tpolk (Nov 7, 2009)

same with rafters, sub tract ridge at top not at wall as wall cut changes vertical dimension at seat cut


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## johnd393 (May 12, 2007)

Well, something went wrong and I don't know what it is. The diagonal corner to corner dimensions were very near equal before putting up the common rafters. Maybe a bunch of micro errors made the ridge too low. All 4 hips looked long by about the same amount. I didn't measure it. 
As to my unorthadox shortening at the lower end. Lets create a sample rafter. Lets forget about the seat cut for a minute. Lets also eliminate any overhang and have the rafter end at the plate outside edge.
Now we cut plumb cuts top and bottom at the calculated rafter lenght. The rafter is too long because of the ridge thickness. I you put it up like it is, it will overhang the plate edge. It's also to high cus we didn't cut a seat cut yet. It's just a board with a angle cut at both ends. You could flip it around and turn it over and it would look the same. We could cut the extra off either end. We aren't screwing up the seat cut cus we didn't even lay it out yet. We can cut the seat for what ever amount of bearing and HAP we want.


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## tpolk (Nov 7, 2009)

no, you have a triangle. top of ridge to top outside corner of plate, horizontal run to center line of ridge(run), vertical to top of ridge(rise). as you raise the triangle at the wall plate to give rafter plumb height you create a horizontal seat cut. depending on how much rafter you want left you can put a seat cut that is 3.5" to the bottom inside edge of the rafter at the inside of the wall plate. what ever you raise the rafter=triangle you have to raise the ridge the same. If the ridge is a fixed point as in it has to be at a certain elevation to meet other roof lines then sometimes you need to shorten the wall height. the hip will be , has to be whatever this common rafter has established in a standard hip roof hope this helps


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## johnd393 (May 12, 2007)

I attached some pictures.
I pulled the nails out of the hip seats and trimmed them some to get the HAP right. Now I'm learning the an added benefit of cutting jack rafters, extra firewood.
I thought I would be better at this. I got 2 jacks to fit but it's not a fit to smile about.


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## tpolk (Nov 7, 2009)

things are looking ok. i know how frustrating a hip can be and I'm sorry I'm not a better typer to get whats in my head out better. do you own a framing square? on the square it gives you lengths for your jack rafters based on the pitch of your roof. it will tell you how much to add to each based on your on center layout. hope this helps


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## johnd393 (May 12, 2007)

I thought it would be fun to build the hip roof. The materials were $1000 cheaper than trusses. This old retired guy can lift rafters by himself. I won't have to do siding up to a high gable peak. I have a framing square, but mostly used a speed square. I used a calculator than checked it against the tables on the framing square. I started with the bottom jacks. For the first two corners, the calculated laid out lenghts weren't working so I trimmed and recut till they almost fit and nailed them in. One thing that was causing problem is, I was trying to layout on the hip exactly where the jacks would go. Then I read in the book to put them where the make a 45 degree angle with the hip. That was easier. The run of a jack is sposed to be the same as its distance from the corner. For 16" you get the same number as the Common Diff on the framing square. When I used 16 1/2 for the run I got a lenght that seemed to work. That extra 1/2 was the sheathing thickness. Three jacks in my discarded pile were close to that. One had the seat cut too deep so I stuck a shim under it. The last 2 bottom jacks went up the ladder once and got nailed in.
John D


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## tpolk (Nov 7, 2009)

:thumbsup::thumbsup::thumbsup:


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