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Old 09-25-2012, 02:20 PM   #1
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Hip Roof Framing Questions


Hi folks,
I'm building a detached workshop, 1 story, 11' x 15' on concrete slab with 6" stem wall and want to frame a hip roof. I've researched a lot on hip roofs and have some books as well. I'll be joisting the ceiling with 2 x 6, 24" oc for type X drywall. I want a 4:12 pitch. The walls are 2x4 framing with 1/2" ply sheathing.

1. I've read that the ridge board and hip rafters must be one size larger than the commons and jacks. So, if I use 2x4 commons and jacks, I would need 2x6 ridge and hips, correct?

2. I'd prefer to drop the hip rather than chamfer to equalize the sheathing plane. Would doing a deeper seat cut dangerously weaken the 2x6 hip?

3. Is a "square" cut made at the plate corners for the hip cuts to match up?

4. Does someone know of other reference material, books, videos, websites on framing hip roofs for capable diy'ers?

Thanks.
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Old 09-25-2012, 02:32 PM   #2
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Hip Roof Framing Questions


give these a try http://www.blocklayer.com/roof/roofeng.aspx

http://www.basiccarpentrytechniques....p%20Roofs.html

seat cuts should not extend interior of the top plate
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Old 09-25-2012, 04:57 PM   #3
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Hip Roof Framing Questions


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Originally Posted by GBrackins View Post
give these a try http://www.blocklayer.com/roof/roofeng.aspx

http://www.basiccarpentrytechniques....p%20Roofs.html

seat cuts should not extend interior of the top plate

Gary,
Thanks for replying. I've been on those websites and they've helped.

But, when I plug in the dimensions of my building in the blocklayer site, I get the warning..."Birds-mouth depth in Hip will be greater than 1/3 hip depth. This can significantly weaken hip. Continue?"

I've tried entering various dimensions to see if the warning disappears. I've changed the birds-mouth seat from 3.5 to 0 and still the warning appears. So, I'm not sure what is going on there.
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Old 09-25-2012, 07:39 PM   #4
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Hip Roof Framing Questions


Does anyone know if it is possible to safely use 2x4 common rafters in a hip roof on a building with these dimensions (see op)?

I've tried the calculations using 2x4 commons with 2x6 hip and I'm finding that the birds-mouth depth in the Hip will be greater than 1/3 hip depth.

The span of the hips are 8' 8 15/32" x 2" x 6" with a HAP (height above plate) as 3".
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Old 09-25-2012, 10:38 PM   #5
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Hip Roof Framing Questions


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seat cuts should not extend interior of the top plate
Gary,

On the hip they always do.
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Old 09-26-2012, 12:25 AM   #6
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Hip Roof Framing Questions


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Gary,

On the hip they always do.

Would you have any recommendations on the 2x size of the commons and hips?

I'm hoping to keep the overall height of the building as low as possible with a hip roof to meet city height restrictions and with 2 x 4 commons at a 4:12 pitch I am just at the limit.

But, the warning I'm getting on the blocklayer's website about the birdmouth depth of the hip is a concern. The hips would be 2 x 6 with 3" HAP.
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Old 09-26-2012, 09:51 AM   #7
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Hip Roof Framing Questions


joe,

when I worked with my dad many years ago he always cut a birdsmouth for an extended hip rafter, like in the attached photo. maybe he was wrong, but it seemed to work fine. We'd set up the main ridge then measure from the hip-ridge connection down to the top plate and he'd figure his cuts. Man was amazing with a pencil and square. Sometimes I thought he could figure out where I went on Saturday night with his pencil and square.

the "notch" for a birdsmouth is not a notch, not in the traditional sense of the word notch. It's not the same as notching a floor joist. It is a bearing seat cut, and the extended rafter is basically cantilevered.

I was taught that if it extended (seat cut) to the interior side of the top plate you run the risk of the member splitting back up the rafter at the point of the interior side of the top plate. Maybe I didn't pay close enough attention in class and understood it incorrectly.
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Last edited by GBrackins; 09-26-2012 at 01:47 PM.
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Old 09-26-2012, 04:02 PM   #8
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Hip Roof Framing Questions


I reworked the figures on the blocklayer website for roof framing and find that with 2x6 commons and 2x6 hips, the overall rise from the wall is 2' 2 25/32". I don't know if that rise is measured from the top plate or from the top of the rafter at the wall. Can someone clarify? If it is from the top plate, that would give me 2 inches of space above the roof frame for sheathing and roofing material (felt & composition and I would be within the city's height limit for this building. If not, then I may need to adjust my pitch to a 3:12.

Also, everything I've read indicates that hip rafters should be one size larger than commons. So, I tried making the hips 2x8's and immediately the blocklayer calculator gives a warning that the hip birdsmouth cut is too deep. Again, this is a 4:12, birdsmouth is 3 7/8" for 2x4 framed walls with 15/32 sheathing. So, can the hips and commons be the same dimensional lumber? Can they all be 2x6? The ridge would be 2x8 by the way.

Last edited by RM575; 09-26-2012 at 08:09 PM.
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Old 09-26-2012, 11:22 PM   #9
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Hip Roof Framing Questions


At first thought rise would be added to the depth of your rafter at the exterior of your top plate. If your plumb cuts on your rafters and jack rafters are greater than the width of your Hips and/or Ridge they must be designed as beams. I don't have time to draw it out.

Are you sure 2x4's will span?
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Old 09-27-2012, 12:32 AM   #10
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Hip Roof Framing Questions


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At first thought rise would be added to the depth of your rafter at the exterior of your top plate. If your plumb cuts on your rafters and jack rafters are greater than the width of your Hips and/or Ridge they must be designed as beams. I don't have time to draw it out.

Are you sure 2x4's will span?
tham
Thanks for the reply. My original post mentioned 2x4's, but I think I can do 2x6's instead for commons. I want to know if I can also use 2x6's for the hips.

The blocklayer program mentioned the "overall rise at the wall." So, your saying that measurement includes the depth of the rafter as it rests on the top plate?
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Old 09-27-2012, 09:00 AM   #11
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Hip Roof Framing Questions


The rafter stand off, added to your rise.

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Old 09-27-2012, 10:50 AM   #12
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Hip Roof Framing Questions


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The rafter stand off, added to your rise.

tham

That's what I thought. So, with a 4:12 pitch and 2x6 rafters, it's too high for the city by 3 inches. I'm not sure they would challenge this, but I don't want to take any chances and have to make expensive changes later.

I measured for a 3:12 pitch, 2x6 rafters, and it looks like it would be 2 inches under the city height limit which would allow room for 1/2" sheathing and some lite weight roofing material. I ran some string to visualize the roof height and slope and the 3:12 looks just as good, if not better, for this small building.

So, do you know if a 3:12 slope requires anything special in rafter sizes or installation? Beams? Double hips? This is a simple building, 11 x 15, 2x4 wall framing, in California, no snow load, mild climate, and no heavy roof material planned.
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