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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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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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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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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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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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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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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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Old 09-26-2012, 04:02 PM   #8
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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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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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Quote:
Originally Posted by Tham View Post
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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The rafter stand off, added to your rise.

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Old 09-27-2012, 10:50 AM   #12
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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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Old 09-14-2015, 11:24 AM   #13
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try this one Hip Roof Calculator
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Old 09-14-2015, 01:09 PM   #14
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Quote:
Originally Posted by RM575 View Post
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?

This is what the IRC would say about it:

Quote:
R802.3 Framing details.


Rafters shall be framed to ridge board or to each other with a gusset plate as a tie. Ridge board shall be at least 1-inch (25 mm) nominal thickness and not less in depth than the cut end of the rafter. At all valleys and hips there shall be a valley or hip rafter not less than 2-inch nominal thickness and not less in depth than the cut end of the rafter. Hip and valley rafters shall be supported at the ridge by a brace to a bearing partition or be designed to carry and distribute the specific load at that point. Where the roof pitch is less than three units vertical in 12 units horizontal (25-percent slope), structural members that support rafters and ceiling joists, such as ridge beams, hips and valleys, shall be designed as beams.
I've always take that to mean if you are using 2x4 commons, then a 2x6 hip rafter (and possibly valley) is required. Also, a 1x6 ridge board. You can always confirm this interpretation with your building department.
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Old 09-14-2015, 06:13 PM   #15
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