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Old 12-18-2005, 01:57 PM   #1
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Some basic question about roofing help!


do you think it's right or wrong?

1. As a rule of thumb, 2/3 of the material must be left above the birds mouth of a rafter.

2. The ridge board should be the same width as the common rafter.

3. Shed roofs are primarily used for porches and additions to a building.

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Old 12-18-2005, 02:22 PM   #2
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Some basic question about roofing help!


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Originally Posted by motd0iyeuem
do you think it's right or wrong?

1. As a rule of thumb, 2/3 of the material must be left above the birds mouth of a rafter.

2. The ridge board should be the same width as the common rafter.

3. Shed roofs are primarily used for porches and additions to a building.
1) OK for rule of thumb. All the stresses can be calculated. The amount of lumber left supporting an overhang after the bird mouth is cut is critical. The rafter should still be full demension at the point where the plate and the level cut start.
2) The demension of the plumb cut on the rafter at the peak is longer than the width of the rafter. Next size up as a general rule. A ridge is customary for framing site built cut roofs but is not critical from an engineering standpoint.
3)That was probably true 70 years ago. Todays designs have multiple roof planes and often include a couple of shed roofs.

That's my opinion anyway. HS.

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Old 12-18-2005, 03:48 PM   #3
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Some basic question about roofing help!


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Originally Posted by hammerslammer
1) The rafter should still be full demension at the point where the plate and the level cut start.
What does this mean?
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Old 12-18-2005, 04:22 PM   #4
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Some basic question about roofing help!


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Originally Posted by AaronB
What does this mean?
The birds mouth has a level cut and a plumb cut. Where the level cut starts, (the point furthest away from where the level cut and the plumb cut meet), the rafter should be full width and resting on the plate essentially giving the rafter full bearing and the rafter strength from wall to ridge is not affected by the birds mouth. <P>

The birds mouth does affect the overhang. If your birds mouth takes 2" out of a 2x6 then your overhang is supported by the 3-1/2" left or a 2x4. That would probably be ok on a 2' overhang but might not be enough on a 3' overhang. <P>

that's is my thinking but as always I'm open criticism.

Last edited by K2eoj; 12-19-2005 at 08:00 AM.
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Old 12-18-2005, 04:48 PM   #5
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Some basic question about roofing help!


So a 2x6 should have a full 5 5/8" resting on the plate?
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Old 12-18-2005, 06:44 PM   #6
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Some basic question about roofing help!


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Originally Posted by AaronB
So a 2x6 should have a full 5 5/8" resting on the plate?
That's how I would do it. I never saw any reason to have an oversized birds mouth. They only serve to transfer all the load vertically and as a reference point. With engineering and or metal hardware birds mouths are not required at all.HS.
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Old 12-19-2005, 07:15 AM   #7
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Some basic question about roofing help!


I never knew that, but I am not a carpenter either. Thanks for the info.

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