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Shed Dormer


fredmorrison's Avatar
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05-01-05, 05:51 AM   #1  
Shed Dormer

I am planning a shed dormer addition that will have 2x8 roof rafters. For 3:12 roof pitch and 8 ft ceiling height I will have to cut a 4-1/8 x 16-1/2 bird mouth in the rafters. This leaves about 3-1/2 inches of meat in the rafter to continue the overhang.

Two questions:

1. Does the bird mouth size sound reasonable?
2. Is 3:12 pitch reasonable?

 
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05-01-05, 01:33 PM   #2  
Posted By: fredmorrison I am planning a shed dormer addition that will have 2x8 roof rafters. For 3:12 roof pitch and 8 ft ceiling height I will have to cut a 4-1/8 x 16-1/2 bird mouth in the rafters. This leaves about 3-1/2 inches of meat in the rafter to continue the overhang.

Two questions:

1. Does the bird mouth size sound reasonable?
2. Is 3:12 pitch reasonable?
1.) NO

2.) Depends upon your snow load.

http://www.awc.org/pdf/WCD1-300.pdf

 
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05-01-05, 03:36 PM   #3  
I can decreased the roof pitch and reduce the bird mouth to about 7/8 x 3-1/2 (the width of the top plate). Does that seem more reasonable?

It seems to me that snow load would be used to size the rafters and not establish the pitch.

 
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05-02-05, 02:28 AM   #4  
Hey Snoonyb, thanks for the awc website referral.

I can see clearly now how to do the dormer...by lowering the outside wall I can increase my pitch. Then attach the ceiling joists to the rafters instead of setting them on the wall to keep the 8 feet ceiling height except for a short distance at the outside wall.

 
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05-02-05, 05:10 PM   #5  
And as I recall, 2/3 of the area must have a min. 6'8" ceiling height.

 
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05-03-05, 01:00 AM   #6  
Posted By: Snoonyb And as I recall, 2/3 of the area must have a min. 6'8" ceiling height.
Good point! I will be able to maintain 8 feet except for 3 feet along the outside wall where it tapers to 7'-4. I haven't figured per cent but I am well over code minimums.

 
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05-03-05, 12:45 PM   #7  
Referring to the awc website:

http://www.awc.org/pdf/WCD1-300.pdf

I noticed that the shed dormer illustration shows the outside wall studs nailed to the sides of the floor joists. I would have expected to see conventional framing with the subfloor extending over a rim joist and a wall with a bottom plate. Although with my remodel project, I would have to either cut off the ends of the existing joists to accommodate a rim joist or fit blocking between the joists.

Is there a reason to frame a shed dormer the way it appears in the illustration?

 
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