measuring common rafter bevel cuts on shed

Old 08-31-06, 08:28 AM
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Question measuring common rafter bevel cuts on shed

I would appreciate any tips for measuring beveled cuts and birdsmouth cuts for shed roof common rafters that will be nailed to an existing roof (not plumb cut and nailed to a ridge board). Is there a simple method for doing this?
Old 09-01-06, 08:34 AM
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I can answer part of your question -

The overall length would be calculated just like any other common rafter, using the point where the shed roof will meet the existing roof as your ridge point, and the vertical wall of the shed dormer as the outside of the span. The pitch of the shed roof will determine the length per foot of span. The rafter end cut and birdsmouth would be cut again using the pitch of the shed roof. For example: If you were adding a shed dormer with a 6/12 pitch to an existing roof pitched 12/12, the rafter end and birdsmouth would be cut just as any other 6/12 rafter, and the length of run per foot of span would be the same as any other 6/12 rafter.

The overall length at the top of the rafter would be the same as a normal rafter for the same span. What is different is the angle where it meets the existing roof sheathing. It seems to me that if you determined the angle for the ridge cut for the existing roof, then added the angle for the shed roof to it, you would have the angle for where the rafter meets the sheathing, using the top edge of the rafter as your pivot point.

Since I don't know the math to properly calculate it (especially since I don't know the pitch of either roof), I would probably cut one like it was joining a ridge, take it up on the roof, then scribe a line on it using the roof as a guide (best done at the edge of the roof holding the shed rafter where the birdsmouth is resting on the top plate of the shed wall, and the top edge of the rafter intersects the roof line). Then I would use that board as a template for all the others.

Not the right way, but it would get it accomplished. Hope this helps.

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