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Old 11-07-2007, 08:56 PM   #1
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Finding angle where two roof slopes meet

I'm building a shed porch roof and tying into the existing house roof. Well, the end of the rafters will, but they will be supported by a knee wall built on top of the existing wall. The existing roof is 5/12 and the shed roof will be 4/12. Does anyone know how to find the angle to cut the shed roof rafters where they will meet the existing roof (other than using a sting and bevel)? I saw this along time ago, but can't find it now.



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Old 11-07-2007, 09:29 PM   #2
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You might try the calculator at this link:

Edit: I don't think that link has the angle you are looking for, I will try and find another.


Last edited by spebby; 11-07-2007 at 10:33 PM.
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Old 11-09-2007, 10:38 PM   #3
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why wouldn`t you run them next to the house rafters and secure them thru the sides,it`s stronger that way
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Old 11-09-2007, 11:24 PM   #4
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I think I would take a sheet of paper and draw the two pitches on top of each other and my cut angle would be looking up at me. It's also quite entirely possible I misunderstood the question. Cliff
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Old 11-10-2007, 04:49 PM   #5
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To avoid the headache of doing all the calculations,which will probably turn into a trial and error episode anyways,why not just do a "field fit".

I take it your laying these rafters directly over the rafters on the existing 5/12 pitched roof,and the knee wall is going to pick up the weight of the roof at midpoint,with a second rafter extending to the existing roof?
If so----
Once you get your knee wall and outside wall up(and plumb),cut rafter to carry this span.Place a string line on the top edge of this rafter,extending it to the existing(5/12) roof,and tack in place.That's the length of the next rafter.The angle along the bottom of the string line is the angle,which can be found with a speed square.
Or you can take a short length(2'-3') of the rafter material you'll be using and,hold it so the string line is flush with the top of it (lay the upper, bottom edge on the roof),than trace along the side of it with a straight edge(another narrower piece of lumber) laid on the slope of the 5/12 pitch.This will take another set of hands.Once you have this angle transfer it to a pattern,cut for length and test it.Use one piece to cut all the other rafters.

We use this method because your working with the actual angle and nothing is ever by the book.

I take it you have some experience so I am taking some liberty with my description.

Any more ?'s just ask.

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