Rafter layout from existing ridge.

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  #1  
Old 08-03-14, 07:21 AM
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Rafter layout from existing ridge.

Sorry if this has been discussed already but maybe I'm using the wrong search terms...

Aside from holding a rafter in place and scribing, what is best way to layout rafter cuts (ridge, building line, and seat cut) to fit an existing structure? For example, an existing ridge board at a fixed height to an existing outside wall also of a fixed height, taking into account the H.A.P. and seat cut (birds mouth). I found lots of calculations and methods for determining ridge beam height for a new rafter layout but not so much the other way around. I'm sure it is just a matter of doing some calculations in reverse but I'm having a hard time grasping the concept.

Thanks!
 
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Old 08-03-14, 07:33 AM
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Hi mw, the pros will be along and I will be watching to see if they have a trick. One of the variables is how deep you cut the birds mouth. On a 2x4 wall (dating myself) I always cut the upper flat to be 3.5", but on a 2x6 wall a 5.5" cut might be more than you want, depending upon your rafter size.

I'm afraid, I fall into the scribing category, it is faster, and I like math.

Bud
 
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Old 08-03-14, 07:36 AM
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Thanks Bud. It's a 2X4 wall so I will be using a 3 1/2" seat cut. The problem with scribing is that it is hard to accomplish by yourself.
 
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Old 08-03-14, 07:54 AM
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Screw a 2x4 vertically to the ridge beam approximately where your rafter will line up with its position on the wall and drag one end of the rafter up just over the peak. Tack the rafter in place, it will now be the face dimension above where it will end up. At the birds mouth the flat portion needs to be lifted the same amount above the wall. Temporarily secure the birds mouth end to another vertical 2x4 and recheck the ridge end to be sure it didn't lift off of the beam.

A little trial and error, but once in place draw the cut lines for top and bottom and you have your pattern. Once you cut your pattern, test it in other locations before you cut all of your rafters.

This is one of those "easier done than said" tasks.

Bud
 
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Old 08-03-14, 08:02 AM
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Thanks... I'll give that a try! Oh... and just to simplify things, I'm not concerned with the rafter tail length. I plan to snap a chalk line and cut them to size afterward.
 
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Old 08-03-14, 08:18 AM
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It is always amazing how difficult it is to build something simply square. Cut everything just as it should be and put it together and struggle to compensate for the irregularities. Let us know how it comes out.

Bud
 
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Old 08-03-14, 11:01 AM
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I would use a framing square to mark the ridge cut and the bird's mouth. I need to know the roof pitch to use this method.
 
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Old 08-04-14, 05:56 AM
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OK... So I've been doing more research on this subject and it seems like the consensus among the pros is that that scribing or something called the "block and string" method are the most practical ways to do this. Can anyone explain or maybe provide a link to this "block and string" method?
 
  #9  
Old 08-05-14, 05:22 AM
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Here is an explanation with pictures:

rafter length
 
  #10  
Old 08-05-14, 03:46 PM
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Thanks! His narrative is a bit hard to follow but I get the picture.
 
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