Roof Pitch/Joists

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  #1  
Old 03-07-05, 09:06 PM
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Thumbs up Roof Pitch/Joists

I'm building a shed with a 3/12 pitch. For the life of me, I can't get the joists to fit. I could scribe them but I really want to do it with a framing square to learn. I think my DIY book left out a few steps for laying out rafters (actually, I think it left out steps for determining ridge board height.)
The length comes out fine. The shed is 93" wide. I divided this in half, 46.5", and stepped this off with a framing square. I added the tail. I cut the birdsmouth at 3 1/2" perpendicular to the building line. I cut back 3/4" at the ridge board. If you hold it up, it looks like it fits fine, but it is above the ridge board.
For the ridge height, I took 46.5/12= 3.875' x 3= 11.625 or 11 5/8. This is too low. I've been over and over this book and recalculated, I'm doing what they say but the ridge board needs to go higher. Is this because of the birdsmouth cut? I'm using a 2x6 ridge board and 2x4 rafters. It looks almost like the rafters would fit if they sat on top of the ridge board rather than be attached to the sides. Or it looks like the rafters would fit if the birdsmouth was cut completely through the 2x4 leaving nothing to support the overhang.
I'd appreciate any input, I think I'm almost there except for the ridge height, like I said I could just hold the joist up and measure the height but I'd really like to know the proper way to determine the height.
Thanks,
Brian
 

Last edited by awesomedell; 03-11-05 at 07:27 PM.
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  #2  
Old 03-08-05, 12:26 AM
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Cut four rafters, two for each end. Nail them in place. Lift the ridge into place and temporaily support while you nail the rafters. Plumb, brace and finish.
 
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Old 03-08-05, 06:12 AM
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DIY book

Is there some reason for using 3/12 pitch for the roof?
That's fairly flat - recommend that you go to at least 4/12 for drainage purposes.

The prior suggestion is good - build the roof off the ridge beam after it is level and centered on the walls. After it's built but not decked, take measurements and look at the computations provided in the book.
 
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Old 03-08-05, 07:47 AM
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The problem is that you didn't add for the H.A.P. cut (Height Above Plate) which is the plumb measurement that is the building line above your level 3-1/2" seatcut.

You said you have a 93" wide building and your using a 2x6 ridge with a 3/12 pitch and 2x4 rafters.

93" - 1-1/2" (Ridge Thickness) = 91-1/2" ÷ 2 = 45-3/4" = (Run)

3/12 = .25 x 45.75 = 11.4375" or 11-7/16" (Rise)

If you measure that H.A.P. cut Plumb Line it will measure 2-3/4" above your 3-1/2" level seatcut line/birdsmouth line. That is what you would add to the 11-7/16" rise.

11-7/16" + 2-3/4" = 14-3/16" should be the top of the ridge.

What happens when you figure a rise is that you are figuring a perfect triangle but you have to add the H.A.P cut plumb measurment. I always figure the rise deducting half the thickness of the ridge, so if you have a run and pitch you can figure the rise and if you drew a triangle it would be on the top of the rafter wher the birdsmouth is /building line and then you add the H.A.P cut plumb line. As mentioned already your best to set the rafters in place and slide the ridge up into it and that will set the ridge height. If you want you can go back later and put a 2x4 under the ridge even though you don't need one you can.

I hope this makes sense to you. If I could post a drawing I would.


Joe Carola
 
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Old 03-08-05, 11:43 AM
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Thanks Guys,
Joe, that does make sense. I knew something was wrong because the length of the rafters came out perfect, they were just sitting above the ridge. The carpentry book I was refering to has no mention of the HAP, although it does have a lot of good info on roofs. Anyway, I raised the ridge board and the rafters fit fine. I removed the 2x4 support I had under the ridge and they fit even tighter since the ridge could move a little (per suggestion).
Thanks guys, I'm in business.

Brian
 
  #6  
Old 03-08-05, 01:48 PM
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Handyone,

What book are you talking about?

Joe Carola
 
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Old 03-08-05, 01:53 PM
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I'm just trying to see if this link works out to a drawing that I made for ridge height but I go from the inside of the wall and add the plumbcut to figure the rise. Let's see if it works.

Joe Carola



http://www.construction-resource.com...ter_length.jpg
 
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Old 03-08-05, 10:56 PM
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That link worked fine Joe. The book I refered to is "Carpentry and Building Construction", by William P. Spence. It's more of a reference than a DIY book. It has over 700 pages from foundations to post & beam.
In the book's defense, I went back and read the pages on run and rise. Although they do not mention HAP, they did say the calculations give you the height from the top of the top plate to the top of the ridge. So I guess they assumed you would know to add the height of the material above the bird's mouth.
Anyway, I learned how to construct a roof. I'm done framing it and it came out nicely. The framing square is an interesting tool.
I think I'm ready for a dutch hip now...

Brian
 
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Old 03-11-05, 07:26 PM
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Excellent post Joe C and the drawing too. Really appreciate the assist as does the other member I'm sure. Keep up the good work!
 
  #10  
Old 03-13-05, 08:06 AM
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Thank You Awesomedell. Drawings always help when trying to explain things in person but on these forums they are really needed sometimes. You should see my jobsites with drawings all over the place but when you see the expression on someones face when your explaining something and drawing it out how it clicks in it's well worth it especially when cutting rafters when you see the rafters go up and it actually llooks like the drawing and plans you can see how and why it works.

Now we'll get Handyone to cut his Dutch Hip


Joe Carola
 
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