Run-Rise Relationship To Rafter Length

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  #1  
Old 11-23-05, 08:15 PM
tafallon
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Run-Rise Relationship To Rafter Length

I thought I would leave some information that I recently learned when determining the correct rise for the roof pitch of an exterior building I am constructing. If you have a better method or better advice, please leave it.

Without being able to draw a triangle, I will try to explain this as best as possible.

Situation: I am building a 14-foot by 16-foot exterior shed, the width being 14-feet. With this in mind, I wasn't sure what the best pitch for my roof should be. I looked everywhere for a good solution, only to finally discuss it with the builder who built my home. He came up with the easiest method, simple math. I also picked up a good little book called "Swanson's Little Blue Book for Roof and Stairway Layout", put out by the Swanson Tool Company in Frankfort, IL. It's a good reference book to have with either project.

My builder told me my home roof was a 10-rise. He recommended at least an 8-rise on my shed. To figure the proper rise, there is supposed to be a simple rule to refer to, the 8/12 or whatever you choose (6/12, 7/12, 8/12, 10/12, etc).

This is how it has been explained to me. With the 8/12 rule, this means the rafter will rise 8-inches for each 12-inch of run, which requires the rafter length 13-inches longer. With a 7-foot total run (the width of the building divided in half, in this case.. the width is 14-feet, so half would be 7-feet), the rafter length is 7 x 13-inches or 91-inches long (just about 7-1/2 feet). The pitch of the roof will be 8 x 12" or for every 8-inches of run length multiplied by 12-inches.

Confused yet? Me too.

Basically, to figure the rise, you take the run length (7-feet or 58-inches), multiply x 8 for a total rise of 56-inches. The total length of the rafter will be approximately 10-feet. For a 10/12, the total rafter length would be approximately 12-feet. Since I only have 10-footers for the rafters, I've chosen the 8/12 rule.

If I'm all wet on this and understood it incorrectly, someone please correct me. I am planning construction of the roof in a couple days. I want to be sure my measure is dead on the money. Swanson's book can be confusing to understand the basic principles.
 

Last edited by tafallon; 11-23-05 at 08:18 PM. Reason: Spelling and grammar errors
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  #2  
Old 11-24-05, 10:38 AM
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Basically, to figure the rise, you take the run length (7-feet or 58-inches), multiply x 8 for a total rise of 56-inches. The total length of the rafter will be approximately 10-feet. For a 10/12, the total rafter length would be approximately 12-feet. Since I only have 10-footers for the rafters, I've chosen the 8/12 rule.
It's a bit challenging to follow this if 7-feet is 58 inches rather than 84 inches. For 8/12 pitch the run is 12, the rise is 8 and the common rafter length is 14.4222. Applying that to 84 inches of run gives 100.955 inches or 8' 4" + 15/16" or nearly 8'5".
The 8'5" is measured to the building line. The 10' material that you have will probably still work, but the rafters typically extend to the fascia (or overhang) line, using the same triangulation. The common rafters are also shortened by half the thickness of the ridge beam. This will make a difference in the birdsmouth plumb cut.

I notice that the soffit material width is not mentioned in your calculations. By planning ahead, you can save time installing standard width soffit material by computing the overhang properly. The drip edge of the roof is usually 12" or more from the building exterior side walls.
For a gabled roof, it is also desirable to plan for lookouts in the gable ends (they are supported by framing set into the rafters) so that the drip edge on the sloping roof edge is away from the exterior end walls.

I am a bit taller than most, so I risk running my head into the drip line if the fascia causes the bottom of the rafter ends to drop significantly. For instance, if the building line is at 8', the rafter drops 8" when the soffit width is 12". This leaves the fascia at 7' 4" approximately.
 
  #3  
Old 11-24-05, 11:08 AM
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According to the "Full Length Roof Framer" by A.F.Riechers, the rafter length, minus 1/2 the width of the ridge for:
10:12=9'7/8"
9:12=8' 71/2"
8:12=8' 41/2"
7:12=8'3/4"

Here are a couple of links which may be informative;
http://www.awc.org/pdf/WCD1-300.pdf
http://www.tpub.com/content/construc...s/14044_59.htm
 
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