rafter sizing

I have played with joist sizing formulas a few times, but don't know about rafter sizing. Could anyone give me some pointers? Where do one find info for snow loads in ones area? I know my area would vary, we go from plenty to lots. I don't know how or if the rafter's pitch affect loads. Or is snow shedding not a factor merely a blessing?
Brandon

You can contact your local building department for snow load information.

Pitch can affect loads, shedding can be a factor depending on what type of roof material you use (metal vs. shingle), as well as exposure, etc. etc. You might also need to calculate unbalanced snow - what happens when the wind helps pile up snow on 1 side of the roof.

If memory serves there was some intro information in Steve Chappel's book:
http://books.google.com/books/about/A_timber_framer_s_workshop.html?id=pElSAAAAMAAJ

[img]http://bks5.books.google.com/books?id=pE...iUzOHV5kmnJpl9b[/img]

The short answer - pitch, roof material, exposure can affect the loads, and you need to calculate those based on the tributary area that each rafter is picking up, over its span, calculated against its length (a 14:12 rafter is longer than a 6:12 rafter over the same span).

I looked in a friend's copy but didn't see anything on rafters. I there a another source for equation for rafters?
Thanks,
Brandon

Brandon:
Go to this site and page: http://www.beamchek.com/publications.html

And download this free booklet called: "Introduction to Load Paths and Wood Design. You download it by clicking on the link that says: "View the Acrobat pdf version." It is a pdf file.

This explains load paths and how to calculate loads.

Basically rafters are considered by their run as the snow load is vertical. The snow load is the "live load" on the roof.
The dead load is figured by the length of the rafter. You combine the two to make the "total combined load".

Then you can adjust things for "snow duration" based on the type of roofing used. Snow slides off metal roofs when they warm up.
So the duration on a metal roof is less then on an asphalt or a wood shingle roof.

I hope that helps you to understand load paths.

Jim Rogers

Variables in equations. What happens in 8 years when the owner or new owner decides they don't like the metal roof, it makes too much noise when it rains, and they put a asphalt roof on? This then changes the load. So do they engineer for the higher load even though it is a metal roof?

How do you size a rafter Tim?

thanks. this will give me give me a starting poin. do people ya'll use the beam check much? I have seen it before. Also, I have wondered if there is a sort of beam sizing/ stresses quiz? So i see how well i am at getting the right answer.
Brandon

I use a lot of history.

Wait, maybe I should say a computer program, a program that encompasses past historical data.

Other factors are tree size available from my stand of trees or what is is on hand. At times, they are tapered, with some sweep or crooks present, even. Some times the right brain is more active in the decision process, as opposed to the left brains logical number driven force, a happy medium is the goal.