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Old February 10th, 2011, 02:11 AM
bobm bobm is offline
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Default rafter span tables/No pitch

Hello All,

I am curious why rafter span tables do not include a spec for roof pitch? Load (live and dead) and clear span are considered but not pitch. It seems to me that the greater the pitch the less stress on rafter but perhaps I am missing something. Thanks Bob
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Old February 10th, 2011, 04:50 AM
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pushkins pushkins is offline
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From an engineers perspective there are, structural engineers often work with pitch calcs. when designing. I think why in residential building your don't see them in span tables is that it's much easier to have a "standard" table covering all pitches. A 3:12 pitch yes will have more live load in regards to snow sitting on it as opposed to a 12:12 with snow sliding off, but the wind loads on the 12:12 is much greater than the loads wind would put on a 3:12.
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Old February 10th, 2011, 06:40 AM
bobm bobm is offline
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Good point, In addition, the length of the rafter will be a little longer for the steeper pitches as well, so there may be somewhat of a balance as you stated.

Thanks Bob
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Old February 11th, 2011, 10:52 AM
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Wgoodrich Wgoodrich is offline
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A rafter span is not figured from the length of the rafter. Instead the distance from the outside wall horizontally to the center of the roof span is the legal rafter span.

The IRC now has in it calculations to address wind load for upsizing rafters required by this calculation. Also the location of your home geographically changes concerning snow load that also affects size of rafter.

Also the ceiling is considered the normal collar tie allowing the rafter size matching the span charts. However if you have say a cathedral ceiling and a collar tie is higher then the rafter is required to be adjusted larger than the chart given in percentage comparing height of ridge versus height of collar tie. Ceiling as collar tie at wall height is 100 % span matching chart. Collar tie installed 2/3 way up to the ridge requires a 50% reduction in span capability requiring a rafter double the size said in the charts in the IRC matching the snow load rating of your area.

Hope this helps

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