Rafter sizing

[Triathlete]

Any sites that can help a guy with sizing rafters?  I'm looking at a 24 foot span with a 4 in 12 slope and thinking that 2x8s on 2' centres would be strong enough.

Btw, I'm in an unorganized township where there are no building codes and I'd rather build my own roof, given the high price of trusses around here.

[Don_P]

awc.org
the quick links bottom right of that page, click on "calculators" and then their span calculator or the printed span tables.

Their publications tab is wealth of info as well

You'll need either ceiling joists tieing the feet of each rafter pair together or a ridge beam sized to carry half the roof load.

[MountainDon]

what he said; here.

[Don_P]

since most people have been building 12/12 pitches we haven't talked much about how the horizontal thrust at the connection between ceiling joist and rafter foot changes with pitch.

For an example I used a 30 pound per square foot snow load, a 10 psf roof self weight load, 24' wide and 24" rafter spacing.
At 4/12 the design shear in the connection is 1440 lbs, about 18 nails, there is that much outward push from the rafters against the restraint of the ceiling joists. Think of it as a toggle, the closer to flat the pitch gets the more powerful a load is in thrusting outward.
At a 12/12 pitch that outward push drops to 480 lbs, about 6 nails

You can play with that using this non official calc, kind of interesting to use it to see the forces at work as the pitch changes.
http://www.timbertoolbox.com/Calcs/slopedrafter.htm

[Triathlete]

Thank you gentlemen.  I can recall the awc site now.  It has been a few years since I've visited it.

Btw, I'm a little surprised that the formula does not take into account roof slope.  As Don points out, the values can change substantially.  And thanks for the inclusion of nailing Don.  Never thought that a 4/12 slope would require so many nails.

[Don_P]

Roof slope is taken care of by using the horizontal run as the span for live load. A rafter for a 12/12 pitch is considerably longer than one for a 4/12 but for a given building width we use the same span, the building width, for either when calculating the live load.

I'm being a little careful there, the dead load is correctly calculated based on the rafter length, those weights of materials are per square foot.

On nailing, the table at the end of the rafter span tables in the codebook gives their nailing schedule. My calc is correct but is often more conservative than their table, the midjoint tension and nail count is sufficient nailing for the rafter heeljoint connection when a typical birdsmouthed stick framed rafter is being used.