http://www.backwoodshome.com/articles/harris23.html

… Today, in almost all parts of the country, one can go to a local lumberyard with a building design on the back of an envelope, watch the technician keyboard all the critical dimensions—spans, snow loads, roof pitch, and so on into the software formula— and, a few seconds later, watch the printer spit out a complete roof truss design with every piece of lumber identified as to size, every joint identified as to strength, and a complete cost estimate to boot.

If you can’t choose between two alternate designs, ask the computer to do both. It’s usually free. The lumberyard or truss-assembly plant provides this service in the expectation of selling trusses that wouldn’t be sold if the potential buyer had to go out and buy expensive engineering services for their design. …

In MY part of the country, that doesn’t seem to be happening.

http://www.cwc.ca/applications/light_framing/roof_design.php

Rafters and Joists

For light duty (typically residential) applications, the allowable spans for roof rafters and joists are given in tabular form in the North American building codes. For applications which fall outside the limits of the tables, the size and spacing requirements must be calculated.

Rafters and joists should be continuous, except where spliced over vertical supports, and should be doubled for extra strength on both sides of openings wider than two rafter or joist spacings.

Roof Trusses

Roof trusses must be designed to limit deflection. This is especially crucial on flat roofs where deflection will result in ponding and a consequent increase in load.

For residential construction, some of the North American building codes list spans and sizes of members of some wood species for some of the more common truss types. Generally, these table apply only to roof trusses of up to 12m (40’wink span spaced up to 600mm (2’wink on center. For longer spans or heavy loads, engineering analysis is required.

Finally found a place to order trusses with a Phoenix location and a lot of cool calculators:

http://www.bradcosupply.com/lumber_math.aspx