I was trying to figure rafter dimension and spacing on a (8/12 pitch)common rafter roof ; 3 bay at 14-8-14, with mid span (purlin plate) support. I was estimating 4.5" x 6" deep EWP or hemlock rafters spaced at 4',but it is probably incorrect. Also trying to find out if the purlin plates at 8" x 10" would be strong enough in EWP, or if I should upsize or switch to oak. Anyone have formulas to share on these calculations? tb

The best way is to buy beam sizing software. Then calculate the rafter sizes and determine what the point load is for each rafter where it lands on the plate. It sound more difficult that is actually is. Once you get the hang of tracing the load path it is very simple. NOTE: This does mot replace a structural engineer! It will how ever make your engineering reviews less painful.

Timber Brained, the design standards and formulas are available from the American Wood Council in the publication "National Design Specifaction." Ed Levin wrote an article on design calculations that is republished in the TFG publication "Timber Frame Joinery and Design Workbook Vol.1" The local building code authority will specify snow loading. Good luck.