There are many variables to this question and it can not really be specifically answered without more information. Roof snow loads may range from 20 psf to 300 psf around here depending on the elevation above sea level. What spacing will you be putting the rafters? The tributary area of each supporting rafter is drastically affected by spacing and span. You also need to know the species of the wood you are using to determine allowable stresses (bending and shear) and modulus of elasticity (used for deflection). A typical code book (1997 Uniform Building Code is what we use) gives allowable spans for Douglas Fir species assuming you are using 2x4, 2x6, etc. You probably want to talk to a local engineer (for calculations)or your local building department to see what they allow. If you are an owner/builder, most building departments will help you out if they can while engineer's charge money. Good luck!