I'm building an outhouse 18ft x 15ft . I am looking to construct a pitched roof with minimum rise, across the 15ft span. Can anyone tell me how to calculate the size of timber I need to use for the trusses and the rafters ? I will be using tiles on the roof . I have conflicting info telling me that 30 degrees and also 18degrees is the minimum pitch.
Trusses are an engineered system, and are not something you should make yourself. If you're getting an inspection you need to purchase trusses from a reputable manufacturer with the engineering to back them up.
Given the size of the building, you could easily field frame it conventionally, and probably for less money than trusses would cost.
A truss is an assembly of several pieces of lumber and metal gussets and is typically triangular...Thus forming the shape of the roof. The truss incorporates the rafters, ceiling joists, and soffits into one pre-made unit. They're made by truss companies. They're a time saver in a lot of cases, especially if you aren't an experienced framer.
Conventional framing (or field framing, as in framing in the field) involves using individual pieces of lumber to form the various aspects of the roof, ceiling, and soffits. This is how most homes are constructed.