Rafters, purlins and board feet from the span, pitch and spacing you set.
Foundation, roof and structure. 5 questions answered below, each with the arithmetic behind it and the limits of what the tool decides. Open the Roof Framing and Board Feet tool.
A roof is measured on its slope, and its slope is longer than its plan. That single fact is why roof quantities differ from floor quantities of the same building, and why the framing takeoff starts from the pitch rather than from the footprint. The tool returns members by length and by volume. Length is what you order from a timber merchant and what you cut; board feet is what a mill or a merchant prices in some markets, and a schedule that gives one without the other leaves you converting at the counter. Pitch changes both, because a steeper roof has longer rafters, more of them per unit of plan area, and more material in the same footprint.
A plan of a hundred square metres at thirty degrees has a sloped area of about a hundred and fifteen square metres, which is the plan divided by the cosine of the pitch. At six hundred millimetre rafter spacing over a nine metre span, with rafters at two point six metres of slope each side, the count is about thirty two. Boards at the sizes you specify then convert to board feet, so the same schedule prices in linear metres for one merchant and in board feet for another. Twenty degrees instead of thirty would shorten the rafters and cut the sloped area to a hundred and six, which is a real reduction in both timber and covering.
Members by length and by volume: rafters, ridge, hips, valleys, purlins and battens, each from the pitch, the span and the spacing you enter. Board feet and linear metres are both returned because merchants price timber differently across markets. What it does not measure is the covering, the fixings or the trusses, since those follow their own rules and their own schedules.
Through the slope. A roof at thirty degrees has a sloped area of the plan divided by the cosine of the pitch, so a hundred square metres of plan becomes about a hundred and fifteen of roof. Rafters get longer as the pitch rises for the same span, and their count per unit of plan area rises with the slope. Both effects are in the arithmetic, which is why pitch is the first input rather than a detail.
Because timber is sold and priced both ways. A merchant may quote a length and a rate per linear metre, while a mill or a specification may price by board feet, which is a volume measure of the nominal section. Returning both from one schedule means the order does not have to be converted by hand, and a conversion done at the counter is where the two figures usually drift apart.
No. Member sizing follows from the span, the load, the spacing and the grade of timber, and it comes from span tables or an engineer's design. The takeoff measures the members a design calls for, at the sections you specify. Where a span exceeds the tables, that is a design question with a real answer, and it is not one a quantity tool should be inventing.
No. The covering is measured by the roofing tool, which converts the sloped area into sheets or tiles at their cover width and lap. The framing takeoff covers the structure that carries the covering. Keeping them apart matters because a change of covering changes the battens and the spacing but not the rafters, and a single blended estimate would obscure that.