Roof From the drawings

Roof Length along the Eaves m

Horizontal Run, Eaves to Ridge m

Measured on the plan, not up the slope.

Roof Pitch °

Roof Slopes of This Size slopes

Two for a simple gable roof.

Membrane From the roll label and the maker's instructions

Roll Width m

Roll Length m

Head Lap between Courses cm

The overlap of each course over the one below; the membrane maker sets it, and it often varies with pitch.

Allowance for End Laps, Valleys & Cutting %

Rolls to Order

— rolls

Membrane length with the allowance, divided by the roll length, rounded up

Take-Off

Slope Length, Eaves to Ridge: — m
Courses per Slope: — courses
Membrane Length Laid: — m
Membrane Length with Allowance: — m
True Roof Area: — m²
Plan Area (for Comparison): — m²
Membrane Area Laid: — m²
Membrane beyond the Roof Area (Laps & Top Course): — %

Pitch, run, head lap and allowance are yours: the head lap and how a membrane is carried over the ridge or fixed at the eaves are the membrane maker's instructions and the local building rules, and the tool has no figure of its own for either. Courses run parallel to the eaves, the full roof length each; the top course is counted whole even where it overhangs the ridge. The allowance covers end laps where a course is longer than a roll, valleys, hips, openings and cutting, so a complex roof needs a larger figure or a slope-by-slope count. **This is a quantity take-off; it does not choose a membrane or check ventilation.**

Roof Underlay & Breather Membrane: Slope Length, Courses, Laps & Rolls for a Pitched Roof — free, with the formula and a worked example, at Textile School.