About this calculator
Estimates the quantities for a gambrel roof (also called a barn roof): a steep lower slope up to a break height, then a shallower upper slope to the ridge, on each side. Models a constant cross-section extruded along the building length with open gable ends — not a hip-style four-sided French mansard, whose angled ends this calculator does not cover.
Formula
half span = building width / 2
lower run = break height / tan(lower pitch)
upper run = half span − lower run
upper rise = upper run × tan(upper pitch)
ridge height = break height + upper rise
lower rafter length = (lower run + eaves overhang) / cos(lower pitch)
upper rafter length = upper run / cos(upper pitch)
rafter count = 4 × (ceil(roof length / spacing) + 1) (lower + upper, both sides)
lower area = 2 × lower rafter length × roof length
upper area = 2 × upper rafter length × roof length
covering area (incl. waste) = (lower area + upper area) × (1 + waste% / 100)
The lower run must be smaller than the half span — otherwise the steep lower slope alone would reach past the ridge centreline before the break height is even satisfied, which isn't a buildable roof; the calculator reports an error in that case rather than guessing.
Worked example
Defaults: 10 m × 8 m building, 60°/20° pitches, 2 m break height, no overhang, 600 mm rafter spacing, 10% waste.
half span = 4 m; lower run = 2 / tan(60°) ≈ 1.155 m
upper run = 4 − 1.155 ≈ 2.845 m; ridge height = 2 + 2.845×tan(20°) ≈ 3.036 m
lower rafter length ≈ 1.155 / cos(60°) ≈ 2.309 m
upper rafter length ≈ 2.845 / cos(20°) ≈ 3.028 m
rafters = 4 × (ceil(10000/600) + 1) = 4 × 18 = 72
lower area = 2 × 2.309 × 10 ≈ 46.19 m²; upper area = 2 × 3.028 × 10 ≈ 60.56 m²
covering incl. waste ≈ 117.42 m²
FAQ
Is this the same as a (hip-style) mansard roof? No. This calculator models a gambrel roof: a constant two-slope cross-section extruded along the building length, with open gable ends. A true French mansard has angled hip-style ends on all four sides — a different, more complex take-off this calculator doesn't cover.
Why doesn't the calculator cite a standard?
No standard was found covering this roof shape's quantity take-off — it's trigonometry and
counting, not a code requirement, so meta.standards is left empty.
Assumptions and limits
Models a gambrel roof (open gable ends), not a hip-style four-sided mansard. The ridge is treated as a zero-thickness line. Rafter and connection sizing are not checked — this is a quantity take-off only; structural design must be verified by a qualified engineer.