Gambrel Roof Calculator

Background.

A gambrel roof changes pitch partway up each side: a steep lower slope rises from the eaves to a knee, and a much shallower upper slope carries on to the ridge. It is the outline of the classic barn and the Dutch colonial house, and builders choose it for the space underneath. The steep lower planes push the roof outward, so the loft or upper storey gets tall walls and usable headroom almost out to the eaves.

For ordering materials the shape is simpler than it looks. A plain gambrel is four rectangles, two per side, each running the full length of the building. Each rectangle's area is the building length times the length of that slope, measured along the roof surface. Add the upper and lower slope for one side, multiply by the length, double for the far side, and you have the roof. The default building, 12 m long with slopes of 2.5 m and 3.5 m, comes to 144 m².

The mistake that costs people a second trip to the supplier is entering horizontal distances. A dimension read off a floor plan, or a tape run across the attic floor, is shorter than the sloping surface above it, and on the steep lower planes the gap is large. Measure along the roof itself, or work each slope out from its rise and run.

The five-sided end walls are walls, clad with siding, and are not counted. Dormers, valleys and skylights are not counted either, and overhangs appear only if your measurements include them. The waste allowance then scales the bare area up to cover offcuts, starter courses along the eaves and ridge capping.

What is gambrel roof calculator?

Gambrel roof area is the total sloping surface of a roof with two pitches on each side, found as the sum of four rectangular planes. Each plane is as long as the building and as wide as its slope length, the distance measured along the roof surface from one edge of the plane to the other. On a symmetric gambrel the two sides mirror each other, so the total is twice one side. It is the figure that sheathing, underlayment and roof covering are ordered from, before any allowance for waste.

How to use this calculator.

1. 01Enter the Building length, the distance along the ridge from one end of the roof to the other. If the roof projects past the end walls, measure to the roof edge rather than the wall.
2. 02Enter the Upper slope length for one side, measured along the roof surface from the ridge down to the break where the pitch changes.
3. 03Enter the Lower slope length for the same side, from the break down to the eave edge, including any overhang beyond the wall. If you only know a slope's rise and run, its length is √(rise² + run²).
4. 04Set Roofing waste to the allowance you want added. The 10% default suits a plain roof; one cut up by dormers or valleys produces more offcuts and deserves more.
5. 05Read the gambrel roof area for the bare surface, and the roofing area with waste as the quantity to order.

The formula.

Area = 2 × L × (u + l), where L is the building length, u the upper slope length and l the lower slope length on one side, all in metres. L × u is one upper plane and L × l one lower plane; together they make one side, and the factor of 2 adds its mirror image. The order quantity is that area × (1 + waste ÷ 100). Because the two slopes enter only as a sum, the split between steep and shallow makes no difference: slopes of 2.0 m and 4.0 m cover exactly the same area as 2.5 m and 3.5 m. Pitch affects the answer only through the slope lengths you measure, since a steeper roof over the same footprint has longer slopes. Every term is linear, so a 5% error in the slope lengths becomes a 5% error in the order. The formula assumes both sides are identical and the cross-section is constant along the whole building; hips, dormers, valleys and openings sit outside it.

A worked example.

Frequently asked questions.

References& sources.

1. [1]International Code Council. International Residential Code 2021. Chapter 9 roof assemblies (code framing context). Retrieved 2026-08-06.
2. [2]National Institute of Standards and Technology. NIST SI Guide, 2008 edition. Appendix B unit relationships. Retrieved 2026-08-06.
3. [3]International Code Council. 2021 International Residential Code. Building construction context. Retrieved 2026-08-06.