Gambrel Roof Calculations

Semicircle

• A gambrel roof is semicircle with the sides formed of straight lines rather than curves. Get an idea of how a gambrel is made by drawing a circle on paper. Divide it in half horizontally. Mark the centers of each side of the half-circle. Connect those center points to the bottom edge and top center with straight lines, and you have the basic gambrel shape.

Reverse Pitches

• Convert that plan into the building’s dimensions, the width or span of the roof, the height of the peak above the walls. Decide on a pitch or slope from the peak to the first angle. That frequently is seven-12 on a gambrel, meaning it slopes 7 inches per foot of run or the distance from the peak to where the second slope begins. Figure the bottom slope the easy way -- reverse that to make the lower slope a 12-seven pitch.

90 Degrees

• For basic gambrel calculations, the angles of each side must add to 90 degrees. That means if the slope of the top portion is 30 degrees, the bottom angle must be 60 degrees. Gambrel roofs usually have a gentler top slope and a more sharply pitched bottom slope. The 90-degree rule applies regardless of the width of the roof sections.

22.5 Degrees

• A simple gambrel formula is to cut all the angles of the rafters that together form the roof at 22.5 degrees. That means the top rafter is cut at 22.5 degrees on both the peak end and the bottom end and the bottom rafter is cut at 22.5 degrees where it meets the top rafter and the same where it connects to the wall. That forms the classic gambrel roof.