Gambrel Roof Calculator

What this gambrel calculator measures, and how it differs from a plain pitch tool

A roof pitch calculator answers one narrow question: for a single slope, how does a 6:12 pitch turn into an angle or a slope factor, and what is the rafter length for one continuous plane. A gambrel is not one plane. It is two planes on each side, a steep lower plane and a shallow upper plane, meeting at a break line, so a single pitch number never describes it. This page works directly in gambrel geometry: you enter the building width, the length, both slopes written as rise per 12 inches, the break height above the eave, and the overhangs, and it returns the real roof surface area, the shingle bundle count, the underlayment and sheathing quantities, the rafter lengths and count, the ridge and break line lengths, and a cost estimate per square.

If you only need one slope and one plane, the roof pitch calculator is the simpler tool. Here we add the second plane and the break, which is what makes the gambrel math different and what changes the material list. A gable roof at 6:12 scales its area by one single factor; a gambrel at 20:12 below the break and 6:12 above it grows area on two different factors at once, and that is the part people miss when they reuse a gable number.

The geometry you need: width, two slopes, the break, and the overhangs

Start from the building width. Half of it is the run from the wall to the ridge line, and the break splits that run into a lower part and an upper part. The lower run equals the break height times 12 divided by the lower pitch, so for a 24 foot building the half width is 12 feet, and with a 7 foot break and a 20:12 lower plane the lower run is 7 x 12 / 20 = 4.2 feet. The upper run is then 12 - 4.2 = 7.8 feet, and the ridge stands 7 + 7.8 x 6 / 12 = 10.9 feet above the eave.

Each plane length is its run times its pitch factor, where the pitch factor is the square root of 1 plus (pitch / 12) squared. Adding the eave overhang to the lower run first gives the full lower rafter. The total roof area is two sides times the roofed length times the sum of the two plane lengths. The table below shows the factor and the angle for the slopes a gambrel most often uses.

Worked example: 24 by 30 foot gambrel, 20:12 lower, 6:12 upper

Take a 24 by 30 foot building with a 7 foot break, a 20:12 lower plane, a 6:12 upper plane, and 1 foot overhangs on every edge. The lower plane length is (4.2 + 1) x 1.944 = 10.11 feet. The upper plane length is 7.8 x 1.118 = 8.72 feet. One side is 10.11 + 8.72 = 18.83 feet.

The roofed length is the building length plus two rake overhangs, so 30 + 2 = 32 feet. The roof surface area is 2 x 32 x 18.83 = 1,205 square feet. Adding the overhangs gives a roofed width of 26 feet and a roofed length of 32 feet, which makes the footprint 26 x 32 = 832 square feet. The roof is 1,205 / 832 = 1.45 times the footprint. A gable at one pitch would not reach that ratio at these slopes, which is the gambrel effect made visible.

From square feet to bundles, rolls and sheets

Roofing is sold by the square, and one square is 100 square feet. Standard 3-tab and most architectural shingles come three bundles to the square. USDA Agriculture Handbook 73 (1955) documents 27 strips per bundle and three bundles per square for 12 by 36 inch strip shingles at 5 inch exposure, and modern architectural bundles list 33.3 square feet each, again three to the square. Our 1,205 square foot roof is 1,205 / 100 = 12.05 squares. Bundles before waste are 12.05 x 3 = 36.15; adding 10 percent waste gives 12.05 x 3 x 1.1 = 39.8, rounded up to 40 bundles.

Underlayment runs about 400 net square feet per 15 pound felt roll, so 1,205 x 1.1 / 400 = 3.3, rounded up to 4 rolls. Sheathing uses 4 by 8 foot sheets of 32 square feet, so 1,205 x 1.1 / 32 = 41.4, rounded up to 42 sheets. The ridge and the two break lines are each as long as the roofed length, which drives the ridge cap and bend trim counts.

Slope, code minimums, and the snow that piles at the break

IRC R905.2.2 (ICC, 2021) allows asphalt shingles on slopes of 2:12 and steeper, and from 2:12 to 4:12 it requires a double underlayment. A gambrel lower plane at 20:12 and an upper plane at 6:12 both clear that bar, but the shallow upper plane sits near the snow holding range. FEMA P-957, the Snow Load Safety Guide (2013), puts the snow angle of repose near 30 degrees, about a 6:12 to 7:12 slope, so a 6:12 upper plane sits right at the line where snow begins to slide while the steep lower plane sheds it well. That drift loads the break line, the weak point of a gambrel.

Design snow load must come from the local code official through the IRC R301.2 ground snow load map or ASCE 7, not from a generic guess, because the break concentrates unbalanced load. Where local criteria flag ice dams, IRC R905.1.2 requires a self adhering ice barrier at the eaves and rakes. A gambrel with a shallow upper plane in a snowy zone should include it, and the eave length below tells you how much eave and rake edge there is to protect.

Rafters, ridge and the framing takeoff

The lower rafter piece is 10.11 feet and the upper rafter piece is 8.72 feet for the worked example. At 16 inch spacing over the 32 foot roofed length, the spaces are 32 x 12 / 16 = 24, so there are 25 rafters per side per plane, or 100 pieces across the four planes (two sides and two planes). The ridge length equals the roofed length, 32 feet, and each break line also runs the full 32 feet, for 64 feet of bend trim in total.

OSHA 29 CFR 1926.501(b)(11) treats any roof steeper than 4:12 as a steep roof, so a 20:12 lower plane demands fall protection above 6 feet: a guardrail system with toeboards, a safety net system, or a personal fall arrest system. Buy rafter stock to cover the longer piece with a tail, so 12 foot boards for the 10.11 foot lowers and 10 foot boards for the 8.72 foot uppers, and add the tail length before cutting.

Gambrel versus other roof shapes

The gambrel trades a steeper footprint for far more usable space under the roof than a gable or hip of the same width. The table below shows how the four common shapes compare on slope and attic use.

Cost per square, and what to hand a contractor

Cost equals the billed squares times the material rate plus the labor rate. The billed squares here are 12.05 x 1.1 = 13.25. At example rates of $140 material and $150 labor per square, materials are 13.25 x 140 = 1,855 dollars, labor is 13.25 x 150 = 1,988 dollars, and the total is 1,855 + 1,988 = 3,843 dollars. A realistic spread is about 20 percent each way: 3,843 x 0.8 = 3,074 dollars to 3,843 x 1.2 = 4,612 dollars.

The rates are inputs, not published averages. The tool ships with placeholder numbers so the arithmetic is visible, and you should replace them with your own quotes and with the tear off cost when old layers must come off. IRC R105.1 requires a permit for roof covering replacement, and a full reroof is not an ordinary repair, so plan for a permit and inspection. Permit practice varies by jurisdiction: some departments exempt ordinary roof repairs, while others require a permit and a final inspection for every replacement, so check the building department that serves the address before you order material. What moves the price most is roof height (a 10.9 foot ridge means steeper, harder access), pitch, the number of old layers torn off, decking replacement, and the chosen material, with metal running above shingle rates.

For a second check, a 30 by 40 foot barn with a 12:12 lower plane, a 4:12 upper plane and a 5 foot break gives a ridge of 5 + 10 x 4 / 12 = 8.33 feet, a lower length of (5 + 1) x 1.414 = 8.49 feet and an upper length of 10 x 1.054 = 10.54 feet, so each side is 19.03 feet, the roofed length is 42 feet, and the area is 2 x 42 x 19.03 = 1,598 square feet. That is 15.98 squares, and 15.98 x 3 x 1.1 = 52.7 rounded up to 53 bundles, and 17.58 x 140 = 2,461 plus 17.58 x 150 = 2,637 for a 5,098 dollar total at the same rates.