Roofing Estimating Software: How to Price a Reroof

Bid a roof off the building footprint and the estimate is wrong before you've measured anything else. An 1,800 sq ft house doesn't have an 1,800 sq ft roof, the actual surface is bigger, and how much bigger depends on the pitch. Add a few hips and valleys and the waste climbs too, quietly eating the margin you thought you'd built in.

This is how roofers actually take off and price a reroof: converting the footprint to true roof area with a pitch multiplier, turning that into roofing squares and shingle bundles, measuring the linear feet of ridge, hip, valley and eave, and pricing tear-off labor on top. One full worked example ties it together at the end.

Roof plane area and the pitch multiplier

A roof pitch is written as rise over a 12-inch run, so a 6/12 roof climbs 6 inches for every 12 inches it runs horizontally. That slope is exactly what separates the footprint (the flat shape you'd trace from a satellite photo) from the true roof area (the surface the shingles actually cover). The steeper the pitch, the more surface it takes to span the same footprint, the same reason a ramp is always longer than the height it climbs.

The conversion uses a slope multiplier. Multiply your footprint area by this number and you get the actual roof plane area, which is the number everything else in the estimate should be built on. InterNACHI's roof slope multiplier table lists the common ones:

• 3/12 (a low, walkable slope): multiplier 1.031
• 4/12: multiplier 1.054
• 6/12 (the most common residential pitch): multiplier 1.118
• 8/12 (a noticeably steeper roof): multiplier 1.202
• 12/12 (a 45-degree roof): multiplier 1.414

Skip this step and price off the footprint, and every job steeper than a low-slope roof gets short-ordered on shingles and underpriced on labor. JobPlumb's roofing estimating tool applies the pitch multiplier automatically once you trace the roof planes and set the pitch, so the area is right before a single bundle gets priced.

Converting square feet to roofing squares

Roofing is priced and ordered by the square, not the square foot. One roofing square equals 100 sq ft of roof area, a unit that goes back to hand-nailed wood shakes and never went away. Once you have the true roof area (footprint times the pitch multiplier), divide by 100 to get squares.

Take an 1,800 sq ft footprint with an 8/12 pitch: 1,800 × 1.202 = 2,164 sq ft, which is 21.6 squares. Round up, since you can't buy 0.6 of a square's worth of material, and a bit of reserve is useful for future repairs anyway.

Shingle bundle counts and waste

Most architectural and three-tab shingles run three bundles to the square, which is the coverage GAF builds its own packaging around. Some heavier laminate or designer shingles ship three, four, or five bundles per square depending on the product, so check the wrapper or spec sheet before ordering rather than assuming three across the board.

Waste is where a lot of estimates come apart on a cut-up roof. Owens Corning's own installation guide puts typical waste in the 2 to 10% range, with the higher end reserved for roofs with more valleys, dormers and hips. In practice, plenty of roofers price closer to 15 to 20% on a heavily cut-up hip-and-valley roof as a safety margin, because every hip and valley cut throws away a triangle of shingle that a plain gable roof never loses. Match the percentage to how broken up the roof actually is, not to a single number used on every job.

Linear feet of ridge, hip, valley and eave

Area gets you the field shingles, but the trim items price by the linear foot and they matter just as much to the bid. Walk the plan, or run the numbers quickly with the free roofing calculator, and measure these separately:

• Ridge: the horizontal peak where two planes meet at the top, capped with ridge shingles
• Hips: the sloped external corners running down from the ridge, capped the same way as the ridge
• Valleys: the internal angles where two planes meet running down to an eave, needing extra underlayment or metal flashing because they carry the most water
• Eaves: the lower roof edge, where the starter course goes and where ice-and-water shield is required in cold climates

Ridge and hip footage price out at roughly the same cap-shingle rate. Valleys need ice-and-water shield or metal flashing running their full length, since a valley leak causes some of the worst water damage a roof can have. Eave length sets the starter course and any ice-dam protection local code requires.

Tear-off and disposal labor

On a reroof, tear-off is its own line item, priced by the square just like the new shingles. Stripping old shingles, pulling nails, loading a dumpster or trailer, and paying disposal fees all scale with the roof area, not the footprint, so use the same squares figure calculated for the new material. A roof with two existing layers takes noticeably longer to strip than a single layer, and that's worth its own line in the bid instead of a guess folded into the labor rate.

A worked example: Dana's hip roof

Dana is bidding a reroof on a single-story house: an 1,800 sq ft footprint, an 8/12 pitch, and a hip roof with two valleys, the kind of cut-up shape that pushes waste toward the higher end. Here's how the numbers build:

1. Footprint: 1,800 sq ft
2. Pitch multiplier for 8/12: 1.202
3. True roof area: 1,800 × 1.202 = 2,164 sq ft
4. Squares before waste: 2,164 ÷ 100 = 21.6 squares
5. Waste allowance for the hips and valleys: 17%
6. Squares to order: 21.6 × 1.17 = 25.3, rounded up to 26 squares
7. Shingle bundles: 26 × 3 = 78 bundles

Add up the line items and Dana's estimate lands around $7,875 before overhead and profit, a number that only holds up because the roof area, not the footprint, drove every quantity in it.

Common mistakes that sink a roofing bid

• Pricing off the footprint instead of the true roof area: on anything steeper than a low-slope roof, this alone can shortfall the shingle order and labor estimate by 10 to 40%
• Using one flat waste percentage on every roof: a simple gable and a roof with six hips and three valleys don't carry the same waste, and treating them the same under-orders material on the cut-up roof
• Forgetting flashing and boots at penetrations: every vent pipe, chimney and skylight needs its own flashing, and missing these on the takeoff turns into a change order and an awkward conversation mid-job
• Skipping a site-measured eave length: gutters, fascia and ice-and-water shield all depend on the actual eave footage, which can differ from the plan once soffit overhangs are accounted for

The same measure, convert, price pattern shows up in every trade takeoff, only the units change. What is a construction takeoff covers the general version, and JobPlumb's estimating hub has the same workflow set up for framing, electrical, concrete and more.

Frequently asked questions

How many bundles of shingles are in a square?

Most architectural and three-tab shingles run three bundles per 100 sq ft square, the coverage GAF's own shingle lines are built around. Heavier laminate or designer shingles sometimes ship four or five bundles per square, so check the coverage printed on the wrapper or spec sheet before ordering rather than assuming three.

What is a roof pitch multiplier and how do I use it?

A pitch multiplier converts a flat footprint area into the actual sloped roof area, since a roof surface sitting at an angle is longer than the ground it covers. A 6/12 roof multiplies the footprint by about 1.118, and an 8/12 roof by about 1.202. Multiply your footprint square footage by this number before converting to roofing squares.

How much waste should I add to a roofing estimate?

A simple gable roof with one ridge and no valleys usually needs waste toward the low end of the typical 2 to 10% range. A roof broken up with multiple hips, valleys or dormers commonly needs more, and many roofers price 15 to 20% or higher on a heavily cut-up roof as a safety margin. Match the percentage to how cut-up the roof actually is instead of using one number for every job.

How do I estimate a roofing job from a plan?

Measure the footprint area of each roof plane from the plan, apply the pitch multiplier for the roof's slope to get the true roof area, convert to roofing squares by dividing by 100, add a waste allowance based on how cut-up the roof is, then price shingle bundles, ridge, hip and valley trim by the linear foot, and tear-off labor by the square. JobPlumb's roofing estimating tool runs this whole sequence once you trace the roof planes on the plan.