Roofing Squares Calculator

What a roofing square is

A square is 100 square feet of finished roof surface — a 10 ft by 10 ft patch. It is the unit the entire trade prices in. Materials are sold by it, labor is bid by it, and when a contractor says a job is “thirty squares” they mean 3,000 square feet of roof. It is not a metric or an engineering unit; it is a shorthand that survived because 100 is easy arithmetic on a tailgate and because a square is roughly what a two-man crew can strip and dry in before lunch.

The trap is that a square measures roof, not house. A 2,000 square foot ranch does not have 20 squares of roof. It has 20 squares of footprint, and the roof on top of it is larger by the pitch multiplier — at a common 6/12 pitch, about 22.4 squares, and at 12/12, over 28. Confusing the two is the single most common estimating error homeowners make, and it runs consistently in one direction: short.

1.  A_plan  =  length x width                    (sq ft of footprint)

2.  M       =  sqrt(rise^2 + 12^2) / 12          (pitch multiplier)

3.  A_roof  =  A_plan x M                        (sq ft of actual roof)

4.  Squares =  A_roof / 100

    then, to order:

5.  A_order =  A_roof x (1 + waste / 100)
6.  Bundles =  ceil( (A_order / 100) x bundles-per-square )

Worked example

A rectangular ranch, 40 feet by 30 feet, plain gable roof at 6/12, no dormers and one small plumbing vent. Standard architectural shingles at three bundles per square. Simple roof, so 10 percent waste.

Footprint     = 40 ft x 30 ft
              = 1,200 sq ft

Multiplier    = sqrt(6^2 + 12^2) / 12
              = sqrt(36 + 144) / 12
              = sqrt(180) / 12
              = 13.4164 / 12
              = 1.1180

Roof area     = 1,200 sq ft x 1.1180
              = 1,341.6 sq ft

Squares (net) = 1,341.6 / 100
              = 13.42 squares

Add 10% waste = 1,341.6 x 1.10
              = 1,475.8 sq ft
              = 14.76 squares
              -> order 15 squares

Bundles       = ceil(14.76 x 3)
              = ceil(44.27)
              = 45 bundles

Notice how far apart the two headline numbers are. The house is 1,200 square feet on the ground and you are buying 1,476 square feet of shingles — 23 percent more material than the floor plan suggests. Roughly 12 points of that come from the pitch and 10 from the waste. On a 10/12 roof with valleys, the same 1,200 square foot house needs about 1,800 square feet of shingles, half again as much, for the identical floor plan.

Waste factor: when 10 percent is right and when it is not

Waste is not sloppiness. It is the material that necessarily becomes offcuts: shingles cut on an angle where a plane dies into a valley, the piece trimmed off at every rake, the starter course made by cutting the tabs off full shingles, and the cap shingles cut from three-tabs on a ridge. A perfectly run crew still generates it. The question is how much your particular roof forces.

How to measure a roof for shingles

There are two legitimate methods. The footprint method is what this calculator uses, and it is the one to use from the ground. The plane-by-plane method is what a roofer does on the roof and what an aerial measurement report produces.

What a square count does not buy you

Field shingles are the largest line on a roofing invoice but they are nowhere near all of it. If you are pricing a job against a contractor’s quote, these are the items that live outside the square count and account for much of the difference between a cheap bid and a complete one:

• ●Underlayment — synthetic or felt across the whole deck, sold by the roll in its own coverage units.
• ●Ice and water shield — self-adhering membrane at eaves, valleys and penetrations. In cold climates this is required at the eaves and is a real cost, not an upsell.
• ●Starter strip and hip/ridge cap — both sold separately and both priced by the linear foot. Measure your eaves and ridges, not your area.
• ●Drip edge, flashing, pipe boots, step flashing — the details that actually stop leaks. Chimney and skylight flashing in particular is where cheap bids get cheap.
• ●Tear-off and disposal — priced per square and doubled if there are two layers. A dumpster is a separate line.
• ●Deck repair — nobody knows how much rotten sheathing is under there until the old roof is off. A quote should state a per-sheet price for replacement.
• ●Ventilation — ridge vent, soffit vent or box vents, sized by attic area rather than roof area. Work that out with the ventilation calculator.

For what all of this actually costs installed, the new roof installation guide breaks down national ranges by material.

Common mistakes

• ●Using the home’s listed square footage as the footprint. Listed square footage is finished living area. On a two-story house it is roughly double the footprint; it excludes the garage but includes an upstairs that has no roof of its own. Measure the outside of the building instead.
• ●Forgetting the overhang. Eaves and rakes extend past the wall and are roofed. On a typical house the overhang is worth one to two squares.
• ●Applying one pitch to the whole house. Attached garages and porches are very often framed shallower. Enter them as their own sections.
• ●Adding waste twice. If a supplier’s takeoff already carries a waste factor, do not add your own on top. Ask which number you are looking at.
• ●Assuming three bundles per square. That holds for standard 3-tab and most architectural shingles, but heavyweight, designer and multi-layer products can be four or five bundles. The exact coverage per bundle is printed on the wrapper.
• ●Rounding squares down. Suppliers sell whole bundles. Round the order up, always.

Where this method breaks down

The footprint-times-multiplier method is exact when every part of the footprint is covered by roof planes at the pitch you entered. That is true of most houses, and it stops being true in a few specific situations worth naming honestly.

• ●Gambrel and mansard roofs. These have two different pitches on the same plane of travel. Split them: treat the steep lower section and the shallow upper section as separate sections with their own pitches and footprints.
• ●Dormers. A dormer adds roof area that is not in the main footprint, and it removes a little of the main roof where it lands. On small dormers those roughly cancel and the error is inside the waste factor. On a large shed dormer they do not — measure it as its own section.
• ●Non-rectangular footprints. Break an L or a T into rectangles that do not overlap, and add each one. Bays, turrets and curved sections need plane-by-plane measurement.
• ●Roofs that are not a single continuous surface. A courtyard, an open atrium or an interior light well is footprint with no roof over it. Subtract it.
• ●Anything under 2/12. Shingles are not an approved covering below 2/12, so a square count is the wrong question there entirely. Those are membrane roofs, priced and installed differently.

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