How to Measure a Roof and Calculate Squares for Shingles
Plane by plane, pitch factor, waste and squares. The arithmetic that decides how much material you order.
- Intermediate
- 1 to 2 hours
- Updated
The short answer
One roofing square is 100 square feet of roof surface. To find it, measure the footprint of each roof plane, multiply by the pitch factor (1.118 at 6/12, 1.202 at 8/12) to get the true sloped area, add the planes together, then add 10 to 15 percent for waste. Divide the total by 100. Shingles are ordered at 3 bundles per square for most laminated products.
Roofing is sold and ordered by the square: 100 square feet of roof surface. Everything downstream depends on getting that number right: how many bundles to order, how much underlayment to buy, how many nails, how many hours the job will take. Order too little and the last course waits for a second delivery. Order too much and you carry the cost of leftover bundles that cannot always be returned.
The mistake that catches most people is measuring the house instead of the roof. A roof is bigger than the footprint under it, because the slope adds length, and the eave and rake overhangs add more. A 1,500 square foot bungalow can easily carry 20 squares of roof once pitch, overhang and waste are counted. This guide shows how to measure every plane, apply the pitch factor, add waste and read the number that comes out.
Once you have your squares, the next step is turning them into an order list. That is covered in how to order shingles and accessories. If you want the price of a full replacement, see roof replacement cost.
Tools
- Tape measure, 100 foot
- Pitch gauge or a 12 inch level and ruler
- Smartphone inclinometer app
- Clipboard with graph paper
- Calculator
- Ladder, set up and tied off
Materials
- Graph paper or a roof sketch template
- Aerial roof measurement report, if ordered
- Manufacturer's coverage chart for the shingle you plan to use
Safety first
- Measure from the ground, from a drone or from a report whenever you can. Walking a roof only to count feet is the most avoidable fall on a job.
- If you do go up, tie off. WorkSafeBC requires fall protection where a fall could be 3 metres (about 10 feet) or more, and pitch and wet moss make a low roof slippery too.
- Never walk a wet, frosty or mossy roof to measure it. A pitch gauge can be used from a ladder at the eave.
Step by Step
Step 1: Sketch the roof and break it into simple shapes
Walk the outside of the house and draw the roof as if you were looking straight down. Mark the ridges, hips, valleys, dormers and any changes in pitch. Label each plane with a letter so nothing gets counted twice or missed.
A gable plane is a rectangle. A hip roof has two trapezoids and two triangles. A dormer is its own small set of planes. The less you try to treat as one big shape, the less error you carry into the order.
Step 2: Measure the length and run of every plane
For a simple gable roof, measure the length of the eave and the horizontal distance from the outside face of the eave overhang to the ridge. Include the overhang: shingles cover it. Eaves typically project 12 to 24 inches, and the rake overhang adds to the width of each end.
If you cannot get a tape on it, pace the wall line and add the overhang, or use a measuring wheel at ground level. A rough measurement is fine for a ballpark, but anything you order from should come from a proper report or a careful re-measure.
Tip: Measure the footprint to the outside of the overhang, not to the wall. Missing the overhangs on a whole house can cost 1 to 2 squares.
Step 3: Find the pitch of each plane
Pitch is written as rise over run, with the run always 12: a 6/12 roof rises 6 inches for every 12 inches of horizontal run. On a ladder at the eave, hold a 12 inch level on the shingles, tip it until it is level, and measure from the level to the roof surface at the 12 inch mark. A pitch gauge or a phone inclinometer does the same in seconds.
Check each plane. Additions, porches and dormers often sit at different pitches from the main roof. An aerial report lists pitch per plane for you.
Step 4: Convert the footprint to true roof area with the pitch factor
Because a sloped roof is longer than its footprint, every plane's flat area needs to be multiplied by a factor that grows with pitch. The common factors are shown below. The factor is simply the hypotenuse of the pitch triangle divided by 12.
If your plane is 30 feet along the eave and 15 feet of horizontal run at 6/12, the footprint is 450 square feet and the sloped area is 450 x 1.118, which is about 503 square feet.
- 3/12: 1.031
- 4/12: 1.054
- 5/12: 1.083
- 6/12: 1.118
- 7/12: 1.158
- 8/12: 1.202
- 9/12: 1.250
- 10/12: 1.302
- 12/12: 1.414
Step 5: Add the planes together and subtract what is not shingled
Add up every plane. Subtract large openings such as skylights and chimneys only if they are big. Small penetrations are not worth deducting, because you will cut around them and the offcuts are waste anyway.
Keep the planes in a simple table: plane label, footprint, pitch, factor, sloped area. That table is also what you will hand the supplier.
Step 6: Add the waste factor
Waste comes from starter, cuts at hips, valleys and rakes, damaged bundles and the off-cut at the end of every course. A plain two-plane gable can run near 10 percent. A roof with several valleys, dormers and hips often needs 15 to 20 percent. Complex or diagonal-pattern roofs go higher.
Some suppliers add hip and ridge cap, starter strip and ice and water shield as separate line items, so do not double-count them inside the waste factor. See the next guide on ordering for how each accessory is measured.
Step 7: Convert area to squares and bundles
If you measure 2,150 square feet of sloped roof and add 12 percent waste, you have 2,408 square feet, or 24.08 squares. Round up to 24.5 or 25 squares.
Most laminated shingles are packed 3 bundles per square, but some heavier designer products are 4 or 5 bundles per square. Confirm on the manufacturer's coverage chart, which is printed on the bundle wrapper and in the product data sheet.
Step 8: Check the number against an aerial report
Aerial reports, made from satellite or drone imagery, give total roof area, area by pitch, ridge, hip, valley, eave and rake lengths, and often a waste table showing the squares at 0, 10, 15 and 20 percent waste. Read the table carefully: the headline number may be measured area without waste.
If your hand numbers and the report disagree by more than a few percent, find out why. A common cause is a missed plane or an overhang that the report treated differently. Reports can also miss trees that hide part of the roof, or confuse a flat porch roof for pitched.
Reading an Aerial Roof Report
Most reports give you a summary page with the total sloped area in square feet, a count of roof facets, the predominant pitch, and linear footages for each edge type. Use those footages to order the accessories: eave length for drip edge and starter, rake length for rake starter and drip edge, hips and ridges for cap, valleys for valley metal or membrane.
Look at the waste table rather than applying your own factor blindly. Reports usually list suggested squares at 10, 15 and 20 percent, and often recommend one based on the number of facets and hips. Remember that a report is only as accurate as the imagery. Trees, recent additions and steep pitches can all skew it.
Fraser Valley Notes
Houses in the Fraser Valley often have a mix of pitches: a steep main roof, a low-slope porch or addition, and a dormer or two. Measure and order each pitch separately, because a low-slope section may need a different underlayment, which changes the order list.
Heavy moss growth on a north-facing slope can hide the roof surface from aerial imagery, and overhanging firs can hide edges. When the imagery is poor, a ground check is worth the half hour.
Common Mistakes
- Measuring the floor plan instead of the roof. Ignoring pitch and overhang can under-order by 15 to 30 percent on a steep roof.
- Using one pitch for the whole house. A 4/12 porch and an 8/12 main roof have very different factors, and averaging them hides the error.
- Forgetting the overhangs. A 2 foot overhang around a typical house adds about 2 to 3 squares.
- Skipping waste or using the same waste on every roof. A cut-up roof with hips and valleys eats far more shingle than a plain gable.
- Trusting a report without checking it. Shadows, trees and unusual shapes cause errors in aerial measurements. A quick sanity check against your own numbers takes minutes.
- Ordering only field shingles. Starter, cap, underlayment, nails and vents are all separate quantities. See how to order shingles and accessories.
When to Call a Professional
Measuring a simple gable is a fair job for a careful homeowner. For a roof with multiple hips, valleys, dormers, mixed pitches or a pitch steeper than 8/12, have a roofer measure it or order a report, because small errors multiply into real money on the material order.
If you are using your measurement to compare quotes, ask each roofer for their square count and waste assumption. Two quotes with different square counts are not comparing the same roof.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Replacement service or the towns we cover.
How to Measure a Roof and Calculate Squares: Questions
How many square feet is a roofing square?
One roofing square is 100 square feet of roof surface. A roof of 2,400 square feet of actual sloped surface is 24 squares. The figure always refers to the sloped area of the roof, not the floor area of the house, so a steeper pitch means more squares for the same footprint.
How do I calculate roof squares from the house size?
Multiply the footprint, including the overhang, by the pitch factor, then divide by 100. For example, a 40 by 30 foot footprint with overhang added is about 1,400 square feet, times 1.118 for a 6/12 pitch is 1,565 square feet, or about 15.7 squares before waste. Add 10 to 15 percent for waste.
How much waste should I add when ordering shingles?
Ten percent is typical for a simple gable roof. Add 15 percent or more for roofs with hips, several valleys, dormers or steep pitches. Every cut at a hip, valley or rake produces offcuts, and starter and cap are separate from this figure. Round up to the next half square.
How many bundles of shingles are in a square?
Most laminated architectural shingles are packed 3 bundles per square. Some thicker designer lines are packaged 4 or 5 bundles per square. The wrapper states the coverage per bundle, and that figure is what governs. Never order from a general rule without checking the actual product.
Are aerial roof reports accurate?
They are usually within a few percent on clear, simple roofs. Accuracy drops when trees shade the roof, when the imagery is old, or when the roof has unusual geometry. Compare the total area with your own rough measurement, and ask for a waste table. A gap larger than about 5 percent deserves a second look.
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This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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