How to Measure a Roof

This is the method to learn. It works on nine houses out of ten, it takes twenty minutes, and it is what an estimator does before he decides whether the job is worth a site visit.

Step 1 — measure the footprint. Walk the outside of the house with a 100 ft tape and record the length and width of each rectangle the roof covers. An L-shaped house is two rectangles. A house with an attached garage under the same roofline is two rectangles. Measure to the outside of the wall, not the edge of the eave.

Step 2 — find the pitch. Hold a 2 ft level horizontal against the underside of a rafter in the attic, mark 12 in along it from the end touching the rafter, and measure vertically from that mark up to the rafter. Five inches means 5/12. If the attic is inaccessible, do the same off a gable rake board from a ladder.

Step 3 — apply the multiplier. Each pitch has one: 4/12 is 1.054, 6/12 is 1.118, 8/12 is 1.202, 12/12 is 1.414. Multiply the footprint area by it and you have the sloped surface.

Step 4 — add the eaves. Perimeter in feet, times the overhang in feet, times the same multiplier. A 160 ft perimeter with 16 in eaves adds about 238 sq ft on a 6/12 roof, which is more than two squares and worth counting.

Step 5 — divide by 100 for squares, then add waste. Ten percent on a simple gable, fifteen on anything with valleys and dormers.

Worked through: a 40 x 28 ft ranch at 6/12 is 1,120 sq ft of footprint, 1,252 sq ft of roof, plus roughly 200 sq ft of eave, so 14.5 sq ft short of 15 squares. Order 16 squares and you have your ridge cap covered too.

Measuring on the roof gets you every plane, every valley, every dormer, and the actual ridge and hip lengths for cap. On a cut-up roof — three dormers, a couple of valleys, a turret — it is the only method that produces a real number rather than an estimate with a fudge factor.

It is also how people get hurt. Above 6/12, a roof is not a floor. Roofers work 7/12 and 8/12 on jacks and planks and stage anything steeper, and they do that because the consequence of a slip on a 9/12 is a fall from twenty feet onto concrete. If you are a homeowner in trainers with a tape measure, the answer above 6/12 is simply no.

Two other conditions rule it out regardless of pitch: a wet or frosty roof, and an old roof you suspect is soft. A deck that has been leaking for two winters will not hold you, and you will not know which sheet is the bad one until you are through it.

Nothing about getting a quote requires you to be on the roof. Any roofer who needs a measurement will take it themselves, and their insurance covers them for it.

EagleView, Hover, RoofR and the rest fly or model your roof from imagery and send back a report with every plane, pitch, ridge, hip, valley and rake dimensioned. They run roughly $20 to $90 for a residential property, and contractors buy them by the hundred.

They are genuinely accurate on pitch and plane geometry — usually within a couple of percent — and they are the right call for a complex roof you cannot safely walk. They also give you eave and rake lengths, which is where drip edge and starter get ordered from and where the ground method has to guess.

Where they fall down: imagery can be years old, so a roof with a recent addition or a new dormer comes back wrong, and heavy tree cover degrades the model. They also cannot see the deck. A report tells you the roof is 32 squares; it does not tell you that eight sheets of sheathing are rotten, and that is the line item most likely to move the final invoice.

If a contractor hands you a satellite report with their bid, that is a good sign — it means the number is measured rather than eyeballed. Ask what waste factor they applied on top of it.

For a homeowner sanity-checking a bid: the ground method. It costs nothing, it takes twenty minutes, and it tells you whether the 28 squares on the quote is honest.

For a simple gable or hip roof you are buying materials for yourself: the ground method, with a walk of the roof only if it is 6/12 or under and dry.

For anything cut up — multiple dormers, several valleys, mixed pitches: buy the report. Fifty dollars against a $18,000 job is not a decision, and the ground method genuinely cannot handle mixed pitches without measuring each plane separately anyway.

The mistake worth avoiding in all three is bidding off the floor plan. The house measures 2,000 sq ft through the rooms; the roof over it is closer to 2,600 once pitch, overhang and waste are counted. That gap is the most expensive arithmetic error in residential roofing, and it is entirely avoidable with a level and one multiplication.