How Do You Measure a Roof for Shingles?

A roof is measured in squares, and one square is 100 square feet of finished roof surface. The measurement everyone gets wrong is that a roof is always larger than the house under it, because a slope covers more area than the footprint it sits on. On a real northeast Oklahoma roof we measured recently the aerial report came back at 4,262 square feet across 15 facets, which is 42.62 squares before a single allowance for waste.

Note: this is a general guide only. Every roof is different, and a measurement taken from the ground or from the air should be confirmed on the roof before material is ordered.

An aerial measurement report gives seven numbers, and each one becomes a line on the estimate. The one we order is a GAF QuickMeasure. These are the real figures from that roof.

The Short Answer

Three steps, in this order, and only the first one can be done from the ground.

Measure the footprint, meaning the area the roof covers seen from above. Multiply by the pitch to turn that flat area into actual roof surface. Add waste for the material that gets cut off and thrown away at every hip, valley and rake. Divide by 100 and you have squares.

Miss the middle step and you will underbuy by ten to forty percent depending on how steep the roof is.

The Pitch Multiplier, Which Is Where Most Estimates Go Wrong

Pitch is written as rise over run: a 7/12 roof rises 7 inches for every 12 inches it runs horizontally. The multiplier converts flat footprint into sloped area, and it is simple geometry rather than a rule of thumb.

The same house needs 6.5 more squares of shingles at 12/12 than it does at 4/12, and nothing about the house changed.

Waste, and Why It Is Not Padding

Every cut at a hip, a valley or a rake leaves an offcut that cannot be used anywhere else. The steeper and more cut up the roof, the more of it ends up in the trailer.

The aerial report on that 15 facet roof supplied quantities at four different waste levels, and its own caveat is worth quoting: the suggested factor “is based on the roof complexity and is provided for guidance purposes only.”

The Worked Example

Start with the report: 4,262 square feet of roof surface, already pitch adjusted, across 15 facets at a predominant 7/12.

That is 42.62 squares. Fifteen facets is a genuinely complex roof, so the 13 percent column applies rather than the 8: 42.62 x 1.13 = 48.16 squares. Material is ordered in whole bundles at three bundles to the square, so that becomes 145 bundles, and the ridge and hip figure of 224 feet is ordered separately as ridge cap because field shingles cut down for ridge is not the same product.

None of that is a negotiation. It is arithmetic, and you are entitled to see it.

Why Your Roofer’s Number Differs From Your Satellite Measurement

It usually does, and the honest reasons are worth stating.

An aerial report measures what a camera can see from above. It can miss a low slope section hidden behind a parapet or a dormer, an addition built after the imagery was captured, and anything under a tree canopy. It reports a predominant pitch, so a roof with two or three different pitches gets averaged. And it measures surface, not access: a roof you can only reach across a neighbour’s fence costs more to work on and the report has no way of knowing that.

We run both, and the aerial report is the one we treat as definitive. It is ordered before the first visit and it is what the material order is built from. The field check is what catches the things the camera could not see, and where the two disagree we go back and find out why before anything is ordered.

What Changes the Answer

Three variables move a roof’s square count more than anything else, and only one of them is the size of the house.

The pitch

The single largest factor after footprint. It also decides whether the roof is walkable, which changes how long the job takes and what safety equipment it needs.

The facet count

A four facet roof and a fifteen facet roof of the same area are not the same job. More planes means more valleys, more hips, more cuts and more waste, and it means more places where flashing details have to be right.

Low slope sections and obstructions

A porch roof or a flat section over an addition may not take the same covering as the main roof, and chimneys, skylights and mechanical curbs each add detail work that does not show up in a square count.

Related Questions

How many bundles is that in shingles?

Three bundles to the square for most laminate shingles, so 48 squares is roughly 145 bundles. How many bundles of shingles are in a square? covers bundle counts and weights by shingle type.

Does the code care how the roof was measured?

Not directly, but it cares a great deal about what goes on the deck once you have measured it. What building code applies to roofs in Oklahoma? covers the provisions that turn into line items.

Why is the number on my insurance estimate different again?

A carrier’s estimate is built from its own measurement and its own scope, and the two rarely match line for line on the first pass. How do you get insurance to pay for a roof replacement in Oklahoma? covers how the documentation gets reconciled.

See Also

The two answers next to this one are how shingles are counted, ordered and priced, which is where a square count becomes a material list, and how long the result is going to last.