Measuring & pitch
How to measure a roof from the ground (no ladder needed)
Everything a roofing order needs can be measured standing on the lawn. Here is the method, the one calculation that matters, and the checks that catch the usual mistakes.
Why you do not need to climb
Roofers measure on the roof because they are already there. For everyone else, the roof is the most dangerous place on the property and the least necessary one to visit for a measurement. A roof is a set of tilted planes sitting on a building whose walls you can walk around, and the tilt is the same for every plane on most houses. Measure the building, measure the tilt, and arithmetic gives you the roof.
That arithmetic is one multiplication. The horizontal area under the roof, the footprint including the overhangs, times the pitch factor, √(1 + (rise ÷ 12)²), is the sloped area of every plane together. Every quantity on this site starts from that number or from the edge lengths you collect on the same walk round the house.
What to measure, in order
You need a 100-foot tape (or a laser measure), a 2-foot level, a ruler, binoculars and a notebook. A helper makes the tape work twice as fast.
- The walls. Measure each wall of the house at ground level, outside face to outside face. Sketch the plan as you go and write each length on it. An L-shaped or T-shaped house is two or three rectangles; measure each.
- The overhangs. Stand under the eave and measure horizontally from the wall to the drip edge or the face of the gutter. Do the same under a rake (the sloped gable edge). Twelve to 24 inches is usual; rakes are often shorter than eaves.
- The pitch. Hold the level horizontally against the underside of a rake board at a gable end (from a stepladder at a single-storey gable, or in the attic against a rafter). Mark 12 inches along the level, then measure straight down to the board. The drop in inches is the rise per 12. A phone inclinometer laid on the rake board gives the angle instead. Do it on each roof section that looks different.
- The edges. Count the eaves (horizontal bottom edges), the rakes, the hips, the valleys and the ridge from your sketch and the binoculars. Their lengths follow from the dimensions you already have.
- The details. Count chimneys, skylights, vent pipes and dormers, and note any wall the roof runs along.
A satellite view helps with step 1 on a complicated plan: the measuring tool in a free mapping program gives the outline of the roof in plan, which is the footprint with overhangs. It does not give the pitch, and the plan area it reports is not the roof area until the pitch factor is applied.
From measurements to roof area
A ranch house measured from the lawn
The walls measure 50 by 32 feet, the overhang is 12 inches all round, and the level shows a 6-inch drop over 12 inches.
- Footprint with overhangs: 52 × 34 = 1,768 sq ft
- Pitch factor at 6/12: √(1 + 0.25) = 1.118
- Roof area: 1,768 × 1.118 = 1,977 sq ft, 19.77 squares
- With 10% waste for a plain gable: 21.74 squares, which is 66 bundles of three-to-the-square shingles
The roof area calculator does exactly this, and the shingle calculator takes the same inputs straight to bundles.
The pitch is the measurement most worth repeating. A rise read as 5 inches instead of 6 changes the factor from 1.118 to 1.083, three percent of the roof; read as 8 instead of 6, it changes it by eight. The pitch calculator converts a rise over any run, or an angle, into the factor, and roof pitch explained covers reading a pitch from a plan or a photo.
The edges, without a ladder
Every linear material on a roof follows one of five edges, and each comes from the plan dimensions.
| Edge | How to get its length | Used for |
|---|---|---|
| Eaves | The house length (or each wall on a hip roof) plus the rake overhangs | Starter strip, drip edge, ice barrier, gutters |
| Rakes | Half the width plus the overhang, times the pitch factor, for each sloped edge | Starter strip, drip edge, rake trim |
| Ridge | The house length plus rake overhangs on a gable; length minus width on a regular hip roof | Ridge cap, ridge vent |
| Hips | Half the width times the hip factor, √(rise² + 288) ÷ 12, for each hip | Hip cap |
| Valleys | The horizontal distance from the inside corner to the ridge times the hip factor | Valley lining and metal |
On the 50 by 32 foot gable house the eaves are two 52-foot runs (the 50-foot wall plus two 12-inch rake overhangs), the four rakes are each 17 × 1.118 = 19 feet, and the ridge is 52 feet. Those three numbers feed the starter strip, drip edge and ridge cap calculators, and the eave lengths are the gutter length.
Checks that catch the usual mistakes
Compare with the living area. The roof area of a single-storey house is usually 1.1 to 1.4 times its floor area, depending on overhangs and pitch. If yours comes out below the floor area, an overhang or the pitch factor was left out; if it comes out double, you probably measured a two-storey house’s floor area instead of its footprint.
Count the planes. A plain gable has two; a hip roof four; an L-shaped house with a hip roof six or more. A hip roof of one pitch has the same area as a gable over the same footprint, but more waste, more cap and no rakes, which why your roof is bigger than your house works through.
Look for a second pitch. Porches, additions and dormers are often shallower or steeper than the main roof. Measure them as separate footprints with their own factor, and add.
Check against a satellite outline. Divide the mapping tool’s plan area by your footprint: the two should agree within a few percent. A larger difference usually means a missed bump-out or a porch roof.
What to do with the numbers
The squares go to the shingle, underlayment, metal, tile and cost calculators; the edges go to the trim and flashing calculators; the waste allowance depends on the shape, which bundles per square and waste explains. Write the measurements on the sketch and keep it: a roofer’s quote should land within a few percent of your squares, and if it does not, you can ask which numbers differ.
When to stop measuring and call someone
If the roof is steep (above about 7/12), more than one storey at the eave, covered in moss or ice, or sagging, measure what you can from the ground and stop there. A roofer or inspector with fall protection can check the rest, and a sagging roof line is a question about the structure before it is a question about shingles.