You can measure most roofs well enough for a material estimate without standing on them. What you need is the horizontal footprint of each roof section, the overhang and the pitch. This guide shows where to take each measurement, how to split a house into sections and how much each kind of mistake changes the result.
What you are measuring
Roof area is footprint times slope. The footprint is the rectangle the roof covers when seen from straight above: the building length and width at the outside of the walls, plus the overhang on every side. The slope part comes from the pitch. So three numbers per section are enough, and all three can be taken from the ground, a ladder at the eave or the attic.
Roof area = (length + 2 × overhang) × (width + 2 × overhang) × √(1 + (pitch ÷ 12)²)
Step 1: sketch the roof and split it into rectangles
Walk around the house and draw the roof from above. Every part with its own ridge or its own slope is a section: the main house, an attached garage, a porch roof, an addition. Draw each as a rectangle and name it. Where two sections meet, decide which one owns the overlapping strip so that you do not count it twice.
Step 2: measure the footprint at the walls
Measure each rectangle along the outside of the walls, at ground level, with a long tape or a laser measure. Measure both directions and check opposite sides against each other. If the house has drawings, the outside wall dimensions on the floor plan are the same figures.
Do not use the living area from a listing. It leaves out garages and porches, and for a two-story house it counts floors that share one roof.
Step 3: measure the overhang
The overhang is the level distance from the outside of the wall to the edge of the roof. At the eaves, hold a level or a straight board against the wall under the soffit and measure out to the fascia, then add the thickness of the fascia board. At the gable ends, measure the rake overhang the same way. Our calculators take one overhang per section and apply it to all four sides, so if eave and rake overhangs differ, enter the average or split the difference in the section dimensions.
Overhangs matter more than they look. On the example house below, leaving out an 18-inch overhang understates the roof by 265 sq ft, 15.0% of the area.
Step 4: find the pitch
The safest way is from the attic: hold a level against the underside of a rafter, mark 12 inches along it and measure straight up to the rafter at the mark. The inches you read are the pitch. A gable wall works too, by measuring total rise and half the building width. The roof pitch guide describes both, and the pitch calculator converts any rise and run.
Step 5: calculate each section and add them up
| Section | Walls | Overhang | Pitch | Footprint | Roof area |
|---|---|---|---|---|---|
| Main house | 48 × 28 ft | 18 in | 6:12 | 1,581 sq ft | 1,768 sq ft |
| Garage | 22 × 22 ft | 12 in | 4:12 | 576 sq ft | 607 sq ft |
| Back porch | 16 × 10 ft | 12 in | 3:12 | 216 sq ft | 223 sq ft |
| Total | 2,373 sq ft | 2,597 sq ft |
For the main house the steps are: 48 + 2 × 1.5 = 51.0 ft long and 28 + 2 × 1.5 = 31.0 ft wide, a footprint of 1,581 sq ft. A 6:12 pitch has a multiplier of 1.1180, so the surface is 1,768 sq ft.
How much does a measuring mistake cost?
| Measurement | Roof area | Difference |
|---|---|---|
| As measured | 1,768 sq ft | — |
| Overhang left out | 1,503 sq ft | −265 sq ft (15.0%) |
| Overhang 6 in too small | 1,677 sq ft | −91 sq ft (5.1%) |
| Pitch read as 5:12 | 1,713 sq ft | −55 sq ft (3.1%) |
| Pitch read as 7:12 | 1,830 sq ft | +63 sq ft (3.5%) |
| Length short by 1 ft | 1,733 sq ft | −35 sq ft (2.0%) |
| Width short by 1 ft | 1,711 sq ft | −57 sq ft (3.2%) |
Two things stand out. A missed overhang is the most expensive error on the list. And being one unit off on the pitch moves the area by only a few percent at this steepness, less than a 10% waste allowance. Spend your care on the footprint and the overhang first.
Shapes the rectangle method does not cover
- Dormers. A dormer replaces part of the main roof with its own small roof and adds side walls. For shingle quantity, treating the main roof as if the dormer were not there and adding the dormer roof as a small section slightly overstates the area, which is the safe direction.
- Valleys and L-shaped houses. Split the L into two rectangles that do not overlap. The area comes out right when both wings have the same pitch. Valley lengths and the extra cutting waste are not part of the area and have to be allowed for separately.
- Hip roofs. With the same pitch on all four faces, a hip roof has the same area as a gable roof over the same footprint. Choose the hip shape in the calculator so that ridge and hip lengths come out right.
- Curved, conical or multi-pitch faces. These need a measured drawing or a professional measurement report.
When to get on the roof, and when not to
For ordering shingles, the method above is normally close enough once a waste allowance is added. Reasons to have someone measure on the roof are a complex layout with many intersecting planes, a suspected difference between drawings and the built roof, or a contract that prices by measured area. That is work for a roofer with fall protection. Nothing in a planning estimate is worth a fall.