How to measure a roof: site visit, plans or satellite imagery
Tape on the roof, the plans, or satellite imagery: how each method works, what it costs you in time and risk, and the math every one of them ends in, worked on a hip roof.
To measure a roof, get the footprint at the drip edge, get the pitch of every plane, multiply plan area by the pitch factor to get true area, divide by 100 for squares, and total the edge lengths by type. You can collect those numbers three ways: with a tape on site, from the plans or a prior report, or from satellite imagery.
All three end in the same arithmetic. Here's each method, when it wins, the math worked on one house, and the mistakes that cost you a load of shingles.
The three methods at a glance
| Method | What you need | Main risk | Wins when |
|---|---|---|---|
| Site visit with a tape | Ladder, tape, wheel, pitch gauge, a second person | Falls, missed lines on a cut-up roof | You're there for the inspection anyway, or the roof is hidden |
| Plans or a prior report | The drawings or the report, a scale ruler | Out-of-date or wrong-scale paper | New construction, or a roof that was measured recently |
| Satellite imagery | The address and a pitch source | Shadows, trees, old imagery, no pitch from above | Quoting from the office before anyone drives out |
Method 1: Measure it on site with a tape
The site visit is the oldest method and still the reference when the numbers disagree. You can do it two ways: on the roof, or from the ground with a ladder at the eaves.
On the roof
- Sketch the roof on paper first, from the ground. Draw every plane, ridge, hip and valley, and number the planes.
- Measure each eave and each rake along the roof surface, plane by plane, and write them on the sketch.
- Measure the ridge, then each hip and valley from end to end.
- Work out each plane's area from its shape. A rectangle is eave × rake. A hip end is a triangle: ½ × eave × the slope distance from eave to the hip peak. A trapezoid side is the average of its eave and ridge lengths × the slope distance between them.
- Read the pitch on each plane with a pitch gauge, or a level and tape: hold the level flat at 12 inches and measure the rise beneath its end.
Lengths taken on the roof are already slope lengths, so areas worked from them are already true area. Don't apply the pitch factor to them again. That double-count is the classic tape-measure mistake.
From the ground
- Run a measuring wheel or a tape along every wall and sketch the footprint with dimensions.
- From a ladder, measure the overhang at an eave and at a rake. Add it to every side of the footprint.
- Read the pitch on a rake from the ladder with a level and tape or a gauge.
- You now have plan area and pitch. Run the math in the next section.
From the ground you miss hidden valleys, crickets and low roofs tucked behind the main one. Walk all four sides before you trust the sketch.
What it costs you. The drive and the time on site, every job. On a steep or cut-up roof it's slow, and every line you skip is a line you guess later.
When it wins. You're already there for the inspection, the roof is under trees or the imagery is old, or you need what no picture shows: soft decking, the number of layers, the flashing condition, parapet heights.
Safety, briefly: use fall protection on the roof, never measure alone, and skip wet, icy or steep roofs. Measure from the ground or from imagery instead. No number is worth the fall.
Method 2: Take it from the plans or a prior report
On new construction there's no roof to stand on yet. The plans are the measurement.
- Get the roof plan from the builder, the architect or the permit set. Elevations help too: they show the pitch symbol, the little triangle marked 6/12 or 8/12.
- Check the scale against a printed dimension before you measure anything. If the drawing says 1/4 inch = 1 foot, a 40 ft wall should measure 10 inches. If it doesn't, the print was resized and every number you take off it is wrong.
- Find out what the outline shows. Many roof plans draw the wall line with the overhang noted separately. Add the overhang to every side.
- Take the plan dimensions, split the roof into rectangles and triangles, and add up plan area. Then apply pitch, plane by plane.
- Take ridge, hip, valley, eave and rake lengths off the plan the same way. Remember that a hip, valley or rake measured on a plan is a plan length, shorter than the real one.
A prior measurement report works the same way, with two checks. Confirm the roof hasn't changed since: an addition, a new garage or a reframed porch. And find out whether the squares already include waste, so you don't add yours on top.
What it costs you. Mostly the chase for the paperwork. The risk is quiet: drawings that were changed during the build, a print at the wrong scale, or an old report on a roof that has since grown.
When it wins. New construction bids, commercial work with drawings, and repeat jobs on a roof someone measured recently.
Method 3: Measure it from satellite imagery
From the office, you trace the roof on an overhead image, split it into facets, label the edges and apply a pitch. The image gives you the footprint and the edge lengths in plan. It can't give you the pitch, because rise isn't visible from straight above.
What you need. The address, and a pitch from somewhere: a site visit, the plans, or a street-level photo of a rake.
What it costs you. Desk time per roof, and no ladder. The risks are the image's: shadows that hide a valley, trees over an eave, flat roofs with no pitch cue, and imagery older than the latest addition.
When it wins. Quoting before anyone drives out, checking a crew's or a supplier's numbers, and pricing jobs that may never close. The short version of the steps comes after the math.
The math every method ends in
Every method lands here.
Plan area
The footprint seen from above, measured at the drip edge, not the walls. Split odd shapes into rectangles and add them.
Pitch factor
Pitch factor = √(1 + (rise ÷ 12)²). For 7/12 that's √(1 + 0.340) = √1.340 = 1.158. Our guide to finding roof pitch from satellite and Street View has the full table and the per-facet logic.
True area and squares
True area = plan area × pitch factor. Squares = true area ÷ 100. Then order squares = squares × (1 + waste). Waste depends on the roof shape, and the waste factor guide by roof type has starting points.
Edge lengths by type
Eaves feed drip edge, starter and ice and water at the eave. Rakes feed drip edge. Ridges and hips feed ridge cap. Valleys feed valley metal and ice and water. Eaves and ridges are level, so their plan length is their real length. Rakes, hips and valleys slope, so they need a factor.
- Rake length = run × pitch factor, where run is the horizontal distance from eave to ridge.
- Hip or valley length = run × √(rise² + 288) ÷ 12, for two planes of the same pitch meeting at a right angle.
Worked example: a hip roof
A single-story house, 58 × 30 ft at the walls, with a 1 ft overhang all around. Four planes, all 7/12.
- Footprint at the drip edge: 60 × 32 ft.
- Plan area: 60 × 32 = 1,920 sq ft.
- Pitch factor at 7/12: 1.158.
- True area: 1,920 × 1.158 = 2,223 sq ft.
- Squares: 2,223 ÷ 100 = 22.2 squares.
- With 12% waste for a hip: 22.2 × 1.12 = 24.9, so order 25 squares, or 75 bundles at 3 per square.
Now the edges:
- Eaves: the whole perimeter, 2 × (60 + 32) = 184 ft. A hip roof has no rakes.
- Ridge: length minus width on a hip with equal pitches, 60 − 32 = 28 ft.
- Hips: the run is half the width, 16 ft. √(7² + 288) = √337 = 18.36, and 18.36 ÷ 12 = 1.530. So each hip is 16 × 1.530 = 24.5 ft, and four hips are 98 ft.
- Ridge cap: 28 + 98 = 126 ft.
Now frame the same footprint as a gable at 7/12. The area doesn't move: still 1,920 × 1.158 = 2,223 sq ft. The edges do. Eaves drop to 2 × 60 = 120 ft, the ridge grows to 60 ft, and four rakes appear at 16 × 1.158 = 18.5 ft each, 74 ft in total. Same squares, a different drip edge, starter and cap order. That's why the edge labels matter as much as the area.
Common mistakes
- Measuring the walls, not the drip edge. The overhang is roof. On this example, a wall-line trace gives 58 × 30 = 1,740 sq ft instead of 1,920. That's 180 sq ft of plan area, or 180 × 1.158 = 208 sq ft of roof, 2.1 squares short before waste.
- Applying the pitch factor twice. Lengths taken on the roof surface are already slope lengths. Only plan measurements get the factor.
- Reading rakes, hips and valleys off a plan as real lengths. They're plan lengths. Apply the rake or hip factor, or you short the drip edge and the cap.
- One pitch for a roof with three. A 4/12 porch on an 8/12 house is a different plane. Measure or set each one.
- Trusting the paper without a check. A resized print, a reframed porch, or a prior report whose squares already include waste.
- Missing the lower roofs. The porch roof under the main eave, the lean-to on the back wall. Count the planes from every side of the house before you total.
The satellite route, in short
If you'd rather not climb, this is the desk version of the same method.
- Search the address and zoom until the roof fills the screen.
- Trace the outline at the drip edge, overhang included.
- Split it into facets, one per roof plane, snapped to the ridges, hips and valleys you can see.
- Label every edge: eave, rake, ridge, hip, valley and the rest.
- Set the pitch from a site visit, the plans or a street-level photo, and override the facets that differ. A free pitch finder does the arithmetic from a photo.
- Read the area, squares and edge totals, check them, and export the report.
Our pillar guide on how to measure a roof from satellite imagery covers each step in depth, including where the imagery fails and what to do about it. If you'd like the screens named button by button, measure your first roof in RoofStruct walks through the builder. RoofStruct gives you free, unlimited self-serve measuring and PDF reports, no card. When a slope hides under trees, upload your own drone or phone photo and trace on that instead.
Whichever method you use, the numbers end in the same place: plan area, pitch, squares and edges by type. Get those four right and the order follows.
FAQ
How do you measure a roof without getting on it?
From the ground with a wheel and a ladder at the eaves, from the plans, or from satellite imagery. Each gives you plan area. Add a pitch from a gauge on a rake, the elevations, or a street-level photo, and apply the pitch factor.
Do I measure roof lengths along the slope or flat?
Either works, as long as you don't mix them. Slope lengths taken on the roof give true area directly. Flat (plan) lengths from the ground, the plans or an image need the pitch factor. Rakes, hips and valleys measured in plan need their own factors too.
How do I turn roof area into squares?
Divide the true area by 100. A 2,223 sq ft roof is 22.2 squares. Add waste for the order: at 12%, that's 24.9, so 25 squares.
Does a hip roof have more area than a gable on the same house?
Not at the same pitch. Same footprint, same factor, same true area. The difference is in the edges: a hip trades rakes for more eave and hip length, and usually takes more waste.
Should a roof measurement include waste?
No. The measurement is the roof as built. Waste goes on when you turn squares into an order, and it depends on the roof shape and your crew's cuts. Keep the two numbers separate so nobody adds waste twice.