How to measure a roof on Google Earth: the ruler and polygon tools, and their limits
Google Earth measures a roof's footprint, not its roof area. The ruler and polygon tools step by step, the pitch factor that fixes the number, and the limits to know.
Google Earth measures a roof the way a tape on the driveway does: it gives you the footprint and the map length of every edge, and nothing about the slope. The ruler tool returns the distance between two points you click; the polygon tool returns the perimeter and area of the shape you trace. Both sit on the ground plane, so a 6/12 roof comes out about 12% small until you multiply by the pitch factor. Here is how to run both tools, what to do with the numbers, and where Google Earth stops being enough.
Which Google Earth, and one setting before you click
Both versions measure. Google Earth on the web has a Measure tool behind the ruler icon: click corner to corner for a distance, keep clicking and close the shape for an area. Google Earth Pro, the free desktop program, has a Ruler window with Line, Path and Polygon tabs, plus a historical imagery slider for stepping back to an older capture when the current one has a tree or a shadow over the roof.
Before you measure, switch the view to straight down and turn the modelled buildings layer off. Any tilt draws the roof leaning sideways, so a polygon that looks right lands partly on the wall. Then zoom until the roof fills the window: at a wide view one pixel can be a foot, and every corner you click is a pixel or two off.
The method is the same whatever tool draws the lines: trace the drip edge, set the pitch, label the edges, add waste. The general version is in how to measure a roof from satellite imagery; this is the Google Earth version of it.
The ruler tool: edge lengths, one at a time
Pick the units first, then click one corner and the next. Work around the outline in one direction and write each length down with what it is: eave, eave, rake, rake. Google Earth does not know an eave from a rake, so label as you go.
For eaves and ridges the ruler length is the length you order to, because those lines run level. Rakes, hips and valleys climb, and the ruler only sees their shadow on the ground. Multiply:
| Pitch | Pitch factor (roof area, rakes) | Hip and valley factor |
|---|---|---|
| 4/12 | 1.054 | 1.027 |
| 6/12 | 1.118 | 1.061 |
| 8/12 | 1.202 | 1.106 |
| 10/12 | 1.302 | 1.161 |
| 12/12 | 1.414 | 1.225 |
A rake climbs straight up the slope and takes the pitch factor. A hip or valley runs diagonally across the slope, climbs more gently, and takes the smaller factor. The formulas are in how to calculate roof pitch.
The polygon tool: footprint area and perimeter
Trace the drip edge, corner by corner, and close the shape. Google Earth prints the area and the perimeter. Read them for what they are.
The area is the footprint. Multiply by the pitch factor to get roof area, then convert to squares. If the roof has more than one pitch, trace one polygon per pitch group and factor each on its own; one polygon with one pitch over an 8/12 main and a 3/12 porch is wrong on both parts.
The perimeter is every edge of the outline added together, in map length. On a hip roof the whole outline is eave, so the perimeter is your drip edge and starter length. On a gable roof it is eaves plus the map length of the rakes, which still need the pitch factor. Do not order starter to the polygon perimeter on a gable roof.
Trace the edge of the roof, not the walls. The overhang is one to two feet on every side, and on a 40 by 30 house tracing the walls drops about 140 square feet from the footprint.
A worked example
A hip roof, 40 by 30 feet at the drip edge, 6/12 on all four planes, confirmed from the ground. Polygon tool: footprint 1,200 square feet, perimeter 140 feet.
- Roof area: 1,200 × 1.118 = 1,342 square feet, or 13.4 squares. With 10% waste, 14.8, so 15 squares.
- Drip edge and starter: 140 feet, all eave, no factor.
- Ridge: 40 − 30 = 10 feet, level, so the ruler length is the real length.
- Hips: the ruler reads each at about 21.2 feet of map length, 84.9 feet for four. Times 1.061 for 6/12: about 90 feet of hip to cap.
Google Earth supplied two numbers, the footprint and the perimeter. Everything else came from the pitch and the geometry, which Google Earth never sees.
Four limits to know before you quote from it
- It measures the ground plane. There is no pitch in the tool. You supply it from a straight-on gable photo, the attic, a Street View look at the rake, or the free roof pitch finder. Without a pitch the footprint is only a footprint, and quoting from it is a 5 to 40% error depending on the slope.
- Lean is baked into the capture. A tall house photographed off-nadir shows its roof shifted off the walls. In top-down view the roof you trace still has the right lengths; in a tilted view the polygon lands on nothing in particular.
- Nothing is labelled, nothing is carried forward. The polygon does not know which edge is an eave, where the valleys are or how many facets you have. Every number is read off the screen and typed by hand, and a transposed digit on the hip length is a short cap order.
- The capture may be old. An addition, a tree grown over the valley, a new porch: all invisible if the capture predates them. Earth Pro's historical imagery can show an older, clearer frame, never a newer one.
Where you trip
- Tracing the wall line instead of the drip edge, and losing the overhangs.
- Reading the polygon perimeter as eave length on a gable roof.
- Multiplying hips and valleys by the pitch factor instead of the hip and valley factor.
- Measuring a cut-up roof as one polygon with one pitch.
When Google Earth is enough, and when it is not
It is enough for a ballpark before the first visit, for a single-pitch rectangle, and for a sanity check on a report: the polygon footprint should match the report's footprint within a few percent, or one of them traced the walls.
It is not enough for a material order or a quote you will sign, because the pitch, the edge types and the arithmetic all live in your notepad. A roof measurement tool closes that gap: trace the same outline over satellite imagery or your own drone or phone photo, set a pitch per facet, label each edge once, and the roof area, squares and line totals come out computed, as a PDF for the supplier. RoofStruct's measuring builder does exactly this, and self-serve measuring and PDF reports are free and unlimited, no card needed. Google Earth is fine for the look; measure the job in a tool that knows what a rake is.
FAQ
Does Google Earth give roof area or footprint?
Footprint. The polygon tool measures the shape on the ground plane. Multiply by the pitch factor for that plane to get roof area; a 6/12 roof is 1.118 times its footprint.
Can Google Earth measure roof pitch?
No. Neither version measures slope. Get the pitch from a gable-end photo, the attic, or Street View, and apply it to the footprint and the sloped edges yourself.