Measured roof diagrams and aerial measurement reports
A measured roof diagram is a scaled drawing of a roof seen from directly above, labeled with the area, pitch and edge lengths of each section. It may come from measurements taken on the roof by a professional, from aircraft or satellite imagery, or from a drone flight. Whatever the source, it describes the size and shape of the roof, not its condition.
What it contains
The parts of a typical diagram
- Plan viewThe roof drawn from above, usually with a scale and a north arrow. The north arrow shows which way is north, which helps you match each side of the drawing to the house.
- PlanesEach flat section of roof, often lettered or numbered, with its surface area in square feet and its pitch.
- Area totalsTotal area in square feet and in squares, where one square is 100 square feet. Many reports add totals with several waste percentages.
- Edge lengthsLinear feet of ridge, hip, valley, eave and rake, and often the lengths where the roof meets a wall.
- PenetrationsA count, and sometimes the positions, of vents, skylights and chimneys. Imagery does not always pick up small pipes.
- Source and dateHow the measurements were taken and, for imagery, the date the images were captured.
Who produces it
Three common sources
Roofers and estimators often measure a roof themselves, on site, with a tape or laser measure and a pitch gauge, then sketch the result or draw it in software. Home inspectors rarely measure roofs in this much detail, because their reports focus on condition.
Measurement services produce aerial reports from high-resolution aircraft or satellite images, usually taken from several angles so that pitch can be estimated along with the outline. Roofers, insurers and adjusters order these reports routinely, and on a clear, simple roof they usually land close to a careful on-site measurement.
Drone reports are built from photos taken during a flight over the house, often combined into a three-dimensional model. In the United States a commercial drone pilot generally needs a federal remote pilot certificate. A recent flight can capture changes to the house that older aircraft or satellite imagery misses.
What it can tell you
Useful for size, silent on condition
A diagram gives every contractor the same dimensions to price against. It lets you check whether a quote's square count, valley length or number of vents is in a sensible range for your roof, and it makes a line item for a chimney you do not have easy to spot.
It cannot tell you what is under the shingles. A diagram does not show how many layers are on the roof, whether the decking is soft, whether the flashing is sound, or whether the attic is ventilated well. A roof can measure perfectly and still leak.
Limits
Where imagery struggles, and where tapes do
Measuring from imagery has predictable weak points, and a good report flags them. On-site measurement has limits of its own: a professional may judge a steep or damaged section unsafe to walk and measure it from a ladder or from imagery instead, and arithmetic slips happen. No method is perfect, which is why comparing two independent methods is useful.
Tree cover
Branches over the roof hide edges and valleys. The report may estimate the hidden part, and it should say so.
Complex roofs
Many small planes, dormers and stacked roof levels leave more room for error in both outline and pitch.
Image age
Imagery can be months or years old. A new addition or a removed chimney may be missing. Check the capture date.
Pitch from imagery
Pitch is estimated from the images rather than read with a gauge. Shallow pitches, and similar pitches side by side, are harder to separate.
Blurred edges
Imagery measures the roof edge as seen from above, overhangs included. Deep shadow or gutter guards can blur where that edge falls.
Scope and rounding
A report may leave out a detached garage or shed unless asked, and figures may be rounded. Check what is included.
Questions to ask
Before you rely on the numbers
- How were these measurements taken: on site, from aircraft or satellite imagery, or by drone?
- If imagery was used, when was it captured?
- Which structures are included? Is the garage, porch or shed in the total?
- Is the total area before or after waste, and what waste percentage was added?
- Was the pitch measured on each plane, or estimated?
- Were any sections hidden by trees, and has anyone compared the report with an on-site measurement?
Check a diagram from the ground, never from the roof
Do not climb onto the roof to test a diagram. Walk around the house and match planes, valleys and chimneys to the drawing, using binoculars for anything small. If a number looks wrong, ask whoever produced the report how it was measured.
See how the numbers are produced
The measurement guide walks through on-site and aerial methods, pitch factors and waste, and why two careful measurements of one roof can differ.
