EstimatorKitFree roof surface take-off
Roof area calculator — section by section
Add a row for every part of the roof — the main house, the wing, the porch, the dormer — give each one its plan dimensions and its own pitch, and read the roof surface in square feet and squares. Dimensions come off a tape at ground level, and the eave projection is added on all four sides of every section, which is where a ground-measured roof usually loses a square. The page ends there on purpose: no cutting allowance, no bundle count and no price, so the figure can be checked against a plan or an aerial report with nothing hidden inside it.
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- No signup
- Own pitch per section
- Eaves added on all sides
- Stops at square feet
12 in to start, which is a common eave projection on a US house, measured level from the wall face to the drip edge. It moves with house style — 6 in on a flat-fronted modern build, 24 in and more on a bungalow or a ranch, so measure yours: it is applied to all four sides of every section you tick, and on a 42 by 28 house a foot of eave adds 144 sq ft of footprint before the pitch has touched it.
| Section | Shape | Length / base | Width / height | Pitch | Qty | Eave | Minus | Sloped |
|---|---|---|---|---|---|---|---|---|
:12 | 1,476×1.118 | |||||||
:12 | 698×1.118 | |||||||
:12 | 99×1.0308 |
A rectangle takes the two plan dimensions under the roof; a triangle takes the base and the distance from that base to the apex, both measured level. Tick Minus for a section that has to come out — a courtyard, or an area a second row already counted.
Roof surface above this building
2,272 sq ft
22.7 squares — a roofing square is one hundred square feet, by definition.
- Walls enclose
- 1,800 sq ft
- Eaves add
- 240 sq ft
- Footprint at the drip edge
- 2,040 sq ft
- Pitch adds
- 232 sq ft
- Sections counted
- 3
How tight is this number
Plus or minus 45.4 sq ft, if the dimensions came off a tape held at ground level. the spread between two people measuring the same house from the ground with a long tape, and it moves with how square the walls are and whether the tape sags across a driveway. That band is 2% of the surface — smaller than any sensible cutting allowance, which is why measuring from the driveway is good enough to order from and walking the roof with a tape rarely pays.
The two places it goes badly wrong are both structural rather than arithmetic: a section left out entirely, and a wing whose pitch was assumed to match the main house. Count the ridges you can see from the street and make sure there is a row here for each one.
Which roof am I looking at, and what do I measure
- Gable —
- two planes, one ridge, a triangle of wall at each end. One rectangle row covering the whole building; both slopes are already inside it.
- Hip —
- four planes, a short ridge or none, no gable wall anywhere. Also one rectangle row: the four planes fold back down onto the same footprint.
- Shed —
- one plane falling from a high wall to a low one. One rectangle row again, with the pitch measured across the fall rather than along the ridge.
- Cross gable, L or T —
- two ridges meeting. Two rows, split along the line where one wing ends, and use the minus tick if the two rectangles overlap at the junction.
The shape decides what you have to go and measure, not how much roof there is. That is why every one of the four above is entered the same way here.
This is bare surface, and it stops here
No allowance for cuts has been added, no material has been counted and no price has been applied. Hips, valleys and dormers do not change the figure above — they change how much of it ends up in the skip, which is a separate decision taken against the roof’s outline rather than its size.
How to measure a roof from the ground
Count the ridges, take the plan dimensions, add the eaves.
Break the building into the shapes you can actually measure
Stand back far enough to see the whole roof and count the ridges. Each one usually marks a section: the main house, the garage wing, the porch, a shed dormer over the back bedroom. Give each its own row and take the two plan dimensions from the ground with a long tape or a wheel, following the walls rather than the roof. Rectangles cover most houses; the triangle row is there for the odd corner infill and for a hip end that is genuinely separate from the block it sits on.
Add the eaves, because the roof is bigger than the house
The dimension you pace at ground level is the building, and the roof runs past it on every side. Measure from the wall face out to the drip edge, put that in the projection field, and tick every section it applies to. A foot of eave on a 42 by 28 house is 144 extra square feet of footprint before the pitch has multiplied anything, which is well over a square of material — and it is the single most common reason a ground-measured roof comes back small.
Give every section its own pitch and read the total
A wing, a porch and a dormer are rarely at the same slope as the main house, and each row carries its own rise in 12 so the multiplier is applied where it belongs. Sections you want out of the total — a courtyard, a flat deck, an area a previous row already counted — get the minus tick rather than a negative number. What comes out is footprint at the wall, footprint at the drip edge, and the sloped surface with the squares beside it.
Technical specifications
| Sections | Unlimited rows, each a rectangle or a triangle with its own dimensions, pitch and quantity |
|---|---|
| Per-section pitch | Any rise in 12 from 0 to 48, applied to that row's footprint alone |
| Eave projection | One figure applied to all four sides of every ticked row; starts at 12 in and takes feet, inches or metric |
| Deductions | Any row can be flipped to subtract — courtyards, flat sections, an area an earlier row already covered |
| Output | Footprint at the wall, footprint at the drip edge, sloped surface and squares, with each row's multiplier shown |
| Stated accuracy | About 2% either way for dimensions taken from the ground, which is under any cutting allowance |
| Deliberately excluded | Waste, materials, bundles and prices — this page ends at square feet |
| Data kept | Nothing; the row list lives in the tab and is gone on reload |
Frequently asked questions
Do I have to get on the roof to measure it?
No, and for a figure you are going to order material from it is not worth it. Everything this page needs is available at ground level: the plan dimensions of each block, the projection of the eaves, and the pitch, which comes off a level held against a rafter in the loft or a rake board you can reach from a step ladder. Two people measuring the same house from the driveway land within about two percent of each other, which is smaller than any cutting allowance the material will carry.
Does a hip roof have more surface than a gable roof over the same house?
No — over the same footprint at the same pitch the two enclose exactly the same area, and so does a shed roof running all the way across. That surprises people because a hip roof looks more complicated and costs more to cover, but the extra cost is in the cutting and the hip cap rather than in the surface. What the shape genuinely changes is the length of the edges, and edges are counted where the material is counted, not here.
How much does a dormer add?
A shed dormer adds its own footprint at its own shallower pitch and takes almost nothing away from the main roof, so give it a row and leave the main section alone. A small gable dormer is nearly a wash: the roof it adds is close to the roof it displaces, and the honest treatment is to skip it in the area and remember it in the cutting allowance, where it belongs. If the dormer is big enough that its ridge reads as a separate roof from the street, it is big enough to earn a row.
The garage is shallower than the house. Does that really change anything?
Yes, and by more than people expect. A 480 square foot garage footprint at 4:12 is 506 square feet of roof; the same footprint at 9:12 is 600. That 94 square foot gap is nearly a square of material, and it goes unnoticed when one pitch is assumed for the whole property because the error hides inside a plausible-looking total. Attached garages and rear extensions are the two places this happens most, and both are easy to check with a level from a ladder.
Should I subtract chimneys, skylights and vents?
Not from this figure. A chimney is a couple of square feet of hole and several feet of step flashing, so deducting it saves less material than the flashing around it consumes; the same goes for skylights, where the curb and the head flashing cost more than the opening saves. The rule that has stood up for a long time is to deduct only a penetration bigger than about 30 square feet, which in practice means a full turret, a light well or a whole covered section — the sort of thing that gets its own minus row here.
Is this the same number an aerial measurement report gives?
It should be within a couple of percent, and where it is not the disagreement is almost always about eaves or about a section one of you did not see. Aerial reports read the roof outline from imagery and are good at finding the section behind the house that you forgot; they are less good at low-contrast additions and at telling a covered porch from a roofed one. Running both and reconciling the difference is a better use of ten minutes than trusting either alone.
Why does this page not tell me how many shingles to buy?
Because the surface and the order are two different questions and mixing them hides mistakes. Surface is geometry and it either matches the building or it does not; the order is surface plus a judgement about cutting, plus a product that has its own coverage, plus a set of edges nothing here measures. Keeping them apart means the area can be checked against an aerial report, a plan or a roofer's own take-off without a waste factor buried inside it.
About measuring roof surface from a footprint
Roof area is one multiplication and two habits. The multiplication is the pitch factor, which is fixed and reliable and which the roof pitch calculator produces from a reading taken in about two minutes. The habits are what separate a figure you can order against from one that is quietly ten percent light: measuring to the drip edge rather than to the wall, and refusing to assume that the garage, the extension and the porch share the main roof’s slope. Both are easy, both are skipped, and both move the answer by more than any amount of care over the decimal places.
What does not move the answer is the shape of the roof, and this is worth being blunt about because half the pages on this subject imply otherwise. Hip, gable and shed over an identical footprint at an identical pitch enclose identical surface — the planes are cut differently and meet differently, but they cover the same sky. Complexity costs money through cutting, through hip and valley trim and through the hours it takes, none of which are area. Keeping that boundary clean is why this page has no waste field: an area with a padding factor folded into it cannot be reconciled with anything, and reconciling is the main thing an area figure is for.
Penetrations are the one place the trade convention looks illogical until you price it. Nobody deducts a chimney or a skylight, because the flashing, the step lap and the cut courses around an opening consume more material than the opening saves. The line most estimators draw is around thirty square feet, below which an opening is ignored. Once the surface is settled, the roofing calculator turns it into a full order, the shingle calculator into bundles, and the roof cost calculator into money. If it is a floor rather than a roof you are covering, the flooring calculator and the carpet calculator take the same room-by-room approach without the multiplier.
Where the section list lives
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — measurements, quantities, the prices your supplier quoted you — is uploaded, logged or kept, which is also why the calculators carry on working on a site with no signal.
The rows you add are held in the page’s own memory for as long as the tab is open. Nothing about the property — its dimensions, its layout, or the fact that someone is pricing a new roof for it — is recorded, and reloading the page clears the list.