Roof area is not the footprint
Roof covering is ordered against the surface the material lies on. On anything but a dead flat deck that surface is larger than the rectangle the building occupies, and the gap is wide enough to stop a job halfway through. Ordering against the footprint is the classic short delivery. The correction is one multiplication; everything else is about getting the right number into it.
Pitch, written three ways
A slope is a ratio, and it survives being written three different ways.
- Rise per 12. Six inches up for every twelve along: 6/12. The run is always twelve, in every system that uses the notation, so only the first number changes.
- Degrees. The angle from horizontal. 6/12 is 26.57°.
- Percent. Rise over run as a percentage — 50% for that same roof. This is the European convention, and the one on road signs. 100% is 45°, not vertical, which is the part people get wrong.
Degrees are the lossy form. Type 26.57° back into the roof pitch calculator and the rise comes out at 6.0013 per 12, not 6: an angle rounded to two decimals no longer names an exact ratio. It changes nothing you cut, but it is why the tool matches a pitch to its band with a tolerance rather than an equality.
The slope factor is the whole correction
For a rise of R against a run of 12, the sloped length is √(R² + 144), and dividing that by 12 gives the slope factor: sloped length per unit of horizontal run. One number converts every horizontal measurement on the roof into its sloped counterpart. Multiply the footprint by it for area, multiply half the span by it for a rafter.
| Pitch | Angle | Slope factor | Area above the footprint |
|---|---|---|---|
| 1.5/12 | 7.13° | 1.008 | +0.8% |
| 2/12 | 9.46° | 1.014 | +1.4% |
| 4/12 | 18.43° | 1.054 | +5.4% |
| 6/12 | 26.57° | 1.118 | +11.8% |
| 8/12 | 33.69° | 1.202 | +20.2% |
| 12/12 | 45.00° | 1.414 | +41.4% |
At the shallow end the correction is smaller than the waste allowance you were going to add anyway. A 2/12 porch roof is 1.4% bigger than the slab under it; that is not worth a second thought. From 6/12 up it is the difference between a full delivery and a second trip.
Measuring a pitch nobody told you
Hold a level against the underside of a rafter and read two numbers off it: how far along, how far up. That is the pitch. The units cancel, so the only requirement is that both numbers are in the same unit — 400 mm up over 1,200 mm along and 32 in up over 96 in along are the same roof, 4/12, 18.43°, factor 1.054.
Real roofs rarely land on a whole ratio. A rise of 430 mm over 1,200 mm is 4.3 per 12, which the calculator labels 4.5/12 because the conventional labels come in half inches — but the angle it reports, 19.71°, and the factor, 1.062, are computed from 4.3 and not from the label. Order against the factor.
What the overhang adds, and where it does not
An eave overhang projects past the wall on the two eave sides, so it widens the span by twice the projection. It does not lengthen the building along the ridge, and the calculator leaves the length alone on purpose: a rake overhang at the gable ends is a design decision rather than a given, and counting it silently would overstate every roof that does not have one. If yours has one, enter the longer length yourself.
A 28 by 40 house at 6/12
Span 28 ft, length 40 ft, 18 in of eave.
| Surface | ft² | Squares |
|---|---|---|
| Building footprint | 1,120 | 11.2 |
| Footprint widened by the eaves | 1,240 | 12.4 |
| Sloped roof area | 1,386 | 13.9 |
Ordered against the footprint, that roof is 266 ft² short — 2.7 squares, close to a fifth of the covering. The same pitch puts the ridge 7 ft above the wall plate and makes the common rafter 17.33 ft from ridge to eave end. Add a 12 in rake overhang at both gables and the area reads 1,456 ft² instead.
A roofing square is 100 ft², and it is what supplier pricing and bundle counts are quoted against, so divide the sloped area by 100. The tool prints that to a tenth of a square and stops there: rounding up is a decision about waste, hips, valleys and starter course, not arithmetic.
The bands that decide the covering
Pitch settles what can go on the roof at all, and that question is separate from quantity. It bites at the shallow end, where the slope factor is doing almost nothing and the pitch still rules the material out:
- Below 2/12, asphalt shingles are not permitted. That roof takes a membrane.
- A dead flat deck is its own case again: the fall has to be built into the deck, and the membrane fully bonded.
- 2/12 to 4/12 takes shingles, with double underlayment for the warranty to hold.
- Above 6/12 the cost moves out of material and into labour — staging first, then fall arrest above 9/12.
So the order is: get the pitch, from the drawing or off a level. Widen the span by the eaves. Multiply by the slope factor. Divide by 100. Then argue about waste.