Ordering shingles from a ground-level measurement
How a sloped roof is always bigger than it looks from below, what the slope factor does to the area, and how to turn that corrected area into the bundles, squares and ridge capping you need to order.
The measurement you already have is wrong
When you measure a house from the ground you get the footprint, the flat rectangle the building sits on. The roof covers that same rectangle, but it does so at an angle. A sloped surface is longer than the flat ground beneath it, just as a hillside path covers more ground than the straight line between the same two points on a map.
The steeper the roof, the bigger the gap between the two areas. A gentle 4 in 12 pitch adds about 5 per cent. A steep 12 in 12 pitch, which is a full 45 degrees, adds 41 per cent. Order shingles for the footprint alone and you will run out well before the last course.
This gap catches people every time because the roof does not look that much bigger from the ground. Your eye sees the footprint, not the slope. The calculator exists to close that gap before you place the order.
What the slope factor is and how it works
Roofers describe pitch as rise per 12 of run. A 6 in 12 roof climbs 6 units for every 12 it travels along. The slope factor is the number you multiply the footprint by to get the true roof area.
The arithmetic behind it is simple trigonometry. The run and the rise form a right triangle, and the slope is the longest side. Dividing that longest side by the run gives the factor. For a 6 in 12 roof the factor is 1.118, which means the roof is about 12 per cent bigger than the footprint.
You do not need to work this out yourself. Type the pitch into the field marked Rise per 12 of run and the factor appears in the result. The degrees are shown too, so you can compare the number with a protractor reading if that is what you have.
Filling in the panel
Footprint length is the measurement along the ridge, the top line of the roof. Footprint width runs from one gable wall to the other, straight across the building. Both are in metres.
Pitch goes in as the rise figure. If you know your roof is 6 in 12, type 6. If someone told you 30 degrees, convert it first or try values until the degrees readout matches.
Roof shape offers two choices. Gable gives you the classic two-slope roof with vertical triangles at each end. Hip wraps around all four sides, which changes how much ridge and hip capping you need.
Coverage per bundle is printed on the wrapper of the shingles you plan to buy. The default is 3.1 square metres, which is close to the common three-bundles-to-a-square rule. If your brand covers more or less, change it here.
Price per bundle and currency are both optional. Fill them in and the result includes a cost estimate alongside the bundle count.
Gable roofs and hip roofs
A gable roof has two sloped faces that meet at a single ridge. A hip roof has four sloped faces and no vertical gable ends. For the same footprint and pitch, both have the same total surface area, so the shingle count stays the same.
The difference is in the edges. A gable roof has one ridge to cap. A hip roof has a shorter ridge plus four hip lines running from the ridge down to the corners. That means more metres of capping material and more angled cuts along every hip, which is why you should add extra wastage when you choose the hip option.
If your roof has a hip on one side and a gable on the other, pick the hip option for the wastage guidance and note the extra capping it lists.
Wastage and when to raise it
The default wastage is 10 per cent. That covers the trims at the ends of each course on a plain rectangular roof with no breaks in the surface.
Raise it to 15 per cent or more when the roof has valleys, dormers, skylights or anything else the shingles have to be cut around. Every angle means a partial shingle that gets trimmed and often cannot be reused. A complex roof with several wings can easily need 20 per cent.
Wastage is not padding. It is the material that ends up in the skip because it was the wrong shape. Under-estimating it is the most common reason for a second trip to the merchant.
Hip roofs deserve special attention. Each of the four hip lines creates a diagonal cut across every course it crosses. The offcuts from one side are the wrong angle for the other, so very little is saved by careful fitting.
Bundles, squares and capping
The result is given in three forms. Bundles are the physical packs you carry home. Squares are the trade unit, each one covering 100 square feet or about 9.29 square metres. Most shingles cover roughly a third of a square per bundle, so three bundles to a square is the usual count.
Ridge and hip capping is listed separately in metres. It is not included in the bundle figure because it is a different product, sold by the linear metre or in its own packs. On a gable roof the capping length equals the ridge. On a hip roof it equals the ridge plus the four hip lines.
Underlay and battens are not sized here. Underlay is sold by the roll in square metres, so use the corrected roof area figure for that. Battens depend on the spacing your shingle type requires.
Working through a real roof
A house measures 10 metres along the ridge and 7 metres from gable to gable. The pitch is 6 in 12. The footprint area is 70 square metres, but the slope factor of 1.118 turns that into 78.3 square metres of actual roof surface.
With 10 per cent wastage the order area becomes 86.1 square metres. At 3.1 square metres per bundle that is 28 bundles, which is about 9.3 squares. The ridge is 10 metres long, so 10 metres of capping goes on the list.
Now change the pitch to 10 in 12. The slope factor jumps to 1.302 and the roof area to 91.1 square metres. Wastage takes it to 100.3, which needs 33 bundles. Five more bundles just because the roof is steeper, and the footprint has not changed at all.
Switch the shape to hip and raise wastage to 15 per cent. The shingle area stays the same, but the extra wastage pushes the bundle count higher and the capping now covers the ridge and all four hip lines.
Finding the pitch without a ladder
If you can get into the loft, hold a spirit level against the underside of a rafter. Mark 12 inches along the level from the point where it touches the rafter. Then measure straight down from that mark to the rafter below. That vertical distance is the rise, and it goes straight into the pitch field.
If you cannot get inside, a phone app that reads angles can give you the slope in degrees from the ground. Look along the edge of the roof and hold the phone level with the slope. The tool shows degrees beside the pitch, so match the two up.
A third option is to count the bricks on the gable end. Each course is about 75 mm high. Count the courses from the eaves line to the ridge, multiply by 75 to get the total rise, then measure the horizontal half-width of the gable. The ratio of rise to half-width gives you the pitch.
Mistakes that lead to a second trip
- Using the footprint as the order area. The slope factor is the entire point of the calculation.
- Leaving wastage at 10 per cent on a roof with dormers or valleys. Bump it to 15 per cent at least.
- Forgetting the capping. Ridge and hip caps are a separate purchase, and running out mid-ridge means another delivery.
- Measuring the building at the base instead of the eaves. Overhangs add to the footprint on every side.
- Mixing up the length and the width. Length runs along the ridge, width runs across the building.
- Ordering bundles from one supplier and capping from another without checking that the colours are from the same range.
Everything stays on your own machine
The slope factor, the bundle count and the capping figure are all worked out inside your browser. No measurements leave your device, no account is needed, and there is nothing stored on a server to delete later. Close the tab and the numbers go with it.