Why a flat plan under-orders every re-roof
A roof is not flat, but the plan it gets ordered from usually is. Pull up the site plan or the aerial photo, measure the length and width of the house, multiply them together, and you have the footprint, not the roof. The roof itself is that footprint tilted up at whatever angle the trusses were cut to, and tilting a surface always makes it bigger. A 30 degree roof needs roughly 15% more sheeting or tile than its footprint suggests. A 35 degree roof needs closer to 22% more. Nobody sets out to under-order a re-roof. It happens because the number that's easy to measure (the footprint) and the number that's actually needed (the sloped surface) are two different figures, and the gap between them grows with the pitch, not with the size of the house.
This calculator does that conversion directly. Enter the pitch, whichever way you know it, then the building's footprint dimensions, and it works out the true roof area from the two together. If you're quoting sheet metal, tile or batten off a plan rather than a physical measure-up, that's the number to be ordering against, not the footprint on the drawing.
The three ways a roof pitch gets written down
A roof pitch shows up in three different notations depending on where it came from, and they describe the same angle without looking like they do. Degrees is the most common on a current building plan or engineering drawing in Australia: a straightforward angle from horizontal, 22.5 degrees, 30 degrees, and so on. A rise-over-run ratio is older and shows up on hand-drawn plans, some truss manufacturer paperwork and a lot of American material, written as a rise per unit of run, such as 5 in 12. Percentage grade is the odd one out, more common in civil and drainage work than roofing, expressed as the rise divided by the run as a percentage rather than an angle. All three describe the same slope. A 30 degree pitch is roughly a 0.58 : 1 rise-run ratio and a 58% grade. None of them is wrong, and a quote that arrives in one notation against a plan drawn in another is a common reason a job gets re-measured before it's priced. This tool converts between the three so you're not doing trigonometry by hand to check whether two documents actually agree with each other.
Older extensions and additions are usually the case where the mismatch shows up. The original house might carry a pitch specified as a ratio on a 1980s plan, the new section gets quoted in degrees off a current engineering drawing, and someone has to confirm the two actually match before the roofline reads as one continuous plane rather than two roofs that were never meant to sit together.
Measuring a pitch without getting on the roof
You don't need to be standing on the roof to work out its pitch, and for most houses you shouldn't be. The safest read is from inside the roof space, if there is one. Hold a spirit level flat against the underside of a rafter or truss chord so it reads level, measure 300mm along the level from the rafter, then measure straight down from that point to the rafter's underside. Run and rise, both in millimetres, feed straight into the rise-and-run mode above and it'll come back with the angle.
If there's no roof space you can safely get into, read the pitch off the gable end instead, from the ground, with a phone. Stand back far enough to get the whole gable triangle in frame, take a level photo square-on to the wall (not from an angle, which skews the apparent slope), and either use a spirit level and phone protractor app against a printed copy or against the screen itself, held against the roofline in the photo. It won't be as exact as a level held on the actual timber, but it's close enough to pick between notations and sanity-check a quote, and it doesn't involve a ladder.
What it's not close enough for is anything you're about to act on structurally, like confirming whether an existing roof meets a minimum pitch for a particular sheeting product, or whether a proposed extension's roof will actually tie into the existing one at a sensible angle. That's a job for someone standing on the roof with proper measuring gear, not a photo from the driveway.
Turning a horizontal run into a rafter length
The horizontal run is the flat distance from the wall plate (where the rafter sits on the top of the wall) to the ridge, measured as if you were looking straight down on the building from above. It is not the same as the rafter itself, which runs at an angle and is therefore longer. The steeper the pitch, the bigger that gap gets: on a low 10 degree skillion the rafter is barely longer than its run, but on a steep 45 degree pitch the rafter is around 41% longer than the horizontal distance it covers. This calculator takes the horizontal run and the pitch and returns the actual sloped length, which is the figure a rafter, a batten run or a length of guttering fascia trim actually needs to be cut to, not the flat measurement off the plan.
Rafter length and the trims that ride on it
It's not just rafters that need the sloped length rather than the flat run. Fascia board, gutter, sarking and the top course of batten all follow the roof plane rather than the plan, so a length worked out from the horizontal run alone comes up short on every one of them. On a simple gable it's one calculation, run in, slope out, but on a hip roof each hip rafter runs at a compound angle across two roof planes at once, and its true length needs its own working, not just this tool's single-plane figure applied twice. Use the slope length here for a straightforward gable or skillion plane, and treat a hip roof's corner rafters as a separate measurement a carpenter or roofer takes off the actual frame.
Plan area versus true roof area
This is the calculation that catches people out most often, and it's worth spelling out in plain terms rather than just handing over the number. Plan area, sometimes called footprint area, is the building's length times its width, as if the roof were laid flat. True roof area is that same shape tilted up to the actual pitch, which stretches every dimension running up the slope while leaving the dimensions running along the ridge and eaves unchanged. Mathematically, true roof area is plan area divided by the cosine of the pitch angle, and because cosine gets smaller as the angle increases, dividing by it makes the area bigger. A flat roof (0 degrees) needs no correction at all. By 45 degrees the roof needs about 41% more material than the footprint. That's not a rounding error, it's the difference between ordering enough sheeting and having to place a second, smaller order at a premium once the shortfall shows up mid-job.
One thing this figure doesn't include is hips, valleys, ridge capping length or the extra cutting and wastage a complex roofline generates over a simple gable. Those are real and they add further material on top of the true roof area this tool returns, they just depend on the roof's actual shape in a way no calculator working from a single pitch and a rectangle can capture. Treat the number here as the geometric floor, the minimum a roof at that pitch and footprint requires, and build your own job's wastage allowance on top of it based on how many hips, valleys and penetrations the actual roof has.
Why the pitch matters beyond the material count
Pitch isn't just a material multiplier. It decides which roofing products are even allowed on a given roof in the first place. Most sheet metal profiles carry a minimum pitch below which water can be driven back up under the laps by wind, and most tile profiles carry their own minimum for the same reason. A roof that reads as borderline between two notations, or that a homeowner has measured from a gable-end photo rather than off the actual rafters, is exactly the kind of number worth double-checking against the manufacturer's stated minimum before a product gets ordered, not after it's on site.
When to stop and get a roofer
This calculator is for working out what a job needs before you ring anyone, and for checking that a quote and a plan actually agree with each other. It isn't a substitute for someone competent standing on or under the actual roof. Get a roofer involved, rather than relying on a photo or a plan reading, whenever there's any sign of sagging or unevenness across the roofline, whenever the roof needs to be walked to confirm structural condition before a re-roof, whenever two roof planes meet at a hip or valley and the true area genuinely can't be worked out from a rectangle and a single pitch, and whenever you're about to commit to a sheeting or tile product and need the minimum pitch and fixing spec confirmed for that specific profile rather than estimated from a photo. If your own site is a roofing business rather than a homeowner working out a job, see how we build websites for roofing businesses that bring in exactly this kind of enquiry.
Where this fits with the rest of the job
A roof is rarely the only trade on a job like this. An extension or a full re-roof usually touches carpentry for the framing and fascia, and if you're scoping that side of the work too, our other calculators cover the timber side of a build the same way this one covers the roof, converting a measurement into the actual material figure rather than the footprint on the plan.