How to Calculate Roof Pitch (UK Guide)

Three words, and mixing up two of them is the single most common error in roof pitch calculations.

Span is the full horizontal distance the roof covers, wall plate to wall plate.

Run is half the span on a symmetrical roof. It's the horizontal distance from the wall plate to a point directly below the ridge.

Rise is the vertical height from the wall plate level up to the underside of the ridge.

The error is using span where you should use run. Take a roof with a 7.2m span and a 2.4m rise. Done properly the pitch is 33.7°. Done with span instead of run, you get 18.4°. That isn't a rounding error, it's a different roof, and it would send you to a completely different tile.

The takeaway: on a symmetrical duo-pitch roof, halve the span before you touch the calculator. If the roof isn't symmetrical, measure each slope's run separately and calculate each side on its own.

Pitch in degrees is the inverse tangent of rise divided by run:

Pitch (°) = arctan(rise ÷ run)

Both measurements in the same units. On most calculators arctan is the tan⁻¹ or atan button.

Rafter length comes free from the same two numbers via Pythagoras:

Rafter length = √(rise² + run²)

That's your eaves-to-ridge measurement along the slope, before you add any overhang at the eaves or deduct for the ridge board.

The takeaway: two measurements give you both pitch and rafter length. If you're up in the loft with a tape, take rise and run once and you've got everything.

A homeowner is re-roofing a 1970s detached with a plain gable roof and wants to know what she's working with.

Up in the loft, span wall plate to wall plate measures 7.2m. So the run is 7.2 ÷ 2 = 3.6m.

From wall plate level to the underside of the ridge board measures 2.4m. That's the rise.

Pitch = arctan(2.4 ÷ 3.6) = arctan(0.6667) = 33.7°

Rafter length = √(2.4² + 3.6²) = √(5.76 + 12.96) = √18.72 = 4.33m

Now the useful part. At 33.7° this roof sits just under the 35° that plain tiles typically want. Not by much, and not by enough to see with your eye, but enough that a plain tile at standard headlap is off the table without going to the manufacturer and asking about an increased lap. Interlocking concrete tiles at a 22.5° minimum are comfortable here. Double Roman at 30° is comfortable. Plain tiles at 35° are not.

She measured a roof and got a number. What she actually got was a shortlist.

The takeaway: the pitch figure on its own is trivia. The pitch figure held against your tile's minimum is a decision.

Here's where most roof pitch guides send people down the wrong path. They treat pitch as a precision problem: get an accurate number, feel good about the accurate number. Almost nobody explains that pitch behaves like a threshold, not a dial.

Nothing about your roof changes between 27.3° and 28.1°. No manufacturer has a rule that fires at 27.6°. The roof performs identically, the tiles perform identically, your order doesn't move. That whole range is one answer wearing different decimals.

What matters is which side of a small set of numbers you land on, and in UK tiling those numbers are roughly 22.5°, 30° and 35°. Those are the typical minimums for interlocking concrete, Double Roman, and plain tiles. Cross one and your options change. Sit anywhere between them and the exact figure is decoration.

That flips the practical advice. If you measure 42° on a Victorian terrace, stop. You're clear of every threshold that exists, a phone app would have been fine, and there is no version of this job where the difference between 41.6° and 42.4° costs you anything. But if you measure 22° on a 1970s trussed roof, you are sitting on top of a threshold, and the difference between 22° and 23° is the difference between a tile you can buy and a tile you can't. That's when you stop guessing and get out a proper instrument.

Precision is worth paying for near a threshold and worth nothing away from one. Almost every guide gives the same advice at both.

The takeaway: work out roughly where you are first, then decide how hard to measure. Not the other way round.

Search for roof pitch and most of what comes back is American, which means most of what comes back is in x:12 ratios. A "6:12 roof" means six inches of rise for every twelve inches of run.

Nobody in the UK uses this. Marley, Redland, Sandtoft and Russell Roof Tiles all publish minimum pitches in degrees. Building control works in degrees. Your merchant works in degrees. Turning up with a 8:12 roof will get you a blank look.

For reference, the Bury roof above converts like this:

Most UK pitched roofs land between 30° and 45°. Older stock tends steeper: Victorian and Edwardian terraces sit around 35° to 45°, partly because slate and plain tiles need the pitch, and partly because that's what the pattern books said. Interwar semis are commonly 35° to 40°. Modern trussed roofs go shallower, often 22.5° to 35°, because interlocking concrete tiles allow it and shallower trusses are cheaper to make and easier to get under planning.

That history is why re-roofing a 1930s semi in modern interlocking tiles is straightforward and re-roofing a 1970s shallow-trussed house in plain tiles usually isn't. The pitch was chosen for the covering, decades ago, and it's still choosing your covering now.

The takeaway: if your pitch and your preferred tile disagree, the tile moves. Changing a roof's pitch means new trusses or new rafters, structural sign-off under Approved Document A, and quite possibly planning permission if the ridge height changes.

Pitch doesn't set your tile count directly. It sets your minimum headlap, headlap sets your gauge, and gauge sets your tile count.

BS 5534:2014+A2:2018, the code of practice for slating and tiling, ties minimum headlap to pitch and site exposure. Shallower pitch means water moves slower and wind-driven rain pushes further up the roof, so the lap grows. A bigger lap means a tighter gauge, more courses, and more tiles for the same roof area.

So a shallow roof costs more to cover than a steep one of the same area, which catches people out. Working out how many tiles you need is a separate calculation, and how many roof tiles per square metre walks through it properly.

The takeaway: get the pitch, then the lap, then the gauge, then the count. In that order, because each one feeds the next.

Most of the pitch arguments I've watched on site come down to two people measuring to different points on the ridge, then insisting the other one can't use a tape. One measured to the underside of the ridge board, the other to the top of the ridge tile. There's 80mm between those on a typical roof, and on a shallow pitch that's the best part of a degree.

The other one is the sag. A roof that's been on since 1908 has settled, and the pitch at the gable end and the pitch in the middle of a long run genuinely aren't the same. If you're near a threshold, the middle of the roof is the number that matters, because that's where the water sits longest and that's where you'll find out you were wrong.

And the reason it's worth caring: a tile spec is only a warranty while you're inside it. Fit a plain tile at 33.7° because the difference to 35° looked like nothing, and you've quietly voided the manufacturer's cover on a roof that'll be up there for fifty years.

The takeaway: agree what you're measuring to before you argue about the number, and take your reading from the middle of the roof, not the end.

The Roof Pitch Calculator takes your rise and run and returns pitch in degrees, along with the ratio and percentage equivalents.

Once you have the pitch, the Rafter Length Calculator gets you the eaves-to-ridge measurement, the Roof Tile Calculator turns it into a tile count, and the Roof Batten Spacing Calculator sets out the battens to BS 5534.