Roof Area Calculator

Measure on plan, not up the slope

This is the mistake that shows up most often, and it always errs the same way: it oversizes.

If you climb up and tape the rafter from eaves to ridge, you have measured the slope length. The pitch factor exists precisely to convert a flat footprint into a sloping surface's catchment, so feeding it a slope length applies the pitch twice. On a 35° roof that overstates the area by roughly a fifth.

Measure the footprint instead. Stand back from the house and measure the ground the roof covers, eaves line to eaves line and gable to gable. If you only have a slope length, multiply it by cos(pitch) to get back to plan. At 35° that means multiplying by about 0.819, so a 5m slope has a 4.1m plan width.

Why effective area is bigger than plan area

Rain does not arrive vertically. In wind it is driven at an angle, so a sloping surface presents a bigger target to it than its footprint does. BS EN 12056-3 handles this with a single assumption: rain driven at 26.6° from vertical.

That produces the effective area rule A = A_H + ½ × A_V, the plan area plus half the vertical projection of the surface. Since a roof's vertical projection is its plan width multiplied by tan(pitch), the whole thing collapses into one multiplier:

pitch factor = 1 + ½ × tan(pitch)

It is worth being clear about what this is not. It is not a safety margin, and it is not a claim about how much rain falls in your area. It is a geometric allowance for wind-driven rain, and rainfall intensity is applied separately afterwards.

The formula reproduces Approved Document H Table 1 exactly

Approved Document H publishes three pitch factors rather than a formula. They are the same rule, pre-computed:

Pitch	Approved Document H Table 1	1 + ½·tan(pitch)
Flat	plan area of the relevant portion	1.000
30°	plan area × 1.29	1.289
45°	plan area × 1.50	1.500
60°	plan area × 1.87	1.866
Over 70°, or any wall	elevational area × 0.5	n/a

Because the two agree to the second decimal place, this calculator uses the continuous formula and accepts any pitch. Very few real roofs are exactly 30°, 45° or 60°, and rounding a 38° roof up to 45° adds around 8% of area you do not have.

Why per-slope area is the number that matters

A gutter drains one slope, not one roof. On a duo-pitch roof the front slope sheds into the front gutter and the rear slope into the rear gutter, and neither one ever sees the other's water. Sizing a gutter on the whole roof's area would put you two sizes too big on both sides.

The same logic makes hipped roofs interesting. The catchment is identical to a gable roof of the same footprint, but it is divided between four runs rather than two, so each individual run carries less. The calculator above splits it out: the two long runs take trapezoidal slopes, and the two hip ends take triangles. Whether that changes your gutter size depends on the flow, which is what the gutter size calculator works out.

Frequently asked questions

How do I find my roof pitch?

Pitch is the angle of the slope, and you can get it three ways. Use a pitch gauge or a phone inclinometer held against a rafter in the loft. Or measure rise over run: from the wall plate to the ridge, take the vertical rise and the horizontal run, then pitch = arctan(rise ÷ run). A 5m run with a 3.5m rise is arctan(0.7), or 35°. Or read it off the drawings if you have them.

Common UK pitches run from 30° to 45°. Modern trussed roofs are frequently 35° to 40°. Plain tiles and slates usually need steeper pitches than large-format interlocking concrete tiles, which is why older roofs often sit at the top of that range.

Do I include the roof overhang?

Yes. The gutter catches whatever the roof sheds, and the overhang sheds into it like the rest of the slope. Measure to the edge of the tiles, not to the face of the wall. On a typical house that overhang accounts for a couple of square metres per slope, which is small but free to include.

Does a hipped roof need smaller gutters?

Often yes. The total catchment is unchanged for a given footprint, but it is divided across four gutter runs instead of two, so each run carries less flow. Whether that drops you a gutter size depends on the dimensions and your local rainfall intensity, and a shorter run can also let you get away with a single end outlet where a longer one would not.

One thing not to do is size a hipped roof by dividing the total by four. The hip ends are smaller than the long runs, and the longer the building is relative to its width, the bigger that difference gets. An even split understates the long runs, which are the ones that decide the size.

Should I use the plan area or the effective area?

Use the effective area for anything to do with drainage: gutters, downpipes, soakaways, rainwater harvesting yield. Plan area is useful for planning, footprint and floor comparisons, and it is what a flat roof's effective area equals anyway, since its pitch factor is 1.0.

What about a roof steeper than 70 degrees, or a wall draining into the gutter?

Approved Document H Table 1 treats those separately: take the elevational area (the area you see looking at it straight on) and multiply by 0.5. The continuous pitch formula is not used there, which is why this calculator stops at 70°. The gutter size calculator handles walls directly.

How accurate is this for an L-shaped or complicated roof?

Split it into rectangles and run each one. Add the results for any rectangles that drain into the same gutter run. Where two roof planes meet in a valley, the valley gutter itself is outside the scope of Approved Document H Table 2 and must be designed to BS EN 12056 in full.