How Guttering Calculator Works

Enter your roof dimensions, pitch, and the calculator works out the effective roof area, converts it to a rainwater flow rate, and matches that against gutter capacities to recommend a size and number of downpipes. It then counts components for your run lengths.

The sizing method comes from BS EN 12056-3. The effective roof area is the plan area multiplied by a pitch factor, since steeper roofs catch more wind-driven rain: a 30-degree pitch increases effective area by 29%. Multiply this by the design rainfall intensity, 75mm per hour for most of the UK (0.021 litres per second per m²), and you have the flow the gutter must carry. A 60 m² effective roof produces about 1.26 l/s in the design storm. For aggregate surfaces beneath your downpipes, you can check our Gravel Calculator.

Downpipes: Where Systems Actually Fail

The most common guttering failure is not the gutter at all but underspouting: a generous gutter choked by too few or too small downpipes. The working rules prevent it: a 68mm round downpipe handles roughly 1.0 to 1.5 l/s, plan one downpipe per 50 m² of effective roof area, and never run more than 12 metres of gutter to a single outlet.

Outlet position is a free capacity upgrade most installers ignore: an outlet in the centre of a run drains water from both directions, effectively doubling the roof area the gutter can serve compared with the same outlet at one end. On a long run, a central downpipe or two end outlets with the fall split from the middle transforms performance without changing a single component size. You can calculate fall slants using our Drainage Gradient Calculator.

Counting Components

A complete order for a gutter run contains more than gutter. Lengths come in 2m and 4m, so divide each run by length and round up. Then per run, plan one running outlet per downpipe, unions to join lengths (one fewer than the number of lengths, since outlets also join), stop ends for every open end, and 90-degree angles at corners. Fascia brackets go at maximum 1-metre centres, with one each side of every joint and corner, and downpipes need a bracket every 1.5 metres plus an offset (swan neck) at the top and a shoe at the bottom.

On fall, set 3mm per metre as the minimum (1:350) and 10mm per metre for a self-clearing run, marking the high point and outlet first and snapping a line between them. Gutters should sit so the roof edge overhangs into them, tiles about 25 to 50mm above. For matching roof tile configurations, check our Roof Slate Calculator.

Worked Example

A gable roof, 8m eaves × 5m eaves-to-ridge in plan per side, at 35 degrees:

• Effective area per side: 40 × 1.35 = 54 m²
• Flow: 54 × 0.021 = 1.13 l/s per side
• Gutter: 112mm half round at 0.9 l/s is marginal; a deepflow profile or a centre outlet solves it cleanly.
• Per 8m run: two 4m lengths, one running outlet, one union, two stop ends, 9 brackets.
• Downpipe per side: 5m of pipe, 4 brackets, offset bends, and a shoe.

That marginal case is exactly the situation the calculation exists for and where the centre outlet trick beats buying a bigger system.