Interactive lesson

Rainwater, step by step

Interactive lesson: from roof to tap — how much water your roof gives, gutters and first flush, the tank in the right place and rainwater in the house.

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The whole lesson as text

1. What the roof gives

The roof is the biggest water source on the homestead; you just have to measure it.

A millimetre is a litre — 1 mm of rain on 1 m² of roof gives 1 litre. In practice you collect 60–90 %, depending on the roof.
Measure in plan — Length × width of the house including the eaves, not along the slope. A 10 × 12 m roof covers 120 m² (about 1,290 sq ft), whatever its pitch.
Roof coefficient — Metal sheet and slate give 0.9; metal is the cleanest. Clay and concrete tiles 0.8, a flat roof with gravel 0.6, a green roof 0.3.
Asbestos cement: not for drinking — Do not drink rainwater from asbestos-cement sheeting. Old sheets are never brushed, pressure-washed, drilled or cut; removal is done by a licensed contractor (in the UK, see HSE guidance).

Why: A 120 m² house roof in lowland Britain or Ireland catches roughly 100,000 litres of rain a year (about 26,000 US gallons). Most of it ends up in the drain, and in August the same family wonders where to find water for the tomatoes — or sits under a hosepipe ban.

The sum: roof area (m²) × rainfall (mm) × roof coefficient = litres. For US gallons: square feet × inches of rain × 0.62 × coefficient.
Do not collect drinking water from a roof with lead flashing or old paint, or from a roof next to a busy road or industry.

2. Gutters and filter

Rainwater is only as clean as the roof and the gutters.

Gutter fall — 2–5 mm per metre, so water does not stand in the gutter.
Enough downpipes — One downpipe per roughly 50–80 m² of roof, so the gutter does not overflow in a downpour.
Leaf guard and screen — A leaf guard or basket with 5–10 mm mesh at the downpipe outlet keeps leaves up top. A downpipe filter with a 0.5–1 mm screen keeps fine dirt out of the tank.
Clean twice a year — At the end of March and at the end of November, after leaf fall. Ladder on firm ground, at an angle of about 1 in 4, someone holding it at the bottom; never on the roof when it is wet, frosty or windy.

Why: Leaves, needles and mud in the gutter rot and give the water colour and smell, and in the tank they become sludge. If the gutters overflow in every heavy shower, you lose exactly the water you need.

Gutter and downpipe size depend on the roof area and how heavy the downpours are; give the supplier the plan area of roof per downpipe.
Cut back trees over the roof: they bring leaves, birds, shade and moss.