Britain has one genuine preparedness advantage and it falls out of the sky about 150 days a year. The water page says stored water runs out; a roof does not. A modest semi-detached house collects more water annually than a family of four drinks.
The catch is that roof water is not drinking water. Getting from one to the other is the whole of this page.
The only sum you need
One millimetre of rain falling on one square metre of roof gives you exactly one litre of water. That is not an approximation, it is just how the units work, and everything else is a correction to it.
Worked through for a real house: a semi-detached with a 45 m² footprint, a clay tile roof, in a part of England getting 900 mm a year.
45 × 900 × 0.80 × 0.90 = 29,160 litres a year, about 80 litres a day averaged out.
The seasonal distribution matters more than the annual total. British rain is reasonably spread through the year, which is kinder than a monsoon climate, but a dry fortnight in June is normal and your store has to cover it. Size the tank for the dry gap, not the annual figure.
What your roof is actually worth
Two corrections people miss.
Not all of the roof drains where you think. A house typically has two or more downpipes serving different halves. You only capture what runs into the pipe you tap, so unless you plumb them together you are working with a fraction of the footprint, not all of it.
The material matters for what comes off it. Clay and concrete tile, and modern coated steel, are all fine. Avoid collecting from anything with lead flashing draining across it, from asbestos cement sheet, or from a roof treated with moss killer. Old bitumen felt taints the water noticeably.
The first-flush diverter
This is the single component that turns roof runoff into something worth storing, and the most commonly skipped.
Between rain events, a roof accumulates bird droppings, dust, moss fragments and dead insects. The opening minutes of a downpour wash all of it into the gutter at once. A first-flush diverter catches that opening volume, holds it aside, and only then lets the cleaner water through to the tank.
Size it at 0.5 to 2 litres per square metre of roof. Use the low end in clean rural air, the high end near a main road, under trees, or anywhere with a lot of birds.
For the 45 m² example, that is 22 to 90 litres of diverted water per event.
The standard build is a vertical pipe closed at the bottom with a slow-release valve or a small drilled hole. It fills first, and once full the incoming water rises past it and continues to the tank. After the rain stops it drains itself slowly, ready for next time.
Building the system
- Screen the gutter. A mesh guard over the gutter or the downpipe head stops leaves and the worst of the solids. This is the cheapest maintenance saving in the whole system.
- Fit the diverter to the downpipe just above the tank inlet, sized as above.
- Feed the tank through a calmed inlet. Water arriving hard stirs up the sediment that has settled on the tank floor, putting it straight back into suspension. Turning the inlet pipe to discharge downward near the base, or fitting a simple baffle, keeps the tank stratified and the drawn water clearer.
- Draw from partway up, never from the floor. Sediment collects at the bottom and a floating or mid-height take-off gives you noticeably better water for free.
- Overflow deliberately, away from the house. Every tank overflows eventually. Decide where that water goes before the weather does, and keep it clear of foundations.
- Exclude light completely. An opaque tank, or one kept in shade, does not grow algae. A translucent container in sunlight becomes green soup within weeks.
For everything that is not drinking, and that is most household water by volume, roof water needs none of this. Flushing toilets, washing clothes, watering the garden and cleaning floors are all perfectly served by water straight from the butt. Separating those uses from drinking water is what makes a modest store last.
Keeping it clean
Twice a year, and after any storm:
- Clear the gutter screens and check the diverter chamber drains.
- Look into the tank. If you can see a sediment layer building, it is time to drain and rinse it.
- Check the overflow path is not blocked.
Once a year, drain the tank fully, scrub the inside with a stiff brush, rinse and refill. If the tank has been standing stagnant, or anything has got into it, shock-chlorinate it rather than hoping.
Mosquito control is the one that catches people out in a warm summer: any standing water open to the air will breed them. A sealed lid and a fine mesh over every opening, including the overflow, solves it permanently.