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Frequently Asked Questions
How heavy is stored water?
A tonne per cubic metre.
A modest loft tank holds a quarter of that.
Concentrated on whatever it stands on.
And it arrives only when full.
So position is a structural question.
Why does stored water not keep?
Because conditions decide what grows.
Warmth and light encourage algae.
Sediment settles out.
And any disinfectant residual decays.
Until there is nothing left protecting it.
What does that imply for a tank?
That it is maintained, not just filled.
Sealed against ingress.
Screened against insects and vermin.
Kept dark and cool.
And turned over regularly.
Can rainwater ever be drinking water?
Not in these systems.
It arrives dirty off a roof.
And is stored without disinfection.
It serves lavatories, machines and irrigation.
Which is what it is designed for.
How are the two supplies kept apart?
By making contact physically impossible.
Not by valves that could fail.
An air gap between them.
Where mains water falls into open air.
Before entering the rainwater side.
Why is storage volume the central decision?
Because supply and demand are unrelated.
Rain arrives on its own schedule.
Demand follows the household's.
The tank bridges the two.
Bigger bridges longer dry spells.
Is mains backup an admission of failure?
No - it is part of the design.
No rainwater system covers every dry spell.
Without a tank nobody would pay for.
Backup lets the tank be a sensible size.
Rather than a worst-case one.
How much can a roof actually collect?
Roof area times rainfall, less losses.
Plan area, not the sloped surface.
Multiplied by annual rainfall depth.
Then reduced for what the roof absorbs.
And what the filters reject.
Does the roof covering matter?
It changes both yield and quality.
Smooth tile and metal shed most of it.
Rough or porous coverings absorb more.
And some treated surfaces shed chemicals.
Which rules them out for harvesting.
Where should a tank be sited?
Close to the downpipes it collects from.
On ground that will take the load.
With access for maintenance.
And a route for the overflow.
All four, not the first alone.