Urban Rainwater harvesting.

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Exactly right Dev.

The calculation I made was based on 50% of the UK average monthly rainfall, so I captured some seasonality, although maybe not the reality of the exact microclimate of that part of London.

Using a catchment area of 50 sqm, and harvesting 50% of it, there would be just about enough to meet demand, without dipping into reserves, which would act as a buffer.

I also calculated on the basis of a frugal 5L per person per day. This equates to 930 L per month 6 people. If a buffer of 3,000 L is put in place, this will effectively give 3 months insurance against un expected drought.

Back to your point, that is precisely why local average rainfall needs to be determined, and also the degree of variation that can be expected too.

On paper it looks viable, volume-wise.

In addition to reservoirs, and ensuring that the roofing surfaces are free from toxic metals, urban preppers will also have to make provision for sterilising and filtering their harvested water before considering it drinkable.