Cleaning a plastic corrugated sheet roof

[WendyS]

We have a garden storage area along the side of our house which is covered with plastic corrugated sheeting. Over the years it has become coated with dirt & lichen & we want to clean it. Any ideas how we can do this, bearing in mind that the guttering into which drains it drains sources into our water butts which provide water for the garden, so we can't use any chemicals. Also, we're in a hosepipe ban area so hosing it with a broom isn't an option.

[Ethel]

Move the water butts if possible so the water is not contaminated.

Use a bowl(s) of hot soapy water - washing up liquid will do - and scrub vigorously with a stiff broom.

It's the only way - and may take several goes.