Black algae spots on patio

[Janet Siddell]

We have numerous black spots, which I believe is algae, on our old patio. We have pressure washed it and used bleach but they will not budge. I have read that Jeyes fluid may do the trick. If anyone can make any recommendations I would be most grateful. Janet

[84-1093879891]

What paving has been used on the patio, Janet? Is it concrete flags or stone? If concrete, are they deco garden flags, or the tougher 'council' type flags used on public footpaths? If stone, do you know what type?

[Janet Siddell]

Tony, We inherited the patio which must be 20+ years old. It is definitely not stone and I would guess by looking at them they are more like the "council" type cheap slabs. Each slab is covered with spots that range from small up to the size of a 2p piece. Funnily enough there are no spots in less exposed parts of the patio, i.e. along the edge nearest to the house where there is a roof overhang. The black spots looks like dirt and "raised" but just won't budge. Janet

[84-1093879891]

Have you tried an acid-based brick/patio cleaner?

As these are older flags, and probably pressed concrete (from what you say) it's unlikely that the acid will do any real damage. You can get some from a decent Builders' Merchant. test it out on a discreet corner somewhere first, just to be sure it doesn't damage the flags, and then, if it's ok, try scrubbing it in with a wire brush on the algae spots - be sure to wear gloves and goggles, though!

[Janet Siddell]

Thanks Tony - I shall give it a go and report back with (hopefully!) progress.

[Janet Siddell]

We have tried the acid patio cleaner (hydrochloric acid)today and it works. However, its really hard work and is obviously lethal to work with so we're doing a few slabs at a time. Thanks again for the advice and at least now we don't have a patio that looks "deceased"! Jan

[84-1093879891]

Deceased or diseased?

It's good to hear it worked for you, Janet. Which 'brand' of patio cleaner did you use?

[Janet Siddell]

Its called "Ever Build High Strength Brick and Patio Cleaner" in a bright yellow 5 litre container. I bought it from a local builders merchants. Its pure hydrochloric acid and great value at only £4.30.

I did, of course, mean our patio looked diseased, but it did once look deceased too, but then you came up with this wonderful idea!!!

[84-1093879891]

I know the stuff, but it's not "pure" hydrochloric acid - it's around 5%, IIRC. "Pure" HCl would posion you when it was emptied out of the container! :o