North-facing stone patio - how can I remove green algae in its early stages?

Last year, from this north-facing stone patio, I removed several years' build-up of green algae and "white lichen circles" with a hired Karcher pressure washer.  It did a good job but took me a long time and used hundreds of gallons of paid-for water.
The green algae is now starting to come back, is there any less violent/wasteful way of killing it before it really builds up again, and, say, repeat this once every year?
Any advice would be much appreciated.
Mike

Posts

  • NollieNollie Girona, Catalunya, Northen SpainPosts: 2,181
    If its sandstone, limestone, york stone, marble, that kind of thing, a weak solution of ammonia and a good scrub with a stiff brush would work, but never any kind of acid cleaner/catch-all ‘stone cleaner’. If its that kind of stone, the surface is easily damaged by over-enthusiastic pressure washing, it breaks up the surface and creates microscopic pitting for the algae to make themselves at home in. Did you mean to include a photo? I can’t see one...
  • Mike160304Mike160304 Posts: 66
    Thanks for your comments.
    I don't know what kind of stone it is, it's a very hard natural stone but I guess that, with the naked eye, I can't see whether the pressure washing has damaged the surface - no obvious damage like you can get with softer mortar/concrete.
    Unfortunately, gentle pressure washing did not shift the heavy, multi-year algae build-up, I was obliged to have the nozzle down close to the surface.
    Thank you for the ammonia suggestion, but I guess I'd better ask around a bit more before I commit myself to attacking the stone, I think.
  • Lizzie27Lizzie27 Bath, SomersetPosts: 3,328
    I use Patio Magic or Algon for our sandstone paving. It's supposed to last for a few months but if it rains a lot, the algae come back. I find it's less hard work, I use a sprayer which works better than a watering can, but it is quite expensive. All round the house including the terraces and steps costs around £40 a go. 
  • madpenguinmadpenguin Isle of WightPosts: 1,518
    Oxygen bleach (laundry) and a stiff brush!
    “Every day is ordinary, until it isn't.” - Bernard Cornwell-Death of Kings
  • JennyJJennyJ DoncasterPosts: 1,369
    Whatever you decide to try, test it on a small inconspicuous area first to check for damage.
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