Dreaded black mould on patio...

[MDMetal]

We've an indian sandstone patio (at least I'm fairly sure) I finally cleaned it so it's looking rather nice (well sorta).

Half the slabs have the dreaded black mould which refuses to shift even with the most powerful pressure washer. What's the best thing people have used to kill it? There seem to be a few products on the market but varying review of all of them.

I'm going to have to lift and relay the patio (lots of wonky slabs due to them being dotted on) so I want to ensure the slabs are perfectly clean before lifting and laying on a new mortar bed. Otherwise I'll get some patio slabs which aren't so easily mould afflicted.

The patio is about 15 years old, we moved in a year ago and doesn't seem it was cleaned regularly

[V10Mike]

Benzalkonium Chloride (BAC50). Available on Ebay. Kills mould, moss, lichen etc.

[Tango13]

www.mosskillers.co.uk

Not sure what their mould killer is like but the moss killer for the lawn worked very well indeed.

[CAPP0]

I have the same stone on my patio and get the same issue. I just use cheap bleach, eg Tesco Value, leave it down for half an hour and hose off.

[TimmyMallett]

Patio Magic.

[Bearings]

I've had good results from https://www.wetandforget.co.uk/ (not linked with them, just used their products).
It's more of a long term cleaner, instead of an instant thing.

[Fatball]

Bearings said:

I've had good results from https://www.wetandforget.co.uk/ (not linked with them, just used their products).
It's more of a long term cleaner, instead of an instant thing.

We have used the same with good results.

[MDMetal]

So I tried bleach as it was readily at hand, applied scrubbed several times and reapplied for about 30min....Zero effect, if the mould had a mouth it would have laughed at me. I e looked at patio magic before but it carefully lists green Algie/lichen which is the wimpy kid brother of black.

I guess I need to up my game. For avoidance of doubt it's this stuff...

[guindilias]

Never tried it myself, but my Mum has an Indian Sandstone patio and the guys who laid it (really well, a great job) recommended the wet and forget stuff - it's only been maybe 5 years, but no black spots - it certainly kills the green stuff easily, and if there is black stuff about it seems to have kept it away!
But, as always, your mileage may well differ.

[blueg33]

If you have pet cats be very wary with the chemicals the main ingredient is very toxic to cats. Patio magic for instance can be pretty unpleasant..

[blade7]

Sodium hypochlorite.

[anonymous-user]

Wet and Forget is brilliant stuff. After searching on here, I gave it a try on our render that had a bit of a green tinge on the north facing side. It looks brand new. I sprayed it on with a knapsack sprayer and left it. No brushing/hosing, etc. It took about five days!

[Kinkell]

Black spot is very difficult to remove and I've tried various solutions without success. Killing moss on monobloc driveway is also difficult long term so somebody inventing a remedy would be rich and famous if they so wanted.

[guindilias]

Sound like the cure for the common cold, or Coronavirus or something... Big fan of hypochlorite here, it kills everything including jeans - not sure how it does on sandstone though.

[Mack11]

I have just done this with PavKlean, you have to really scrub it in with a hard brush and leave it for as long as possible. It did work for me but I diluted it less than was recommended as they were stubborn stains.

[Candellara]

I have exactly the same problem (black spot on Indian sandstone) and i started a thread some weeks ago.

Tried neat bleach - doesn't touch it

Tried Patio Magic - doesn't touch it

Tried Wet & Forget - doesn't touch it

At a loss as to what will remove it (apart from removing the slabs and getting them blasted and then sealed)

No chemical seems to work on mine

[MJNewton]

Tony McCormack and his Paving Expert site is widely considered to be something of an authority regarding all-things paving. It'd be worth reading his page on the subject of lichens / black spot in case there's anything there not already covered.

[elanfan]

Iron sulphate kills moss algae and the like. Cheap as chips from a garden centre. Comes in a powder which you dilute. Spray or watering can it on. 24 hours later it’s dead.

[robwilk]

elanfan said:

Iron sulphate kills moss algae and the like. Cheap as chips from a garden centre. Comes in a powder which you dilute. Spray or watering can it on. 24 hours later it’s dead.

I would not try this on slabs it will leave rust marks on Indian sandstone I have them on my patio where the lawn weed and feed missed the grass at the edges.
I used Sodium Hypochlorite on my black spots and it removed 50% of them maybe if i had left it on the slab longer than overnight it might have got the rest.

Edited by robwilk on Saturday 20th June 16:24

[journeymanpro]

Do not use iron sulphate on hard standing. It will stain it rust coloured, possibly permanently.