Years ago we had a new patio laid in Indian Sandstone. This stone comes in slightly irregular shapes and sizes, nominally all the same but, just slightly out in length and width but, thickness in particular. Hand cut, this is to be expected. Also, one side is smooth the other side is a nice riven finish. Riven looks nice, but holds the water after rain and in the winter this freezes and the stone is slightly prone to little pieces breaking away. The smooth side would not have done this so much I expect and as a bonus, it would be nicer to walk on. Yes, riven can be a trip hazard especially for the old, infirm or, dare I say it, the slightly inebriated on a warm summers evening!
Over the years the only real work done has been twice annual cleaning with a pressure washer which makes a huge difference. Firstly in the Spring then again late Summer, also the pointing between the stones has had to be redone a few times over the fourteen years we have had it! However, Indian sandstone, like all other sandstone and limestone, suffers with moss and lichen and that is the main problem. The moss cleans off ok with the pressure washer but the Lichen! It just grows into black spots and nothing but a wire brush (impractical) will get rid of it and it spreads and spreads. This totally ruins the look of the patio. So much so, I was considering having it ripped up and replaced! The power of Google and the internet... I decided a few years back to really sort this out once and for all so we "pressure washed" the patio to see how well it had fared over the winter. The result, although impressive at removing surface grime, did not please us one bit. The black spots were every where!
The picture on the right shows the difference just by pressure washing and how bad the sandstone gets over winter!
General consensus of opinion on the internet is to use bleach, nice thick neat bleach and leave it to work for some time. Repeating as necessary every few days. The results were poor! After three lots of bleaching and scrubbing very little difference was noticed. Really bad areas, and the picture (right) shows a better area NOT a bad area, showed very little change. Stronger bleach was needed was my first thoughts... but where from?
More searching on the 'net bought up a suggestion. Sodium Hypochlorite, similar to what is used in swimming pools and by some farmers is readily available from Ebay but so expensive, luckily, I found a local farm supplies shop selling 25 litres of 15% hypo for about £18.00 so some was quickly purchased. The result was pretty amazing!
See the before and after pictures below and see what I mean.
The picture (left) is after pressure washing and one treatment with Sodium Hypochlorite. The lower picture is of the same slabs after a second treatment! I for one am suitably impressed! I am now treating every last stone for the second time. Some areas are superb others I am going to try a third session on them but if they do not improve then I will wait for a nice day, pressure wash a quick sweep of the patio, then once dry I will treat the whole patio with a sealing solution.
Now this post was originally posted April 2014 on a blog I had going back then. The good news is I now do this procedure only once a year and I can do my whole patio, and paths in about an hour and a half!! I have refined it to a very simple procedure.
So what's the procedure? It's not rocket science.
Lightly pressure wash the patio to get the worst of it off, don't spend long on it at all...
Let it almost dry and spray with a pump sprayer a neat solution of hypo all over...
Leave it to work for about an hour and a half. You may see it foam slightly?
Periodically dampen the solution with a fine mist from the hosepipe. This keeps it working and not drying out.
Wash the patio thoroughly with a hose. Remember this may well kill grass and plants so make it really wet to weaken the solution. I had little problem.
Take good care of yourself, wear suitable old clothing, gloves, goggles etc and keep the hose nearby in case you need to suddenly wash yourself down!
BE SAFE!
This method also works great on concrete/slabs/pavers and no doubt other materials but I suggest NOT on Tarmac but on all patios, do a test piece first for safety's sake! You have been warned...