Slate tile outdoors

[SYNCMANATL]

Been asked to repair a black/grey slate, under a covered patio. The slate has a layer of "clay" on the surface, very easy to scrape off with a fingernail. I suspect it was never sealed. It makes "mud" when wet. Is it possible the acid wash maybe, and clean then seal? or is there a better option. The patio is upstairs, on regular deck framing, with a belly in the middle. But thats for another time.

 

[Dan]

I mean a good wash first would be wise acid might want etch a bit and if it does you'll never get it back a nice color.

Wet it clean it mop it up with a washboy, do it again until the water is clear.

Then whatever is left might need treatment individually in each area.

The belly in it must be something expanding under it and / or the actual substrate warping for some architectural reason. It might be alright and worth cleaning up but also bear in mind if you do all that and you find cracks and things then it ideally needs replacing.

But I think you knew that anyway.

Always test on a small area if you're going in with anything with a high (or low, alkaline - if i got that the right way around) PH level.

In your opinion, what's causing the belly mate?

 

[Dan]

P.s. how long has it been laid? And what sort of traffic is on it? I'm wondering whether 25 year old slate might have weathered (even under cover) a bit too much for acid. 40 year old even more so. Etc

 

[SYNCMANATL]

Its about 14 years old, the dip is from day one, just bad construction. Its a 20x20 corner of a rectangular house, under the roof. Exactly same area, below is the "basement" area porch. Pressure treated ply, over joists, 1/2" cement board. Only the homeowner and kids use it. Wife is a bit ADD and the paste irritates her, not so much the dip. Husband squeegees water out if rain makes it in. I HATE SLATE.

 

[Dan]

Haha fair enough mate. I wonder if just a really good (commercial) jet wash would clean it off, maybe a domestic one would be safer. Know if anyone has tried that on it?

 

[Ken Bruty]

If it were mine, the first thing I would do is use a mild alkaline cleaner such as LTP Grimex to remove that residue….slate is easy to clean and maintain with the correct regime, I’m opposite in that I love slate, fitted correctly and looked after it can be beautiful, the sad thing is most people aren’t advised correctly how to clean and maintain it making it a real pain to live with. Once you have a base sealer properly applied such as LTP Mattstone you can use a topical cleaner/sealer/colour enhancer to maintain it. Also I would never use acid to clean it as most slates are calciferous and react to acid.