Advice on indian stone patio

craigwright

Hello,

We have recently had a gazebo fitted with an indian stone patio for the floor. Today we noticed something weird - there is a wet, regular pattern on some of the stones, but they are in part of the gazebo that doesn't get wet. I'm worried that the wet is coming up from the ground and something is not right with how the builder laid them. (I'm not convinced they are of the best quality either). I have attached a photo, so hopefully someone can put my mind at rest or tell me what's wrong.

B3

It looks like where cement was dolloped before the slabs were laid.

Lizzie27

I would expect that they will dry out soon, have you had a lot rain since they were laid?

plant pauper

If it doesn't have a DPC membrane, the dollops of adhesive will be wicking damp up from the ground surely...? No?

Wayside

People like to dollop mortar, but I'd have thought you'd need to lay these on a bed, as usually this type of sandstone is quite thin.  It's a bit of an odd pattern, you'd expect corners and middle if they were dollops.

craigwright

Thanks everyone. Yes, I thought it might be cement, so that could be it. But is it a problem? Will it go away over time? 

They used some cement mixed with something else to aid drying. I don't know much else about it.

We have had a lot of rain in the past few days, but this has to be coming from underneath, because this area of the patio is completely dry otherwise. The marks weren't there before it rained and I'm just a bit worried that something's not right underneath.

Nollie

It looks like your builder has used an inappropriate but all too common technique - spot adhesion or dolloping should never be used with any natural stone - you need a full wet mortar bed application. I always use flexible adhesive and back-butter the slabs because it creates a good bond and a cushioned surface to absorb the knocks and footfall.

Dolloping creates an unstable base for what is often a fragile material. Quality, calibration and grading of Indian SS varies enormously and tbh yours doesn’t look like it’s top grade - I can see areas that are likely to flake off or ‘delaminate’ which is common in the seconds and even lower seconds reject grades.

Dolloping also creates voids for moisture to collect and the dollops not only wick up moisture, but fine cement particulates and are likely to permanently stain - no amount of scrubbing (don’t use acid cleaner whatever your do!) will remove them if this happens as they are right through the stone.

Fire

@Nollie - now here is someone who knows her paving techniques.

plant pauper

Yip!
What @Nollie said. 

craigwright

So what should I do about it? I'm getting the feeling that the whole lot needs to come up and be replaced - the builder supplied the stones, so it wasn't like we chose them. 

Nollie

Hi, Craig, There is a outside chance that whatever he mixed with the concrete mortar has bonded the particulates sufficiently that you may not get permanant marks via migration to the surface, or that the migrating mineral particulates will dissipate over time, this does happen sometimes, if you are lucky. Without knowing more about how it was laid and the materials used it’s tricky to know.

Has the stone been sealed and did he use a penetrating breathable sealant as he should have done? If so, you could try scrubbing with a powerful, alkaline sandstone cleaner - I would recommend Lithofin products for cleaning and then resealing after a good drying out period - minimum two weeks - but there are other brands. If he has used a surface, impenetrable sealant this is more difficult.

If the above doesn’t work, the only other solution is to remove the floor and relay new stone properly, using the correct methods. The only advantage of the incorrect spot fixing is that it’s easier to remove! I see a barney with the builder coming, he will probably deny it’s his fault, say he has always done it that way, never had any problems blah blah. So you need to arm yourself with all the facts from reputable sources - building, stone federations, pavingexpert.com etc.  There are lots of examples online of the effects of spot fixing on porous natural stone.

I would also advise anyone not to rely on their builder to choose and supply natural stone because you don’t know what you are getting and they will always cut corners, but what’s done is done. 

Hope that helps, good luck!