Chinese granite paving - Dampness

[Harry Boy]

I have had a chinese granite patio laid. The slabs are 1000x500. Unfortunately due to the weather the slabs were laid in January but the grouting could only be completed in May. I have been advised that I should seal the flags with a proprietary sealer but at the moment, despite a week of dry weather, the flags are still indicating that there is some damp particularly round the edges.
Two questions a. is the damp merely in the flag
b. what would the effect be if we sealed the flags as they are?
I cannot imagine that, with intermittant rain during the summer, the flags will ever register totally dry

[rumpelstiltskin]

What did you lay the slabs on?? I.e is there a lot of moisture coming up from underneath??
It could also mean that the grouting (What did you use?) is more pores than the slabs, so the grouting holds the moisture and slowly releases it into you unsealed slabs?



Let me know

[Harry Boy]

The flags were laid on sand and cement on top of a full bitumen roof.
Before grouting the gaps etc were dried out by the use of an external vacuum device.

The grouting was I assume the same mixture. The grouting very probably is more porous than the flags s I have tried treating it with Hydrorep penetrating sealer, which is what was recommended for the final finish.

[rumpelstiltskin]

The flags were laid on sand and cement on top of a full bitumen roof.
Before grouting the gaps etc were dried out by the use of an external vacuum device.

The grouting was I assume the same mixture. The grouting very probably is more porous than the flags s I have tried treating it with Hydrorep penetrating sealer, which is what was recommended for the final finish.

Hi Harry

Harry, just off the top of my head and without having had the chance top look at the job, I am guessing

[a] There is water still trapped underneath the flags. External vacuum device???

Using this device, implies that there was water underneath already....so perhaps in the small recess and places were this device could not create negative air pressure to suck up the water, the water is still in there.

[c] Are the flags lower than the height of the surrounding bitumen roof edges??
Because if they are, which I am sure they must be, then perhaps as it rains water is finding its way into unsealed areas i.e. [where the outer edges of the patio meets the inner edge of the bitumen roof] and finding the hollow gaps underneath the flags and cement base, water is then being absorbed by the patio, and or being sucked in via capillary action.

[d] In a normal situation Water would drain into the ground, but since the bitumen roof is water proof moister and water has no where to go but up.



Perhaps get a pro in for a second opinion....not the same peole who laid them!!!

I hope I have helped shed spme light on the matter and not complicated things...

[Harry Boy]

Hi rumpelstiltskin,

thanks for your comments. i think your analysis is probably correct and there is still some water leaching out.

My only concern now is whether we will be able to seal the granite to protect against stains etc, particularly as we are very near the sea.

Harry

[Tony McC]

I'll bet these are G603 silver-grey chinese granite. Bloody notorious for wicking moisture from the sub-layers to the surface.

[Harry Boy]

Thanks again for the advice.

I don't know the code number but they are silver grey and come from a Chinese supplier.

I will speak to my supplier on Monday to see if he can confirm the code.

In the meantime is there any potential problem with the wicking or will it eventually cease if the moisture underneath eventually gets taken?

Harry Boy

[Tony McC]

The problem of capillary wicking can be a serious problem for some sealants. Thomsons' Patio Ruiner, for example, goes milky if there is *any* moisture present when it is applied.

It is becoming standard practice for these susceptible flagstones to be sealed on the underside prior to laying. This is alleged to reduce the appearance of damp patches on the surface once laid, but I still have reservations. It does work in some cases, but I've also been shown instances where the flagger insists he slurry sealed the underside with PVA/cement, but the disfiguring damp has still appeared some weeks later.

I'm not sure if a post-installation sealant would be beneficial. My gut feeling would be that it is more likely to trap in any residual moisture. I'd be tempted to wait until we've had a week or 10 days of warm, dry weather and hope that would be enough to evaporate most of the moisture.

[Harry Boy]

Thanks for that.

We had roughly 10 days of dry weather with only early morning dew and some of the flags did dry out others had dry centres.

I will do nothing until later in the year when maybe they will have dried further.

Meanwhile I will refer the supplier to this site so they can see what you are suggesting is good practice.

Harry boy

[Harry Boy]

Now that the weather has settled in to rain followed by hot sun we are noticing a smell from the flags. It not that pleasant and is definitely coming from the flags themselves. Has anyone any experience of this?

[Tony McC]

I wonder if it's a residue from the mandatory 'pest control' that is applied to all imported stone?

[Harry Boy]

can this be eliminated?