Posted: Tue Jul 18, 2006 10:17 am
Dear All,
I don't know if anyone can offer me some advice? I have nearly completed laying a granite slab patio at the back of my house (~40m2). The slabs are ~3 cm thick (and a mixture of 0.6x0.6m and 0.3mx0.6m) light coloured granite layed with minimal gap directly onto sharp sand without any cement. My problem is that there are numerous darkish damp patches, some extending across multiple slabs. This is despite almost a week of continuous sun shine. My wife, not surprisingly, does not like the dark patches....I assume that water is being drawn-up through the granite slabs by capillary action, and that the sand is either wetter in some places than others, or certain sets of slabs make better contact with the sand.
The patio is just beneath a large metasequoia which has shallow roots which will no doubt lift parts of the patio over time - hence my desire to avoid concrete - I am resigned to correcting levels every few years. The patio also has minimal slope (about 1:200) away from the house. This slope matches an existing neighbouring flush concrete base of an old garage. The lack of concrete below the patio, and the minimal unfilled gaps between the slabs seemed like a good idea to prevent water from pooling on the patio - indeed, even during several heavy downpours we had no problem with drainage.
Is there anyway I can prevent this uneven coloration? I contemplated sealing the top of the slabs - which would also prevent rust discoloration - but, the slabs would end-up dark and the moisture would be trapped. I now think that sealing the underside and edges of the slabs might help. That way evaporation should lighten the slabs and water ingress from below would be reduced. I know it will mean lifting all the slabs, but.....Anyway, am I about to make another mistake? Should I use a PVA compound, an acrylic varnish, an oil - or even lay a plastic membrane directly beneath the slabs (I think that might cause a drainage problem).
Any advice would be most welcome.
Christof
I don't know if anyone can offer me some advice? I have nearly completed laying a granite slab patio at the back of my house (~40m2). The slabs are ~3 cm thick (and a mixture of 0.6x0.6m and 0.3mx0.6m) light coloured granite layed with minimal gap directly onto sharp sand without any cement. My problem is that there are numerous darkish damp patches, some extending across multiple slabs. This is despite almost a week of continuous sun shine. My wife, not surprisingly, does not like the dark patches....I assume that water is being drawn-up through the granite slabs by capillary action, and that the sand is either wetter in some places than others, or certain sets of slabs make better contact with the sand.
The patio is just beneath a large metasequoia which has shallow roots which will no doubt lift parts of the patio over time - hence my desire to avoid concrete - I am resigned to correcting levels every few years. The patio also has minimal slope (about 1:200) away from the house. This slope matches an existing neighbouring flush concrete base of an old garage. The lack of concrete below the patio, and the minimal unfilled gaps between the slabs seemed like a good idea to prevent water from pooling on the patio - indeed, even during several heavy downpours we had no problem with drainage.
Is there anyway I can prevent this uneven coloration? I contemplated sealing the top of the slabs - which would also prevent rust discoloration - but, the slabs would end-up dark and the moisture would be trapped. I now think that sealing the underside and edges of the slabs might help. That way evaporation should lighten the slabs and water ingress from below would be reduced. I know it will mean lifting all the slabs, but.....Anyway, am I about to make another mistake? Should I use a PVA compound, an acrylic varnish, an oil - or even lay a plastic membrane directly beneath the slabs (I think that might cause a drainage problem).
Any advice would be most welcome.
Christof