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Landscape Gardening

Patio damp looking patches before sealing

My patio is currently being laid and it is taking several weeks as the tradesman I am using is only here on weekends, I have noticed that even after very hot dry weather for weeks there appears to be damp patches in the areas that were laid just over a week ago, is it safe to seal them or should this be sorted out first? Could this be water coming up through the base (its mortar on top of hardcore, the ground was previously quite soggy in winter) or is it some kind of staining? The more recently laid part doesn’t seem to have this problem. Here’s two pics of the damp part and one of the good part

3 Answers from MyBuilder Landscape Gardeners

Best Answer

You have a problem. You have mint fossil paving or camel dust or similar which is a very soft sandstone. Your problem is that the tradesman has only spot bedded the slabs down, known as dob and dab or 5 spot method, this means that they have not been laid on a full mortar base just dabs of mortar here and there. The problem with this is that the slabs react to chemicals within the mortar and show up these patches as a full bed is not present so it shows up where he has only robbed and dabbed, a full mortar bed must always be laid or this will happen, especially with the lighter slabs. Sometimes it fades sometimes it does not. We always seal the under side of the slabs before laying to prevent efflorescence and the like. I would not seal the whole patio but do a test slab on one of the worst slabs and seal with a solvent based sealant like PRO-410 Paving Sealer, only do one slab and see how it reacts you may need 2 coats but it might blend in with the stain, just do one slab for now, it may not work but worth a try. As I said previous the staining can sometimes fade over summer hot sun helps, but to be honest it shoddy work from your tradesman who should know better if they were a half decent paver. Sorry I can't help further. Failing that he needs to relay the patio and seal beneath.
Good luck
Paul of Mulberry Cottage gardens.
Ps you can see in the first pic by the cement mixer that there are hollows under the middle of the slabs, he has only bedded the four corners and not the whole slab this is why you have this problem, its not damp, he is pulling a fast one... Sorry


Answered 25th Jul 2018

It seems that the problem you have is due to the cement underneath the slabs.
Firstly slabs should be under sealed to help with the bonding process and the slabs should be laid on a.full bed of mortar if not the mortar tends to show through as wet pacthes.


Answered 26th Jul 2018

Agreed above.Many times I’ve come across this problem.First point as said is the slabs haven’t been laid on a full bed of mix.Popular mistake with laying is a five spot method.Not ideal for laying slabs as ground moisture will show through uneven as you stated.Secondly slabs should be pre coated underneath with “sbr” or a slurry mix to help adhesion to the mortar bed.It depends on what type of slabs are laid.For instance,some slabs naturally stain with the ingredients they are made of.sometimes the minerals in the slabs can come through when laid.


Answered 27th Jul 2018

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