We recently had a patio completed, 30sqm. The slabs are 20-30mm, quite heavily rivened slate for what it's worth. Ranging across 3-4 sizes.
The Builders were brilliant with it, and when realising how poor the sub surface was ended up going as deep as 300mm in some places - 200mm throughout otherwise. This was all whacker plated down in layers, followed by a solid 40mm mortar bed which the slabs were laid on. I watched them do a good chunk of it so know the quality of the foundations.
Edging was done to a high standard and then we agreed to joint it with a Nexus Projoint V75 commercial grade/7.5tonne rated 2 part epoxy Resin - just because I like to 'over engineer' wherever I can.
All was well and true, until I walked across it in socks. Turns out one of the largest slabs - 900mm x 600mm has a hollow knocking noise from underneath an edge of it, there is also a very fine hairline crack between jointing compound & the slab edge approx 400mm long where this hollow noise is.
This knocking noise is only from approx 15% of the slab, the other 85% seems solid. There is no noticeable movement in the slab itself (that I can see or feel), just this knocking noise & hairline crack between it & jointing.
I have tried to pin down the builders over the last couple of weeks, but through the grape vine heard they went on holiday & have two full extensions on their books at the moment. I therefore have had no luck in getting them to discuss this with me - a real shame seeing how friendly & hard working they were.
Now I am torn between up the antee & either getting someone else to pop the slab up and relay it on a slurry (or try it myself without cracking the slab) and spending £60 for the jointing compound vs leaving it as is, trying to seal this hairline crack and see how it goes.
Any advice? I am particularly anal when it comes to things like this, so may be drastically over thinking it - i admit. Just looking for peace of mind from better educated folk?
They were very good when here, but after 2-3 weeks of chasing I think they have washed their hands of me. A shame as we had a good working relationship. I do know they have 2 big extensions on the go.
The slab is still solid, no movement (touch wood), and with the resin being £60 a bag + the risk of cracking the 20mm slab im not sure it's worth the risk?
Tony McC wrote:Hollow, echo-ey sounds when tapped usually indicate a void beneath the flagstone. These can be relatively small voids, but the flagstone acts like a drumskin and amplifies the sound, sometimes making it sound as though there might be a mineshaft under there!
Rocking is when the flagstone visibly moves, usually along the diagonal, from corner to corner, top right to bottom left; top left to bottom right.
Voids can occur even with a full bed when the bed preparation isn't quite as exacting as it could be, so would be less of a concern. Rocking is NEVER acceptable.
The trick of pouring in a slurry is fair enough, if it rectifies the problem, but it should never be used as an excuse to avoid re-laying a loose or rocking flagstone.
Anyone insisting on sealing paving before jointing in order to prevent staining occurring while they carry out the jointing really shouldn't be allowed to carry out the jointing as they obviously lack the necessary skills. A professional would be able to joint *any* paving, using *any* form of mortar, without causing significant staining - that's what makes them professional.
Thanks Tony, that's what I was hoping to hear. Any visible sign of movement and I'd be a lot more proactive on this front. I spent last night on hands & knees inspecting the edges & judging any risk of movement and I really couldn't see/feel anything beyond this tiny hairline crack between slab & jointing and this hollow noise from one end of it.
Inspecting the rest of the patio again on hands, knees & knocking to the point of nigh on bloody knuckles (Lord knows what the Neighbours thought...) reveals the rest of the patio is totally solid, so I can only hope & assume this is a small localised shrinkage of the mortar underneath.
I will give it until towards the end of summer, wash it all & inspect again then seal it with a good penetrative sealer (from this site & forum's recommendations) and see how it goes from there.
