Dry laid slabs. Fixing the wobble?
Discussion
I've got a typical new-build patio of naff 18" square concrete slabs laid on what appears to be sand. There's no mortar, at least.
I'll be thinking of replacing the lot in a few years with proper flagstones and mortar, but in the meantime I'd like to sort out the wobbly ones. I estimate about 25% rock slightly when walked on, which is no big deal, but a little annoying.
Should I lift them, backfill with more sand and hammer them down? Is there a better way? Should I leave as-is?
The sub-base appears to be sound, incidentally. The patio's been in five years now and there's not an iota of sinkage. It's all perfectly level. When it comes to replacing, I imagine I'll just remove the slabs and the sand, wacker down the sub-base again and then mortar in my new flagtones?
I'll be thinking of replacing the lot in a few years with proper flagstones and mortar, but in the meantime I'd like to sort out the wobbly ones. I estimate about 25% rock slightly when walked on, which is no big deal, but a little annoying.
Should I lift them, backfill with more sand and hammer them down? Is there a better way? Should I leave as-is?
The sub-base appears to be sound, incidentally. The patio's been in five years now and there's not an iota of sinkage. It's all perfectly level. When it comes to replacing, I imagine I'll just remove the slabs and the sand, wacker down the sub-base again and then mortar in my new flagtones?
LR90 said:
Should I lift them, backfill with more sand and hammer them down?
If the aim is just to fix what you have then this is how you do it. Not really any better ways to achieve it, unless you feel like experimenting with foam injection slab levelling & stabilisation? (Yes this is a real thing, but not a serious suggestion for your paving!)I await other suggestions involving massive reconstruction producing a result that could double as tank parking.
My slabs are mix of 3x2 and 2x2 are layed on sharp sand. None are wobbly.
I put a bed of sand down. Firm if by walking over it back and forth on my heels. Then I use a bit of wood just under 2ft in lengthnnand draw it across the sand to get a level bed to lay the slab.
Lay slab. Put straight edge over the slab and surrounding slabs to check for level. Once I know whether the slab is too high or low etc I lift it, add or remove sand then try again.
When I was laying slabs row by row I made a tool. a 5ft length of wood with a 2" piece attached at one end. Then I put sand down slightly higher than the level I wanted.
So I could lay it tool on the slabs that were down and the 2" piece would give me a bed 2" lower than the existing slabs. Often the next row went down with no correction needed or at least minimal correction.
I should add that the bed of sand my patio was laid on was on top of a hard clay subsoil. No hardcore used. Obviously if softer ground conditions sand on it's own is not enough.
I put a bed of sand down. Firm if by walking over it back and forth on my heels. Then I use a bit of wood just under 2ft in lengthnnand draw it across the sand to get a level bed to lay the slab.
Lay slab. Put straight edge over the slab and surrounding slabs to check for level. Once I know whether the slab is too high or low etc I lift it, add or remove sand then try again.
When I was laying slabs row by row I made a tool. a 5ft length of wood with a 2" piece attached at one end. Then I put sand down slightly higher than the level I wanted.
So I could lay it tool on the slabs that were down and the 2" piece would give me a bed 2" lower than the existing slabs. Often the next row went down with no correction needed or at least minimal correction.
I should add that the bed of sand my patio was laid on was on top of a hard clay subsoil. No hardcore used. Obviously if softer ground conditions sand on it's own is not enough.
LR90 said:
Should I lift them, backfill with more sand and hammer them down? Is there a better way?
It's worth mixing a little bit of cement in with the sand to make it firmer. Be carefull hitting the flags - those 450 ones are usually very susceptible to cracking. As well as helping to prevent ants, the cement means that you don't have to hit the flags quite as hard - use some wood and a rubber mallet if possible.If it's just sand without any hardcore base then you get this happening from the worms burrowing around underneath which over time creates voids and your slabs tilt. They can be fine for months or even years without issue. That's a big part of the reason why you need a decent depth of hardcore. Lifting them to add more sand and them cement them in is only a short-term fix as you're not addressing the underlying issue.
What's under the sand might be relevant in the long term, but maybe less so if the long term goal is removal!
If it was a few one off slab, I would be look at lifting and reseating them individually, but if it is 25" I do wonder if it wouldn't be easier just to relay the lot.
Either lifting the lot carefully keeping them in the right order, leveling out the sand, and relaying, or lifting say 8ft at one end and doing a rolling relaying process across to the other end if its a much larger area.
If it was a few one off slab, I would be look at lifting and reseating them individually, but if it is 25" I do wonder if it wouldn't be easier just to relay the lot.
Either lifting the lot carefully keeping them in the right order, leveling out the sand, and relaying, or lifting say 8ft at one end and doing a rolling relaying process across to the other end if its a much larger area.
Tisy said:
If it's just sand without any hardcore base then you get this happening from the worms burrowing around underneath which over time creates voids and your slabs tilt. They can be fine for months or even years without issue. That's a big part of the reason why you need a decent depth of hardcore. Lifting them to add more sand and them cement them in is only a short-term fix as you're not addressing the underlying issue.
Depends on the underlying ground. No worms under my patio when I relaid it this year. Only relaid to get rid of an awkward height step. Ground is hard clay/stones. It was good for over 25 years without worm voids.As an aside relaying nearly 70 slabs and adding 3" of sand over the whole area took me around 20 hours over a few days. And gave me a new respect for anyone who earns a living doing this stuff full time. Hard work.
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