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Another patio one!

  • 21-06-2022 8:43pm
    Registered Users Posts: 3,337 ✭✭✭

    I followed the recent Patio thread with interest and there seemed to be a lot of knowledgeable posters :) so now I'm looking for them to cast an eye over this and advise!

    This is my patio, it was lovely when first laid but within a year some of the slabs had started to sink and it was unlevel with edges sticking up and sunken in other parts. This coincided with Covid hitting so the guy who did it wasn't working etc etc but eventually did come back and look, lifted the slabs saying he'd be back and hasn't been seen since! This was despite the 3 yr guarantee lol and before anyone asks this was a totally above board job, full email trail and bank transfers of all payments. I was very pleased with him and the job initially but he has been uncontactable since problem arose.

    Anyway I wanted to get it into some sort of usuable patio for the summer as it was totally useless last year with the slabs lifted but for various other reasons we weren't in a position to use it much anyway, now we want to use it!

    As you know it's impossible to get anyone to do anything these days and even then but I managed to get one person who said whole patio had to be taken up and bedded down again in concrete this time as he pointed out where it is sinking and rising in other parts. To be fair he's probably not wrong but this was an expensive garden revamp day one and the budget is just not there to spend big money redoing the patio totally.

    What is the fix for this? Is it concrete base? If so then I suppose it's pointless just getting the lifted slabs relaid somewhat level to make it useable for the time being at least. Thoughts and suggestions please :)


  • Registered Users Posts: 179 ✭✭bfclancy2

    looks like was laid on fine stone, probably not ideal for those slabs, and the stone wasn't restrained at the edges by a concrete haunch or kerb so it has shifted, quick solution to get you through until it can be relaid is set the slabs back down on a bed of mortar

  • Registered Users Posts: 46,063 ✭✭✭✭muffler

    I don't understand how or why the ground below the slabs moved like that.

  • Registered Users Posts: 3,337 ✭✭✭phormium

    Neither did the guy who put it down! He said he never saw it happen before which to be honest I might believe he hadn't seen it but doubt it has never happened, inclined to agree with first poster that a concrete edge all around under or restraining the slabs would have helped as it's obvious these days that the level is now falling away to back and side, the centre slabs look like they are rising up but it's more the back is falling. The other side is bounded by gravel area so there seems to be more support there and it's not obviously sinking.

    In hindsight this is up at the back of the garden in a raised area so a lot of soil underneath it and it's bounded by a large laurel hedge so I suppose the ground level is unstable in that I presume in dry weather the roots of the substantial hedge are going to suck up a lot of moisture from the ground so it could possibly expand and contract depending on weather or maybe that's a rubbish theory!

    It doesnt look as good as the last pic these days as when the slabs were stacked up then the weeds grew in spring in the bare patches totally covering them so had to lay cardboard and put back the slabs roughly to hold it down to smother the weeds so it's a mess at the moment.

  • Registered Users Posts: 31,067 ✭✭✭✭Lumen

    I'm guessing either the topsoil wasn't completely removed and has washed away under the stone, or maybe a tree was removed and the roots have rotted.

    I had subsidence in a corner of my garden after a large cordyline died. There was quite a large area affected due to the spread of the roots.

  • Registered Users Posts: 46,063 ✭✭✭✭muffler

    Yeah, tree and some hedge roots can cause movement in the soil but Im surprised at the depth of the ground at that point. Im wondering if that area was ever artificially raised previously

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  • Registered Users Posts: 3,337 ✭✭✭phormium

    The area in question is the same for many many years, no trees removed, hedge planted maybe 20 odd years ago. It's a very large site and stepped up to back garden which is quite long and this is at the very back/top of it, field on other side of hedge.

    Lot of topsoil removed but not sure how deep it goes as the soil there is lovely stuff for planting in, everything grows very well, former field itself!

    I couldn't see how the topsoil if some is left under it washed away anywhere, no where to go, it's nice and dry up there in general, water just soaks in nicely and doesn't pool around the place.

    Either way they are buckling and uneven and if the only solution is a concrete base then I assume it will need to be thick enough not to crack if anymore movement. Access isn't great either as when the job was done there was a sort of ramp built to allow small digger up there but then existing wall was raised and new steps put in so lugging up stuff/mixer to mix concrete won't be easy! Plus I don't want them to destroy the lovely path/steps/plants etc on the way up 🤔

  • Registered Users Posts: 4,660 ✭✭✭standardg60

    Sorry OP only seeing this now. The issue is that too much soil was removed and whatever was used to backfill has settled further over time.

    If the patio had been laid in concrete you might have gotten away with it as the concrete would then act as a raft and hold together, but as it was laid on crushed stone it will probably continue to sag and lift.

    The second landscaper is right as you know. Best scenario to have it usable is to relay what's been taken up in a 50mm bed of concrete and hope that keeps the rest in place.

    I know i bang on about it but there was really no need to excavate soil that's been in place for years, and this is a prime reason why.

  • Registered Users Posts: 757 ✭✭✭C. Eastwood

    There is a need to remove the topsoil- in most areas down 150 mm deep as it contains vegetation and organic materials, which will decay and shrink down over time, when covered over. This will usually result in differential settlement.

  • Registered Users Posts: 4,660 ✭✭✭standardg60

    In this case i'd imagine a lot more than that was removed, coupled with the patio not being concreted down.

  • Registered Users Posts: 3,337 ✭✭✭phormium

    So all in agreement that concrete is way to go! Now that I don't trust anyone who seems to know what they are doing what sort of depth of concrete should we be looking at? Anything else I should be asking the guy about the job, want to be as sure as possible it doesn't split and crack and end up in same boat further down the road.

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  • Registered Users Posts: 4,660 ✭✭✭standardg60

    50mm should do the job, hopefully there shouldn't be any other major settlement, and as I've said the concrete will form a bond over any minor dips.

  • Registered Users Posts: 984 ✭✭✭Still stihl waters 3

    I still don't understand why lads are still laying slabs on sand or 5 mill grit, anytime I've done a patio I've dug it out and concrete it, 3 or 4 inches with mesh and a strong mix of mortir to lay the slab, if anyone wants it done any other I'll tell them I won't be back if it sinks, I'll whack it in stages and do it but for the sake if a few hundred euros more to do it right you'd be as well off to not do it at all or wait until you've the funds to do it right

  • Registered Users Posts: 31,067 ✭✭✭✭Lumen

    The idea being advanced here is that it's impossible for a base of properly compacted aggregate to be stable over time, and that a concrete slab (or at least cement binder) is needed.

    That seems ridiculous. We build motorways over layers of compacted stone, but that's insufficient for a few slabs?

  • Registered Users Posts: 3,337 ✭✭✭phormium

    Actually oddly enough I have two old patios and a large porch with slabs also, I notice the oldest patio which is here since 1991 has become more uneven over time and while still flattish unlike this new one there are definite variations in the edges of slabs that were not like that always.

    My porch which is surrounded by pillars/path so totally enclosed and roofed but is open is very level but yet there is one slab that has also come up slightly on one side, cannot think of any explanation for that, I have only noticed it this past year or so, wondering is there an ants nest or something down there!

    Third patio is very small, one of those interlocking circle type and was put down maybe 20 yrs ago, hilariously by myself and it's the flattest of the lot! Nothing has moved and all slabs remain level, I repointed them last year with some good stuff as the filling was never a great job but it's pretty perfect if I say so myself. It was on sand only as I knew no better, other one is I presume sand as well, it's definitely not concrete and the porch slabs are on sand definitely at one side as there unfortunately is a manhole under there that sometimes has to be accesssed, the rest might be on concrete, they have not budged other than one that has defintely risen up slightly recently.

    Talking about roads motorways in fairness probably have a serious layer of stone not to mind the top coat. Local ring road around here built few years back is constantly sinking in places, two repairs alone in this past year and there is another obvious dip there again in a different position. Every road in the country would be lovely and flat if they stayed the way they were day one but there is a fair old few dips and bumps when you drive around them!

  • Registered Users Posts: 4,660 ✭✭✭standardg60

    I'm certainly no expert on motorway construction but anytime i've seen a section of road that's been dug up for pipe laying etc. and then refilled it's always sunk and has to be relaid after a year or two

  • Registered Users Posts: 31,067 ✭✭✭✭Lumen

    But that's presumably because it's practically difficult to effectively compact the unbound backfill in a trench.