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Rocking slabs and problem pointing?! Advice sought...

Discussion in 'In the Garden' started by Grouseman, 9 Oct 2016.

  1. Grouseman

    Grouseman

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    Hi All. We've just had a new patio laid - 90m sq in total, so a bit of a beast. In general I'm happy but before I pay my builder I wanted to get some advise on a couple of niggles that are bothering me. I'm unsure whether to accept as being normal or ask him to re do.In a handful of areas, there are hairline cracks, between pointing and slab. Are these likely to be a problem in the future - weeds, widening due to ice expansion etc? Also some of the slabs don't appear to have bedded down properly and you can hear them moving when you walk on them or the kids ride their bikes over them. It is slight, as it is not necessary visible. Again, is likely to get worse over time? The slabs were laid on a flat mortar base. I've posted a handful of photos of the cracks below. Thanks in advance!

    File_000.jpeg File_004.jpeg File_002 2.jpeg File_001 2.jpeg File_003 2.jpeg
     
  2. r896neo

    r896neo

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    Be very wary here. The minute cracks are most likely caused by movement in the flags. If the flags are moving in any way they need to be re-laid. It will not 'settle' or bed in or be cured by repointing or any other reason the builder may come up with.

    I would lift one of the rocking ones and check the bedding. It should be a full mortar bed with no voids, not dots or dabs here and there.

    The only other cause of the minute cracking would be shrinkage if it was pointed on a very hot dry day. In which case it would need redone in affected areas. However I would strongly suspect movement.
     
  3. Grouseman

    Grouseman

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    Thanks r896neo! The below picture shows the base - is that a mortar base? If so, any idea as to what may be causing the movement? From memory the guy who laid it (who seemed quite meticulous) applied what looked like PVA to the underside of the slab and then put it in place but I must admit, prior to pointing there was quite a lot of rocking of the slabs. The application of the pointing does seem to have minimised that - rather than feel them moving, you can hear movement when you walk on them but from what you're saying, that may only be a superficial 'fix'?!

    IMG_2654.JPG
     
  4. r896neo

    r896neo

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    I'm surprised as the mention of him priming the slabs with pva (was probably sbr a waterproof equivalent) implies he is not just a cowboy however leaving numerous slabs rocking means he has either a terrible bedding technique or very dry bedding both of which imply he is not skilled or experienced?? Perhaps there was a labourer doing some of it here and there and laying poorly.

    As I said there is no cure to rocking slabs they must be lifted and re-laid. The pointing will slightly mask the problem but it is already deteriorating because of the underlying problem.

    Its concerning that someone carried on with pointing despite rocking flags as this shows either ignorance or an attempt to hide the problem.

    If this is widespread throughout then a total relay is the only solution. If its only a few here and there then relay the affected.

    As I said before its worth lifting one or two to check the bedding.
     
  5. Grouseman

    Grouseman

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    Thanks again r896neo! I wonder whether it was the dryness, as from memory it was a dry surface he was laying them onto. How wet should the base be? Like wet cement?
     
  6. r896neo

    r896neo

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    There is no absolute correct way, some people prefer a semi dry mix, personally I prefer a wet mix similar to a bricklaying consistancy to avoid the issues of poor adhesion.

    A dry mix doesn't spread when you tap a flag down onto it whereas a wet mix flows out as the pressure comes onto it providing a nice even bedding. A dry mix the bed must be bang on before laying and so takes a higher degree of skill.

    It just sounds like they have not been very well laid to be frank.
     
  7. WabbitPoo

    WabbitPoo

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    Pointing must never be used as an aid to sticking down slabs......This will need to be lifted and re-laid. Properly.
     
  8. Grouseman

    Grouseman

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    Thanks for the advice to date - much appreciated! Based on the above I'm now discussing a reduced fee with the builder. What would you suggest the cost to lift the slabs, lay a new wet base, re-lay the slabs and point would be? I appreciate costs vary across the country but I want to understand a ball park figure so that a fair amount can be agreed that will allow me to cover the costs of another contractor to do the work. Patio is 90m sq. Thanks in advance.
     
  9. r896neo

    r896neo

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    What did they do? Did they dig out and prepare the sub base? What was there before? Who paid for skips? If they screwed up laying it are you confident the sub base etc is done properly?

    Find out where they bought the stone and get a quote for how much it was. Then as an estimate for the laying and pointing including labour and materials I'd say around 1500-2000 depending on where you are.

    This does not include the cost of lifting and removing the old mortar which is a bit of an unknown time wise although should be very easy if they used a dry mix but breaking up and removing it all and stacking the flags will require a skip and probably between a day and 2 days for two men.

    You may also need to allow a couple of hundred for new flags for new cuts and breakages that cant be reused.
     
    Last edited: 14 Dec 2016
  10. DIYnot Local

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