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Green Building Bible, Fourth Edition
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    • CommentAuthorMarkyP
    • CommentTimeJul 18th 2016
    my extension slab was poured last week, the concrete was very late and I could see the team were rushing to finish and go home and the slab finish isn't great. There are some ridges in the slab where they rushed floating it so this has left me with some high spots/ridges. I'm putting 125mm PIR over slab, then 75mm screed on that. Before I lay the PIR should I address the slab surface? I need to get a long straight edge on it to see how high the ridges are, they look like they might be about 20mm or so (maybe more in one area). I'm not sure how flat the surface under the PIR needs to be prior to insulating.
    The issue is that your screed become effectively load-bearing, instead of load-transferring if it has got voids underneath.

    I might, if there are many voids, consider reinforcing mesh in the screed, but I am not a concrete expert.
    • CommentAuthorMarkyP
    • CommentTimeJul 18th 2016
    I was thinking of trying to address it before I insulate. Things I've thought of are to grind the ridges off (with a dust vac attached), or use some sharp sand to fill the lows as I lay the insulation, I could also use some smoothing compound to even things up. All of these are a chore, and would have been avoided had the guys spent an extra 20mins floating it properly.

    is there an accepted tolerance for a slab finish? I'm wondering if I should question it with the contractor. I dont expect concrete to be perfect, especially when the drawings show insulation and screed over, but you'd think it would be considered standard practice to leave a flat, smooth surface.
    • CommentAuthortorrent99
    • CommentTimeJul 18th 2016
    It'll be on the BBA cert, AFAIR for Celotex it's 5mm over 3m.
    • CommentAuthortony
    • CommentTimeJul 18th 2016
    Needs dressing with sand and cement dry mix will be OK hollows are not good as screede will crack, ridges cause hollows.
    • CommentAuthorMarkyP
    • CommentTimeJul 18th 2016
    what sort of mix would you use Tony? 5:1? when you say dry, you mean no added water at all? thanks
    • CommentAuthortony
    • CommentTimeJul 18th 2016
    May be wet or damp sand but no water is ok and easier.

    I sprinkle ant powder round the perimeter of all areas too
    Lime the cement quickly dissolves the Alu foil surrounding PUR allowing the Pentane gas to escape more quickly.
    PUR is 90% closed cell/10% open cell so water gets into the 10% open cells its effectiveness is reduced.
    So be sure you protect the PUR.
    • CommentAuthorDarylP
    • CommentTimeJul 19th 2016
    ...avoid voids under the PIR wherever possible, if you don't want to risk cracking the screed.
    As VH said, the cement in the concrete is caustic, will dissolve the foil-faced Cx PIR....so protect the Cx (if it is foil-faced PIR?) and give the PIR boards a constant bearing, with no flex/bend possible.
    Depending on your agreement with the contractors, you may have some room to push for a reduction? Was there a tolerance in the description of the finished slab?:smile:
    • CommentAuthorMarkyP
    • CommentTimeJul 20th 2016
    thanks for the pointer on protecting the PIR. I am going to use foil backed boards so I'll lay some 500 gauge poly sheeting over the slab dressing mix before I put the PIR down. With Tony's dry mix suggestion I reckon it will only take me an hour or two to dress the slab before I insulate.

    One to chalk up to experience, next time I'll be watching more closely.

    thanks for all the replies.
    • CommentAuthorMarkyP
    • CommentTimeJul 3rd 2017 edited
    thought I'd update on what I did with this...

    I ground the high ridges down. I found a dust shroud for my 115mm grinder which could be attached to a vac, it was surprisingly effective, virtually no dust at all. And the grinder took off the narrow ridges with ease. Then I followed Tony's advice and used some dry mix to fill the worst of the hollows. I then sprayed down with a light mist of water and left for a couple of days, which stiffened the dry mix up and meant it didnt get moved around when laying the DPM prior to insulation.

    Having spent all this time getting the slab flat I then find that many of the sheets of PIR are slightly warped. I applied two offset layers of 70mm celetox but they were rocking all over the place, really not good. I read someone else having faced the same issue and who suggested relief cuts about half way through the problem PIR sheets on the top layer, along the axis of the wobble, this seemed to help. And while the sheets are still going up and down under foot as the UFH pipework is laid, I'm fairly confident the weight of screed will flatten them out. All the sheets can be pushed flat with a dab of the foot so I reckon 120kg per square meter of pumped wet screed will see it all settle nicely . I also read that two layers of PIR is better than one thick layer in terms of settling once the screed goes on.

    UFH is going down now and then the liquid screed is being pumped on Monday.
    • CommentAuthorbarney
    • CommentTimeJul 3rd 2017
    Seal all the gaps in the insulation with good quality tape before the screed arrives - otherwise the liquid screed will start to fill the voids and actually allow the sheets to float up - bringing the UFH with it unless you have it really well pinned to the insulation and the insulation isn't rocking

    It does happen, I had at least 4 school classrooms ripped up and re-laid as you could actually see the top surface of the UFH grinning through the screed - a bit of excavation showed why - curled up insulation basically

    As mentioned, a bit of reinforcing mesh tends to work wonders to keep it all down and allow the screed to actually be a screed


    • CommentAuthorcjard
    • CommentTimeJul 4th 2017 edited
    One to chalk up to experience, next time I'll be watching more closely.

    Next time buy self levelling concrete and you don't need to hire a team of lads to mess it up for you; you can level it on your own/with a trusted labourer!
    • CommentAuthorcjard
    • CommentTimeJul 4th 2017 edited
    Posted By: barneySeal all the gaps in the insulation with good quality tape before the screed arrives

    A plastic slip membrane (500 gauge sheet, available in 2m wide rolls - doesn't require unfolding, just put the roll down and kick it out to overlay) should be fitted and taped together before the screed is installed. Ufh should be fitted over the top of the membrane. Don't worry about the ufh clips puncturing the membrane. Fill the ufh system with water to test for leaks before the screed is poured. Don't allow the screed to come into contact with the alu foil on the celotex either.

    Markp, using multiple thin layers of insulation is the right idea, don't worry about the odd bit of wobble/flex. Under floor is a great place to use seconds quality insulation as we don't care if it's bent out of shape. Www.secondsandco.co.uk
    • CommentAuthorMarkyP
    • CommentTimeJul 4th 2017
    <blockquote><cite>Posted By: cjard</cite>Don't worry about the ufh clips puncturing the membrane. Fill the ufh system with water to test for leaks before the screed is poured.</blockquote>

    I've taped the insulation with foil tape, and applied a clear 500 guage seperating layer over the top. However, I noticed the rocking of the boards has already split the tape in many places with the plumbers clambering all over it while they laid the UFH pipe work. I'd read the UFH clips were self sealing but there are clear holes in the 500 guage where many of the the clips haven't gone in cleanly, I can't see how screed wont flow through these?
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