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Products: Primer to use before tiling over anhydrite screed

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  1.  
    • CommentAuthorRobinB
    • CommentTimeDec 30th 2009 edited
     
    I believe I need to do this priming but I've no idea how much of the stuff I'll need or what it will cost or the best place to get it. Has anyone used it? The only instructions I've found are to keep applying it until no more will sink in.

    I have another area which will have a 18mm wood floor laid over. I was thinking of waiting a few months and sticking it down- as the wood-flooring people suggest, however I've also been told it would be better to put down a DPM and float it over that and not worry about it drying out. I also saw cwatters suggesting a dry forumula for the screed - any idea on relative cost of wet and dry screed.

    All screed will have a low level of UFH.

    Sorry if this is a "how long is a piece of string" question, and thanks for any answers!

    RobinB
    • CommentAuthorowlman
    • CommentTimeJan 1st 2010
     
    Hi Robin,
    You could try using a decoupling matting, which is dry floated onto the screed before tiling. Or a product such as Schluter "DITRA" which is stuck to the floor and then tiled onto. These membranes are usually about 3mm thick. There are also some quite viscous acrylic primers available. In all cases, personally, I'd wait for it to dry out sufficiently first, especially the wood flooring, failure to do so is courting potential trouble. If finished floor heights are not a problem you could first glue/screw treated battens to the floor and then nail the wood flooring to the battens in the traditional way.

    Mike
    •  
      CommentAuthordjh
    • CommentTimeJan 4th 2010
     
    "nail the wood flooring to the battens in the traditional way"

    so they squeak, you mean? :bigsmile:
    • CommentAuthorRobinB
    • CommentTimeJan 4th 2010
     
    Thanks for comments re. decoupling matting.

    I wasn't planning to nail onto battens as I want the floor boards in direct contact with the screed to maximise heat output from the UFH set in the screed.

    RobinB
    • CommentAuthorowlman
    • CommentTimeJan 4th 2010
     
    I've never had that happen, djh. I use secret nailer, as used throughout N American Continent where they lay acres of the stuff that way. The benefit being, it allows the wood to adjust to seasonal humidity changes.

    Are you using an engineered flooring product Robin?

    Mike
    • CommentAuthorRobinB
    • CommentTimeJan 4th 2010
     
    Planning to use a solid oak which is being sold as OK over UFH and I'm planning to have extra spacing on the UFH so lower than average temp. Any sort of nailing over UFH makes me a bit nervous!
    • CommentAuthorJulian
    • CommentTimeJan 4th 2010
     
    Hi Mike
    how are you? If I recall Silka do an adhesive (possibly silicone type and I think in a skeleton gun cartridge) for wood flooring to go directly onto solid floors. Have you ever used that? Personally I am skeptical about sticking timber to solid floors and if space allows I would batten and secret nail as you say. If you can batten Robin the nails won't go near your UFH pipes and you hit them in on the diagonal through the tongue. For the tiled area Robin I would go for matting as Mike suggests. Tilers always say SBR for just about everything but I hate the stuff.
    • CommentAuthorowlman
    • CommentTimeJan 4th 2010
     
    Hi Julian,
    I'm fine thanks and a happy new year to you, I'm lugging wood by the barrowload up here in N'Yorks during this cold spell but the system is holding up.
    Robin, as a woodworker I do have a bit of an aversion to UFH and directly glued wood floors despite what the purveyors of the products say, call me old fashioned. As Julian said you don't nail into the floor but into the battens, - say 1"x3" or 22mm WBP ply ripped into strips, the nails don't penetrate to the screed. If you do glue however, personally I'd go for an engineered flooring, they vary in construction but usually something like a 12-15mm ply base with a 4-5mm hardwood veneer. The finished floor is indistinguishable from a solid floor and over the years will even take 2 or 3 sandings and refinishing. I believe they are more accomodating of the fluctuating underfloor temperatures. If you really want to be on the safe side though, keep the UFH temps low and think about flooring with boards that have microbevelled edges. These tend to visually hide the gaps that appear as the boards contract, but they can be notorious dirt traps. You could also backfill between the battens to add thermal mass perhaps covering everything with a membrane before finally nailing down the boards.
    • CommentAuthorRobinB
    • CommentTimeJan 5th 2010
     
    Mike and Julian very much for your helpful comments,
    • CommentAuthorTimber
    • CommentTimeJan 5th 2010
     
    Laying a wood floor over a damp screed/slab is really really bad news.

    The timber flooring needs to be conditioned to the in service moisture content so it doesn't move too much causing issue.

    If the screed is drying, you will have a high moisture content/RH and so the timber flooring, if installed at a lower moisture content will expand and perhaps cause 'humping' etc and then contract again once the screed drys down.

    If you install it at a higher moisture content it will contract when dry and pull away from the walls.

    Not going to work very well either way!

    Timber
    • CommentAuthorwomblepaul
    • CommentTimeJan 24th 2010
     
    Its my understanding from researching the same issue that if you use wood on an UFH screed you must make sure the timber is in direct contact with the screed. Any airspace will insulate and prevent the heat rising. I guess if you lay the flooring on battens and there is a small gap between each board it would be okay. But if the flooring is butted up tight and laid on battens I think you will have problems.

    Check with your UFH supplier.

    Paul

    PS there have been a thread or two on this matter

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