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Waterproofing Concrete Slab Roof

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  1. #1
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    Default Waterproofing Concrete Slab Roof

    I am building a garage and it has a concrete slab roof which I will tile over as it will become a verandah. Before I tile over the slab I would like to waterproof the slab. There seems to be a lot of waterproofers and penertrating sealers. What do people recommend that I waterproof/seal the slab with? (something readily avaliable from the local hardward?)

  2. #2
    Resigned SilentButDeadly's Avatar
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    Bondcrete.
    Joined RF in 2006...Resigned in 2020.

  3. #3
    1K Club Member Pulse's Avatar
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    You need a proper membrane, Davco K10 has good reports, make sure it is the same company as the glue you are using. Also the membrane thickness is important so make sure the amount you buy suits the coverage rate on the bucket. I've only ever used bondcrete to seal dusty surfaces, don't think it professes to be a waterproof membrane.

    Cheers
    Pulse

  4. #4
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    Hi,
    Step 1, clean and dust the slab.
    Step 2, Seal the slab.
    (People DO commonly use bondcrete for this. Bondcrete is a PVA based product, VERY similar to common PVA glue. Many builders use it for expressly this purpose. Having said that, PVA breaks down in water, and according to the manufacturer should not be used in wet areas. If you then waterproof over it, the PVA shouldnt get wet, so you shouldnt have a problem. Alternatively you could use another sealing agent, which isnt water soluable. I think you can use latex alternatives to boncrete, like lanolatex from davco, but many sealers would be fine also, as long as they dont chemically react with your final waterproofing. If you are concerned, then the supplier of the waterproofing, will also be able to sell you their recommended sealer. For what little its worth, I have used boncrete, lanolatex, polymer tile sealer, concrete sealer, and damn near anything else the boss found in his shed, to no ill effect 8 years down the track.
    Do you need a sealer on a new clean slab? I am not a waterproofer, but have done it dozens of times, and if a sealer isnt used, the air trapped in the concrete causes little moon craters on the waterproofing at first coat, and you have to be very careful to fill them all at second coat. Sealing it also makes the waterproofing faster to do for this reason.
    Finally ....
    Step 3.
    Waterproof the slab.
    If it is going to be tiled over, the product does not need to be trafficable or UV stable, and I regularly use Bostiks Ultraseal R, which is a 1 part, solvent based polyeurothane waterproofing, requiring 2 coats, checking for moon craters between and ensuring they are filled. The finished surface is quite strong, although not classified as trafficable and quite flexible.
    If it is not going to be tiled for a long while or at all, or you are going to put a floating deck over it, it will need to be UV stable and preferably light trafficable.
    I have used and am happy with a product called Tremco TP90 which is a 2 part epoxy waterproofing. It requires machine mixing, which is just a mixing rod on the end of sds drill. I just use one of those $80 ozito kanga drills from b@nnings. Although having to mix a product is a pain in the @ss it actually brushes and rolls on very easily, and because its a 2 part epoxy it dries quickly.
    Again, it must be at least 2 coats, but the finish is harder and fine to walk on, and is UV stable.

    Enquire from a waterproofing supplier, at least you know you will get the right product that will do what you need it to do. Or just ring someone like Davco, Tremco, or Bostik , talk to a rep, get their recommedations, and ask them where to buy it from locally.

    Cheers

  5. #5
    Stop Leak njsamson's Avatar
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    Default Waterproofing flat concrete rooftop

    Quote Originally Posted by Markt View Post
    I am building a garage and it has a concrete slab roof which I will tile over as it will become a verandah. Before I tile over the slab I would like to waterproof the slab. There seems to be a lot of waterproofers and penertrating sealers. What do people recommend that I waterproof/seal the slab with? (something readily avaliable from the local hardward?)

    The best available product for a flat concrete roof as described would be a Flexible EPDM Membrane. The best of those membranes on the market in Australia is the Prelasti EPDM which is made by Pirelli in Belgium.

    The Prelasti Membrane is one of the easiest waterproofing membranes to fit in the world and it comes with a 20 year warranty.

    The membrane comes to order in a single sheet 1.5mm thick to reduce any weak points in the membrane where you have to cut and reseal using other products.

    Once the membrane has been layed on the concrete and glued around the edges you can tile straight over it creating a complete seal.

    For more info on Prelasti waterprofing email leigh.weller@njsamson.com

    Cheers

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