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Rendered Dome Finish - Waterproofing - Forno Bravo Forum: The Wood-Fired Oven Community

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Rendered Dome Finish - Waterproofing

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  • Rendered Dome Finish - Waterproofing

    Hi All

    Looking for some advice on the finish for my dome. I have insulated the dome with vermiculite/cement 10:1 and a single coat (so far) of 3:1 standard plaster sand/cement render. I have another coat of render and then a final finish coat which I was planning to colour using oxide powder.

    My question is whether a standard 3:1 render will provide adequate protection from the elements during the winter months (Adelaide Hills, South Australia). I don't want water getting through and soaking the vermiculite between the dome and brick outer skin, or soaking the render enough for it to steam up and crack during firings,

    Has anyone used any of the waterproofing additives around, or painted the rendered dome? Still early stages of curing for me, so don't know how hot the outer skin is going to get yet.

    Appreciate any advice.
    Attached Files
    ------
    Poops

    Currently building first oven with red brick base
    FB plans plus Russell Jeavons book

  • #2
    Re: Rendered Dome Finish - Waterproofing

    HI
    It looks like the way the oven is at the moment that it will catch all the water, you still have to do some work I take it?

    The waterproof additives work just fine, but you wont be able to paint the dome with water based paint after if you use it.
    The English language was invented by people who couldnt spell.

    My Build.

    Books.

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    • #3
      Re: Rendered Dome Finish - Waterproofing

      That's right, still some work to go. The photo is not the best angle, but the brick skin has been filled with vermiculite around the dome to within about an inch of the top of the last course. The first coat of render is flush with the top course, and was a rough coat to hold the vermiculte insulation onto the dome.
      I still need to finish the render, which will direct any rain down the sides (hopefully ....)
      ------
      Poops

      Currently building first oven with red brick base
      FB plans plus Russell Jeavons book

      Comment


      • #4
        Re: Rendered Dome Finish - Waterproofing

        I am at about the same spot as you and plan on asking the same question for my roof. I was thinking of using an acrylic modifier (or they call it white glue). I used this stuff for a skim coat to fill the holes on my concrete counter. But I don't know if that is what one is supposed to use. I will look forward for someone else to jump in.

        Mike

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        • #5
          Re: Rendered Dome Finish - Waterproofing

          Hi mate , It looks like your building an enclosure? or are you going half brick dome top which would be pretty unique. Have you covered the bricks in ceramic blanket before covering with vermiculite?.
          As Far as waterproofing render I would use Bondal Silasec Ive used it to build ponds for years and it works well as a waterproof render
          cheers James

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          • #6
            Re: Rendered Dome Finish - Waterproofing

            If you add a waterproofing agent to the outer shell you risk locking in the moisture as well as keeping it out. This can result in the steam creating pressure in the insulating area and cracking the outer shell. I prefer to dry the whole thing out, about 10 decent firings, then add an acrylic elastomeric product, which can handle the expansion and also make it waterproof. you can paint over this in any acrylic exterior paint.
            Kindled with zeal and fired with passion.

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            • #7
              Re: Rendered Dome Finish - Waterproofing

              The outer layer of my dome is clay brick. I picked up a brick sealer from the company that sold me the refractory brick. I sealed the dome after firing it a number of times. The sealer is solvent based so I needed it to dry a couple of days before firing it... the water now beads on the bricks. Some sealers for natural stone may also work because on the label they say they can be used for brick and mortar. I'd pick up the sealers from an independent stone/slate tile company. Big box stores have the cheaper/poorer sealer products.

              Has anyone any tips for protecting a brick dome for winter.... ie a winter with lots of snow and cold temps?

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              • #8
                Re: Rendered Dome Finish - Waterproofing

                Hi All

                Yes the pic is as high as I'm going with the brickwork, with the top of the dome exposed. The rendered top has the profile of a bell really, with any water then directed over the sides of the brickwork.

                The dome itself is insulated only with vermiculte (mixed about 12:1 with cement) over the top of about a 1" thick fireclay render (hope that will be okay!)

                I've now completed all but the final coat of render, which I was planning to colour with brown oxide power. I've had four curing fires so far with another couple to go before the final render coat. Hopefully most of the moisture will be out by the weekend.

                I have some of this stuff which I was planning to add to the render, although I'm not entirely clear on the ratio to use added to a 3.5:1 sand cement mix

                http://www.lanko.com.au/lanko/images...rdatasheet.pdf

                I'm not used to mixing by weight ..... (any advice is appreciated).

                Will post some updated images after the weekend (flue going on this weekend too).

                Getting close now!
                ------
                Poops

                Currently building first oven with red brick base
                FB plans plus Russell Jeavons book

                Comment


                • #9
                  Re: Rendered Dome Finish - Waterproofing

                  Hi Poops,
                  I think you should cover the finished render coat for a week to keep the moisture in it so that it cures nice and strong. Probably then wait a few more days for it to dry out before firing again.you may also want to consider adding lime to the render mix as it increases flexibility. A render with cement and sand only is more brittle and does not have any flexibility.
                  Last edited by david s; 09-05-2011, 01:31 AM.
                  Kindled with zeal and fired with passion.

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                  • #10
                    Re: Rendered Dome Finish - Waterproofing

                    Hi poops
                    My dome was sealed with a coloured render finish....no problems there.
                    The brick entrance however was not and in the wet summer was to porus for the conditions finally used a "bondall" sealer which works well....you can see the water "bead" on the surface. It darking the colour only a little bit and doesn't make the surface glossy

                    Regards Dave
                    Measure twice
                    Cut once
                    Fit in position with largest hammer

                    My Build
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                    • #11
                      Re: Rendered Dome Finish - Waterproofing

                      Hi David S

                      I am in the process of building my oven and I was intrerested in your comment that adding lime to a render increases its flexibility.
                      What ratios of lime to other components would you use?

                      Also I have been told that adding fireclay to a mortar mix also increases flexibility of the mortar. Do you have any experience with this also?

                      Look forward to hearing from you.

                      Comment


                      • #12
                        Re: Rendered Dome Finish - Waterproofing

                        Originally posted by david s View Post
                        Hi Poops,
                        I think you should cover the finished render coat for a week to keep the moisture in it so that it cures nice and strong. Probably then wait a few more days for it to dry out before firing again.you may also want to consider adding lime to the render mix as it increases flexibility. A render with cement and sand only is more brittle and does not have any flexibility.
                        Hi David S

                        I am in the process of building my oven and I was intrerested in your comment that adding lime to a render increases its flexibility.
                        What ratios of lime to other components would you use?

                        Also I have been told that adding fireclay to a mortar mix also increases flexibility of the mortar. Do you have any experience with this also?

                        Look forward to hearing from you.

                        Comment


                        • #13
                          Re: Rendered Dome Finish - Waterproofing

                          I use 4:1:1 silica sand,cement, lime in one coat about 15mm thick. If you do more than one coat each subsequent coat should contain more lime than the previous one. Not sure about the addition of fireclay. It may have a similar effect to that of lime, but it may also create problems re shrinkage as clay always shrinks on drying.
                          Last edited by david s; 09-10-2011, 04:24 PM.
                          Kindled with zeal and fired with passion.

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                          • #14
                            Re: Rendered Dome Finish - Waterproofing

                            Hi David,

                            Thanks for the advice.

                            Your thoughts on using clay makes sense.

                            Considering you have experience with the mixture you suggest, I thinks I will go with that.

                            Also as I intend applying a number of layers, would increasing the lime by say 20% each time, be too much?

                            Cheers

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                            • #15
                              Re: Rendered Dome Finish - Waterproofing

                              I think more like 10%
                              Kindled with zeal and fired with passion.

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