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I'm building an outdoor 'workbench' for tabletop BBQing / pizza oven / gardening. The countertop will be tiled with 6mm thick, 165mm square porcelain tiles using a flexible exterior-suited adhesive.

Do I need to support this with 18mm plywood PLUS 12mm cement backer board (eg Hardiebacker 500), or would 12mm cement backer board over a wooden frame provide sufficient strength and rigidity on its own? Or should I just use the 18mm plywood?

In an ideal world, I would like to be able to - very occasionally - remove the countertop so that I can maintain the rest of the structure and move it to get to the areas behind and underneath (it's on a patio). Not frequently, but I'd like it to be possible. The countertop is 2.4m x 0.8m. My tiles weigh around 30kg. The plywood weighs 20kg. My assumption is that even if I fix the plywood over a baton structure, it would flex too much when lifted.

If I add 12mm concrete backer board this adds a further 10kg or so. Will an under-framed (2by1), edged, ply-cementboard-tiles structure be sufficiently rigid that the countertop can be carefully lifted and moved on its own? Would I be better off with just frame-edge-cementboard-tiles and skip the ply?

My ply is 18mm class 3, but not marine ply. I will seal it, especially the edges. I live in Scotland so we have -20C to +30C and a lot of rain. The tabletop BBQ / pizza oven will likely cause the highest temperatures.

The countertop will also have a one-tile-high splash-back. I can make this part of the countertop piece (adding weight, but also rigidity along that edge) or I can construct them as separate pieces and keep them unjoined.

The rest of the structure is conventional stud-work in 5by2 C16 timber, painted.

If I can skip the ply and lose the extra 20kg that would be great. And if there's no way it's going to be viable to remove the top without popping tiles, that would be useful to know.

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    Just a comment the only difference in marine plywood is the quality of the veneer (smaller tighter knots) having worked in a ply wood plant that was the difference same glue. You do need strength plywood with backer then tile will be your best option. I use thinset to help “glue” the backer to the plywood then screw together after that then set to glue the tile down this will make a long lived surface once sealed in a shower or an outside surface. I don’t know about where you live but in the Pacific Northwest it rains more often than most locations in the world but not as often as my shower. – Ed Beal Aug 22 '20 at 0:25
  • :) it probably rains for longer outdoors though. – Jasen Aug 22 '20 at 2:11
  • Thanks Ed & Jasen - yes, it rarely rains-then-freezes in my bathroom either, but I’m overall reassured by the fact that it’s used in showers. I also realised that unlike in a bathroom or kitchen, failure will only result in me needing to redo the countertop - nothing important will be damaged. – Lindz Aug 22 '20 at 7:58

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