I have heard to use a high quality silicone between each piece of densshield around all joints and corners and also a dab over each penetration including screw heads, around shower niches, around the mixing valve, and around the shower head outlet. A true silicone never really hardens and this would form an elastic property when I tape and "mud" the joints, also per densshield directions.

Is this a concern? Obviously the silicon should be used with modesty just enough to seal the joints but I will be applying enough that I am comfortable it's watertight.

  • The silicon as a base for the tape and mud. It's not a solid surface like the rest of the densshield. I feel like the silicone would make the thinset bubble up and not BOND with the board in those areas. – Nic Jul 13 '18 at 16:07
  • I agree with isherwood's answer, quite a few years ago I had to clean up a mess where the home owner thought it would be a good idea to silicone the backer board, he did a fantastic job of laying the tile after grouting and starting to walk on it every or almost every tile broke loose and was only being held in my grout, ripped everything out including the backer, we were lucky I was able to use a large belt sander to strip the thinset off the tiles that were imported and redo the floor. You don't want much silicone as it will prevent the thinset from bonding.+ – Ed Beal Dec 13 '18 at 20:29

Your silicone seals should never be wider than about 1/2", and you should avoid smearing caulk onto the face of the backer board. Your mortar and tile will span those small gaps without an issue. Keep in mind that the boards should all be securely fastened to the framing such that there's no movement with respect to each other.

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  • The only spot I have wider than 1/2" is where the recessed niches butt against the densshield. I got a bit sloppy with the silicone here because I was trying to level it out flat. I may take some sand paper to it if it's too wide. So screw holes are probably just fine to silicone over...? I don't want to use an expensive membrane coating since I already spent extra for the densshield itself hoping that would save costs there. I imagine as long as I tape all the joints and mortar them, then when it comes to tile I will have waterproof (and protected) joints! Probably ceramic or porcelain. – Nic Jul 13 '18 at 20:41
  • You should use mesh tape and a water barrier like red guard to seal seams and corners, Mortar on the joints is not waterproof. i use it on the screw heads as well. – Alaska Man Dec 13 '18 at 21:19

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