Sink shower try into the joists

24 Nov 2013
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United Kingdom
I'm installing a new shower and the ceiling height is very limited (2070mm from floorboard to celing) so was thinking I could gain 18mm of height by installing my low profile tray a bit like a wetroom tray (like in the video below). In brief, it removes existing floorboards around the tray, fixes batons to the existing joists 18mm below joist height then fixes 18mm plywood onto those batons flush with the joists.

I'm wondering is it too much work and risk for too little gain? One thing is that video doesn't explain how to support the tray at the edge where the edge runs parallel to the joists and inbetween them. Nor how the existing floorboards are supported at that point. I'm also thinking it would be difficult to screw noggins in at exactly 18mm below the joist so might end up with uneven floor? (but maybe the mortar would even that out?)

But those worries aside, is it something reasonable to do or would a pro never bother with anything like this? I would use 18mm WBP ply (not marine as in the video) so that _hopefully_ the subfloor would be flush with the existing joists. Then I was planning to simply add a layer of mortar as per my shower tray instructions with the tray on top of that.

Any comments on the detail of this or general ones about whether its a good or dumb idea and why would be most gratefully received !
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It's very doable with a bit of thought but to gain 18mm? I really don't think that is worth the trouble.
We do very similar with Bette pressed steel shower trays. Take out the floor then rebate the tray into the joists to allow the "pressed fall shape" to be level with finished tiled floor height.
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For your consideration - we have low profile shower trays like this at work, and they are a pain. They look good but the powerful showers quickly overwhelm them and before you know it water is flowing everywhere and flooding the block...

Moral.. make sure your drainage is top notch and can cope with the water volume .

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