Bathtub box

[Gib Bogle]

I've been making a box for a new bathtub in the redo of our bathroom. The tub is acrylic (where the water goes) wrapped in some sort of reinforced resin (presumably fibreglass). The rim wraps around, with a vertical edge about 3cm deep. Fools rush in, etc., and without seeking any tips online I decided to make a box out of 12 mm ply, with 4x2 in the corners and along the edges to prevent any bowing. The box came out quite nice, and is certainly rigid and strong enough.

The slight problems I'm now facing are that (1) the floor is not very even, so the box doesn't sit flush everywhere, and (2) the underside of the tub edge that meets the ply is also not very even (the fibreglass layer varies a bit in thickness. Yes, it's a Chinese tub, like virtually everything of this kind sold here.) The box, walls and floor will be tiled, by the way (by a very good Chinese tiler!).

I'm open to suggestions, but my current plan to make the tub a snug fit on the box is to epoxy some thin wooden strips that make a tight fit between the box side and the rim overhang in each place where there is some movement, at the same time as holding the tub down in that place with a heavy weight (there is some flexing). I haven't decided on the best approach for the floor fit, but something along the lines of a gap-filling adhesive (e.g. urethane) seems feasible.

[The Natural Philosopher]

1/. stiffen the tub with more fibreglass or car body filler and bits of wood,. 2/. seal all the gaps with silicone 3/. tile over everything to cover the gaps and grout the last few mm.

[Andrew Gabriel]

I can't picture what you've got well enough. The frame I constructed around the two exposed sides of my bath (also on 4x2) is not connected to the bath, but goes up behind the rim, so any splash-over can't run behind it. I expect a bath to move a bit with people and water, and not having to make a waterproof seal between moving parts seemed like a good thing.

[Nightjar]

There also ought to be a solid, usually thick plywood, base glassed in underneath, with feet that take most of the weight.

The sides that came with my bath were designed to leave a gap above the floor when they were fitted to the supporting framework. The gap was then filled by a separate piece fitted behind the main side, to accommodate uneven surfaces, although that was not a problem for me, as I replaced the floor as well.

My bath came with a wooden batten glassed in around under the rim, to give an even surface and something to screw the supports into.

Colin Bignell

[Tim Watts]

My acrylic bath has about 5 or 6 wooden blocks under the rim which are the official support points.

I made an open frame out of 2x2" nominal for the uprights and base. As the bath rim as not wide enough to take 2" depth plus ply + tiles, I used 1x2" on edge for the top rail and chamfered the 2x2" uprights down to 1x2" over the top 6".

I was able to trim each upright to be a tight fit - that frame is enough to take the weight of an adult and makes screwing (and removing) a bit of tiled ply easy.

Back to the OP's problem - if you have the ply, what I would do is to shim the ply you have with glazing shims (only because these are available in steps on 1mm). Or use frame packers - same again. When you find the right thickness, glue the shim onto the ply with some silicone or epoxy. You could use wood shim if you can be bothered to cut some.

[soup]

Really? I would have thought a silicone sealant would be better at the tub/tile interface. Grout dries solid whilst silicone cures flexible so will allow for any slight movement of the tub when being filled/emptied. grout will crack after a few filling/emptying cycles whilst silicone will tend to flex. So tile to tile grout as normal, tile to tub silicone.

[Gib Bogle]

There isn't.

[Gib Bogle]

Sounds like a good plan. I need to find out where I can buy the shims or packers. This scheme will not eliminate all movement, however. Conceivably there could be up to 1 mm of movement, e.g. when someone sits on the rim. Is it inadvisable to permanently connect the tub to the frame, e.g. with epoxy? Clearly this could create a problem in the future when someone (hopefully not me) wants to take it apart. If the shims are wood I could use a belt sander on them (after glueing) to improve the fit.

[Nightjar]

Unless it is remarkably solid (mine is 8mm acrylic and has a board with five feet) you are likely to find the bottom flexes when filled with water. It might well be worth also building something to provide support to the bottom of the bath.

Colin Bignell

[Gib Bogle]

I imagine that it will flex when full of water and a body or two. But should I care?

[Nightjar]

It makes it more difficult to keep a watertight seal around the edges and, personally, I don't like the feel of a bath that gives when I step into it.

Colin Bignell

[Tim Watts]

I would use silicone. ie silicone the packer to the underside of the bath rim and fit the ply. The packer will rest on the ply while the silicon sets but will not be bonded to it. Use double sided tape or a wek glue like UHU to hold the shim to the ply while the silicone goes off.

Silicone over a few cm2 gives plenty of support.

[Gib Bogle]

I take your point about the seal. I will explore glueing wooden blocks to the underside. Sizing them correctly will be a small challenge.

[Gib Bogle]

Thinking about your silicone suggestion ... I have a maximum gap of about 2 mm to handle. Perhaps silicone is all that I need. My thought is to apply a generous layer of silicone to the tub rim in the areas where there is movement, cover the box edge with cling-wrap in those places, and place the tub with some weight in it to ensure that it sits down fully. The silicone will fill any gaps, and will have sufficient area to prevent excessive pressure. This would meet the aim of making the tub removable, while avoiding the complications of spacers. Any problems with this plan? I understand that silicone comes in a variety of forms - which would be the most appropriate for this use, do you think?

[Tim Watts]

No - seems fine.

If you were going to be fussy, a low modulus (unsquishy) silicon - but I would use any *decent* grade, eg Siroflex.

[Geo]

I watched an american DIY video (Old House) where they layed a few inches of plaster (by the bucket) in the box and bedded the bath in that.