A real DIY question

I doubt if any sealing method is going to survive for long if the flex is to large.

Cannot you not limit the amount of flex by supporting the bottom of the bath at the ends with blocks of wood and or a square wood frame at the edge (non-tap end) or a wood T support at the tap end.

I've never had any long term success with tape.

Silicone sealer is probably the best way to go but fill the bath with water before you start.

You want a triangular bead of silicone for the best results (rather than a concave section that you may get using a wet finger to smooth off.) I highly recommend getting a set of cloned Fugi silicone profiling tools (many clones around for a fiver or less)

Let me add a little to this: counterintuitively, silicone does not stick well to cured silicone, so building up in layers won't work well. The silicone needs to all go on in one session, then leaving plenty of time to cure.

Water will find its way through practically any sealing attempt. I had trouble with what should be a nice, easy job: sealing the bottom edge of a shower panel to the top of a shower tray. Both nice, smooth surfaces... which was the problem. My eventual successful solution was to run two strips of masking tape about an inch apart along the shower tray where the frame rested, then roughen the surface between with emery paper. Another run of emery along the shower panel bottom, then a big run of silicone all along the roughened part and plonk the shower panel on top. A bit of smoothing (*without* using any moisture, water gets trapped in even mould-resistant silicone and produces mould under the surface) and it's been OK since.

The problem with acrylic baths is, as you have observed, they flex. I suspect even support all the way along the sides would just result in the centre of the bath sinking and the edges twisting away from the wall. Possibly the answer is (incompressible) support under the bottom of the bath, maybe just in the middle.

The basic problem is that to get at the back of the bath to install a support there is well-nigh impossible, unless I had very long, very flexible arms, but your idea of a support under the bottom might well be a feasible partial solution. I will be at the flat at the end of February, I will put several different pieces of wood in the car before i GO, I just happen to have some offcuts from a recent fencing job. After that, the tape and silicone might just work.

Thanks to both of you.

that is nothing I saw a ground flat where the bath was open to the under floor space in other words the deveoper floored around the bath...water didn't stay warm very long

Not much help to you because you can't remove the bath but it might help someone happens upon this at some future date - when I installed a bath about 20 years ago the instructions said to fix a batten to the wall along the long edge to address this question, then fill the bath with water before applying silicon, as suggested in another post.

The first thing you do with an acrylic bath is stop the bloody thing flexing. Mine are covered in slabs of chipboard/MDF.whatever held on with car body filler. No flex at all. You should be able to do this if you have reasonable access from underneath. Coat the board with body filler, poke upte bath's backside and wedge till set

The next thing is to seal BEFORE you tile. So you can finish at a neat grout line with no nasty visible silicone.

I have recently stripped and redone ALL the edge of bath tiling to fix a leak. Which WAS coming through the ceiling below. The result of a bad decision to tile over a slab of wood used as a gap filler. Its now got kingspan foam to fill the gap. That wont swell when damp.

Or build a crude mechanism or jig to push a slab of material up there

You must do it somehow, or remove the bath and start over.

but your idea of a support under the bottom might well be

It will certainly help - a slab of material glued underneath with the ubiquitous car body filler and then supported with e.g. wooden props will help enormously

Don't follow. How do you finish/fill the gap between the tiles and the bath?

Remove the bath, fix supporting battens to the wall and prop them down to the floor (with something to spread the load if not over a joist), refit the bath, fill with water, apply silicone around the edge and bed quadrant beading into the sealant (using Fugenboys as appropriate), tidy-up, wait 24 hours and enjoy the result of your handiwork.

I’d check the ‘feet’ that support the bath are correctly supported.

They should NOT be directly on floorboards or (worse) chipboard flooring, especially between joists.

If they are, add some substantial bits of thick wood, supported by the joists, so the feet rest on those.

It MAY be possible to do this without removing the bath. Adjust one foot at a time, fit the new wood, and lower the foot.

When all the feet are supported, fill the bath to the rim, reseal, leave to dry.

No promises it will will work 100% but I think it is worth a try.

I think he's assuming that the bath is fitted flush against the wall

However, I don't rate the chances of grout between tiles and bath because, however well supported, the bath will move a bit - and grout is pretty inflexible.

Supporting plastic baths underneath works pretty well. Obviously don't use anything rottable.

Yep. I have a 6mm thick bath that has generally been stable, but another one that was a bit cheaper came with a few slabs of chipboard bonded to the base, but needed way more added to the sides before installation in order to feel 'solid' when you got in.

Grout

Its purely decorative, as the silicone has done the real job unless you have really beefed up the bath so there is no flex at all at that juncture.

Silicone makes a *flexible* seal. If there us no flex, you don't need it. No other joint on a tiled wall requires silicone, why should it be required at the bath edge? Because the bath is a cheap piece of flexible s*it, that's why. Silicone looks out of place, and attracts mould. So I hide it out of sight and make sure my baths are rigid, no matter how the supplier supplied them. Plate the whole f****ng thing in slabs of scrap chipboard glued on with gap filling car body filler. And if your floor flexes , brace it to the wall with diagonal struts. That's what I needed to do in one bathroom.

One everything is shipshape and Bristol fashion THEN get installing and tiling

No personal experience, but this looks interesting:

Chris

In 40 years of applying silicone I've never had any with mould.

I've never really trusted the cheap metal frames baths come with and always supported the base with wood blocks. I've also always supported the top underside edge of baths either by screwing battens to the wall or making a wooden frame for the edge that doesn't mate with a wall.

When I bought this house, the existing bath felt very wobbly to me - it actually wasn't bad at all, but I'd spent all my life until then, living in a house with an enamelled, cast-iron bath.

Nowadays, there is no problem - my wife struggles to get over the side of a bath (she's short and disabled), so we've taken ours out and we now have a shower, with a base only 30mm high, cast in a rigid, stone resin and bedded on mortar.

They seem wedded to a £10.75 delivery charge.