Leaking shower

If it's been doing that for some time, are you sure that the floorboards under the shower aren't rotting through? And if it's right at the edge of the wall, the joists might be checking too.

It would really be worth looking at any wood before doing any remedial work. > My plan is to fill the wedge-shaped void behind the shower tray with

Concrete is not waterproof unless specifically treated. Why not use a waterproof expanding polyurethane foam? Many examples here:

Then perhaps use a sink trim all round, although it depends on how deep the grout is between the tiles. A little extra sealant to fill any gaps might be required.

For the record, most expanding foams are open cell and allow moisture through. Many of the search results on that term aren't actually waterproof (some are 'weatherproof' - not sure what that means but likely not serious quantities of spray), so check your results carefully.

I recently ordered a can of this (at £15 a can, not what they're currently charging):

Theo

What I did, and it seems to be holding so far., when my bathe ledge developed a leak was to go and get some Celotex from Wickes, and some foaming gorilla glue. I could have used a can of expanding foam I guess...and cut the Celotex to fit the gap and glued it all in place with the foaming gorilla. This sealed the gap and was flexible enough not to crack when the bath was full of me and water.

But I ran some flexible caulk down the seams just in case and replaced the tiles over it.

The solid grout has cracked slightly after the first path use, but no water has got through at all.

By all means then add a decorative layter over the top - I already had tiles.

I really wouldn't use cement - too brittle - will crack - but a strong mix of about 2 sand to one cemet is pretty much 100% waterproof since the cement fills all the gaps between the sand grains

My technique, adopted all over four bathrooms I built in this house is to use slightly flexible gap sealing and waterproofing and then add decorative tiling on top. So I seal the appliance edge to the wall/floor/whatever with silicone BEFORE adding the tiling.

Concrete an mortar is simply to rigid. Hence my advice to use something more flexible like expanding foam if you can bear the mess, or something else that is waterproof sealed in place with foaming glue.

I really dislike beading and silicone on show. My ideal is to get the waterproofing and structural work done and then add decoration and not have to worry about e,g. tile grout cracking slightly.

A test of water absorption of various spray foams, conclusions inconclusive but the closed cell pond foam worked out a bit better:

Even if what you say is true, a shower enclosure is no worse than e.g. a window cill in the rain etc.

But I think most foam is closed cell, or it wouldn't expand, would it?

It's much more humid and receives less wind to dry it, and open cell foam works like a sponge to collect and transfer moisture. It needs a flow of air to dry, which is something a window cill doesn't lack but you aren't likely to find behind the wall in a poorly ventilated bathroom.

No, the cell structure is whether the voids are separate or connected. A plastic dish sponge is open cell in that the water can travel between the voids, even though the plastic itself is not permeable. If you dunk a sponge in water it'll soak it up, if you dunk a piece of expanded polystyrene (closed cell) it'll float and not absorb water.

Of course, there's no such thing as 100% open or 100% closed cell and the void size varies, so open cell spray foams don't soak up water anywhere near as readily as a sponge - but they still let moisture through and aren't waterproof.

Seems that most one-part foams (ie what you get in a can) are open cell, and two part foams (where they're mixed in the spray gun) are closed cell. But a lot of the foams available in the UK don't actually say, and two-part foams are relatively rare.

(Soudafoam 2K is a cheaper 2-part one, datasheet says it's 81% closed cell)

Theo

The stuff that your local energy advisor wanted to spray on your roof to replace the old style fibreglass loft insulation, and then make it impossible to get home insurance, was probably open cell.

Indeed - it absorbs and holds moisture against the timbers so they rot.

PIR foam board (celotex etc) is closed cell so it doesn't do that.

Theo

For you final sealant consider CT1 (toolstaion etc.)

I've used it and it is impressive stuff, but they used to market it as "the only sealant you'll ever need" but now they seem to be marketing half a dozen different sealants for different jobs ...

They market it as 'even works under water', but when did you ever need to apply sealant under water?

SF have a clone called OB1:

Theo

Quite often if you're tying to stop a window leaking for example.

While something is wet sure. But IIRC they showed it sticking two breeze blocks together while submerged in a fish-tank.

I have never tried it myself, so I am just throwing the idea out here for others to comment on: Would mixing PVA into the cement make it waterproof permanently after drying?

I have found it waterproof if enough cement is used to fill the gaps between the sand. I am sure other things would work as well, possibly at a cheaper price.

Not sure, but I'd guess it wasn't necessary for the construction of concrete ships. Just a strong concrete mix I'd have thought, 3 sand:1 concrete or some such.

PVA tends to re-emulsify when wet so I cannot see it making concrete waterproof. You can get waterproofing additives to mix into mortar that work well the only issue is for a small job it will be rather expensive. I can attest to the additives waterproofing as my timber garage sits on a single brick foundation on the concrete base. The groundwork’s people who prepared the base used waterproofing additive in the mortar and there has not been the slightest bit of damp let alone water penetrating.

Richard

That looks seriously interesting stuff. Way more expensive than anything else I've ever used before but probably well worth it.

Thanks,

Nick

The "even works under water" claim never did Milliput any harm. Er... has anybody actually seen any Milliput recently?

OB1? Shouldn't the company be called Star Wars, not Screw Fix?

Nick