How to fill large gaps left by plumber before tiling shower?

[Squish]

We've just had our loft converted including adding a new shower room. I'm supposed to be doing the tiling which I've done before with no problems so thought this would be no trouble either but now I'm stumped! Basically, we bought the shower tray before the plastering was done so got what we thought would be the correct size to fit the alcove (on the advice of the builder), but turns out it was a little too wide so the plumber had to gouge out all the plaster along the bottom of the wall to get the shower tray in. So now we have an area at the bottom of both side walls that will need to be filled somehow before tiling & I'm not sure of the best way to do this. Each gap is about 1cm deep, 2cm high & 70cm long.

I was thinking just polyfiller but it's a fairly large gap for that. Also given that it's at the bottom of the shower I was wondering if I should go for something waterproof to help minimise the risk of leaks? Is there a specialist product for this kind of thing? Or could I even just use the tile adhesive or grout as a filler???

A bit of googling on this has worried me as I keep finding info about how you should be waterproofing the entire area behind the tiles in a shower (e.g. using special plasterboard or tanking) which hasn't been done, it's just ordinary plasterboard & plaster as far as I'm aware. The builder (who seems to be a very competent & experienced bloke and has completed the conversion to a high standard) knew this alcove would be a shower so I've trusted him to do it correctly but now I'm a bit concerned it's not been done right... feel a bit like I've opened a can of worms looking for a solution to what I thought was a simple problem!!

 

[Squish]

Just to add - a bit more reading has clarified some of the waterproofing issues for me; I had thought "tanking" involved a lot of messy work by an expert (previous experience with tanking was when we had dampproofing done in our old house & the bloke used something which looked like cement to tank it) but I now realise you can just buy stuff to paint on to the wall to waterproof it before tiling which seems much more doable!

Still confused about filling the gap though - most info I can find on the web about this seems to be referring to either much larger or much smaller holes! And even then a lot of the advice seems conflicting!

 

[AdamCH]

Are you sure you don't want to get a tiler in to finish the work?

If your just looking to repair the plaster then why not use Wickes one coat plaster?

 

[foxhole]

If you have a plasterboard wall then it needs tanking, though should have been constructed with aqua panel or similar as far more secure than plasterboard which can fail even when tanked.
What size tiles are you using?

 

[Squish]

I had a feeling someone might say "get a tiler in"! If I was doing significantly more than just a shower cubicle or using expensive or specialist tiles I probably would, for reasons of time as much as anything. When I posted my question I was starting to wonder if it might be a good idea as it was all looking a lot more complicated than I thought it'd be, but if I can deal with the waterproofing issue & fill the gap adequately then I don't see why it's beyond a competent DIYer. I've done tiling before and (if I do say so myself) I'm pretty good at getting it looking neat & have had no problems down the line with any of it so I know I'm capable of doing a decent job. I'm not one of these DIYers that just launches into jobs without being fairly certain I know what I'm doing, hence the question on here.

I'm using "bog standard" 15cm tiles, so nothing fancy or difficult to work with.

How long does the one coat plaster take to dry? I'm hoping to get started on the job asap (as soon as I'm sure I know what I'm doing!) so would prefer something I can tile over within a day or two if possible.

One of the reasons I was wondering about whether there was something waterproof is because the shower tray will basically be recessed into it & obviously I can't paint the tanking stuff under the join so if the silicone failed & any water seeped under the edge it would damage the plaster. Is there a better solution to this?

 

[RobinClay]

Oh ! I've just asked a similar question at //www.diynot.com/diy/threads/gap-between-shower-tray-wall.410860/#3184217