repair shower trap cracking from underneath?

[charliebean]

Thanks everyone for your help, it has been really useful.

We've got the new tray in and ready to patch up the plasterboard. 3 new pics here:
http://imgur.com/a/r0jDV

We have two problems:
1. We can't get hold of 15mm moisture resistant plasterboard anywhere. Looks like we may have to order a sheet online which with delivery will be about £50 just for this little patch! Once we have hold of some, we are wondering how to fit to the awkward shape in on the right hand side. Any tips?

2. The new tray has a slightly different drainage trap to the old one, it looks a better design but the old elbow joint that connected the trap to the pipework doesn't fit. It's not a slimline tray but I think this is the same trap:
http://www.discountedheating.co.uk/...ZLkfszbd5vJteaLJH-N1d-4H3y96AufJrsaAkCs8P8HAQ

The drain pipe we need to connect it to is 40mm. I'm not sure how we're supposed to measure the trap outlet correctly to ensure we buy the right connector. Any advice on this would be appreciated.

 

[Madrab]

Just use 12mm board and 3mm packers on the studs, should be fine.

Most shower traps have a standard 40mm compression end so not sure what it is that doesn't fit unless you're trying to fit that elbow onto it directly? You can't use that one as it's not for connecting to a compression end, you'd be looking to cut a little of the waste pipe back, use a standard compression elbow or better still a solvent weld then a little length of 40mm pipe that will then fit into the traps compression outlet.

 

[charliebean]

Just use 12mm board and 3mm packers on the studs, should be fine.

Most shower traps have a standard 40mm compression end so not sure what it is that doesn't fit unless you're trying to fit that elbow onto it directly? You can't use that one as it's not for connecting to a compression end, you'd be looking to cut a little of the waste pipe back, use a standard compression elbow or better still a solvent weld then a little length of 40mm pipe that will then fit into the traps compression outlet.

Good suggestion about using packers. B&Q is the only local place to have moisture board and it's 12.5mm - I can't find 2.5mm packers, but will 3mm as you suggested be ok? I guess it will just mean pressing the tiles in a bit more when tiling to make it sit half a mm lower alongside those already fitted? I plan to buy these:
http://www.wickes.co.uk/Wickes-Assorted-U-Packers-150-Pack/p/214865

What you say about the plumbing makes perfect sense, but I'm not sure there is space to do it. Currently the waste pipe is close to the trap outlet and at a 90 degree angle, so I can fit an elbow but there wouldn't be any space to put a piece of 40mm pipe between it and the trap outlet. Am I right that this ought to do the job?
http://www.screwfix.com/p/floplast-wc27-universal-compression-waste-conversion-bend-90-40mm/38002

 

[Madrab]

Just make the packers, if you have a circular saw, or just use 2 x 1mm packers and then use a little more adhesive.

Without seeing the pipework it's difficult to say, but that coupler with go from a plain end of the waste pipe through 90Deg and into the compression end of the trap. I wouldn't use Floplast though, go with a McAlpine MT4L fitting.