Shower room walls

I am currently building a shower room with a studwork wall.
I intended facing the wall on the shower side with 12mm exterior ply (instead of plasterboard) and then tiling with a waterproof adhesive and grout. The bloke in Jewsons, however, suggests I use marine ply at 50 per sheet (8x4).
Is this really necessary or should I even just go with plasterboard. I think Wickes do a waterproof version.
Anybody got any suggestions.
Many t's
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Bilbo Baggins wrote:

I did this earlier this year. I used 3/4" WBP (water & boil proof) ply, which was around 22/sheet.
The number of times I hear "use marine ply", almost always from people who really have no idea what marine ply is... There is no need to use marine ply for this application. The ply shouldn't get wet in the first place - if it does, WBP is plenty.
Don't forget to prime the ply with PVA before tiling. Use a decent waterprrof adhesive and grout.
--
Grunff

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per
Don't use plywood at all ... use Knauf Aquapanel ....... totally waterproof.
Rick
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per
I agree that using normal plasterboard isn't a great idea... if it gets at all wet it just disintegrates. Although it will be fine if you're confident it will never ever get wet (ie you have perfectly sealed shower/tiles now and in the future, which in turn means lots of extra supporting stud work to ensure the plasterboard forms a completely rigid substrate for tiling on to.)
I'm a big fan of the Wickes stuff ("Aquapanel"), and would certainly recommend it for use in showers. Think it's about 17 quid for a sheet 900x1200 mm (the only size it comes in); you're supposed to use special scrim tape and fixing screws with it too. A lot cheaper than exterior ply, and only marginally more difficult to work with than plasterboard (you can still 'score-and-snap' it).
David
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