How do I prepare marine ply base before fitting shower tray?

[Beverly27]

I will install a marine ply base, which will be screwed into the joists and noggins shown in the photo, upon which I will (with some assistance!) mount the shower tray. Should I also glue the marine ply down, or will screws suffice?

The manufacturers recommend mounting the tray on a weak sand/cement mix, which I will use. Do I need to prime the marine ply before applying the sand/cement mix? I've read conflicting advice about this, and if I should do this, what primer is best to use?

Also, do I need to seal the gaps around the edge of the marine ply base, where it sits in the 'gap', and if so, with what?

 

[Burnerman]

Plenty of screws - say 5 x 50 - will be fine.....the essential thing is to have the base flat so there is no rock on the shower tray at all.
A sand and cement screed is fine, all the tray needs is a good level bed to rest on as a cushion. Prime the timber if you like, it will reduce the absorption time of the mix.
A Mira rep told me that a good bed of silicone is just fine too - so long as the tray is perfectly level for drainage and doesn't rock.
John

 

[Beverly27]

Plenty of screws - say 5 x 50 - will be fine.....the essential thing is to have the base flat so there is no rock on the shower tray at all.
A sand and cement screed is fine, all the tray needs is a good level bed to rest on as a cushion. Prime the timber if you like, it will reduce the absorption time of the mix.
A Mira rep told me that a good bed of silicone is just fine too - so long as the tray is perfectly level for drainage and doesn't rock.
John

Cheers John, that’s helpful. Any idea if I need to seal around the edges of the ply, after I’ve sat it in the gap and screwed it in? I don’t suppose you have any suggestions as to primer? Thanks again.

 

[Diver Fred]

Seal the ply with PVA (50/50 with water) including the edges.

Before you set the tray in place put 'Classi' seal around the edges of the shower tray that are against the walls. The backerboard / tiles / shower paneling goes on the room side of the seal tape. It's a final solution to water leaks into the wall.

 

[Beverly27]

Seal the ply with PVA (50/50 with water) including the edges.

Before you set the tray in place put 'Classi' seal around the edges of the shower tray that are against the walls. The backerboard / tiles / shower paneling goes on the room side of the seal tape. It's a final solution to water leaks into the wall.

Diver Fred, thanks. I already have an Abacus No More Leaks kit that does the same job as the Classi seal, so thanks for reaffirming that I'm taking the correct approach. Thanks for the sealing advice. What's the purpose of doing this?

 

[foxhole]

I would use cement board under the tray.

 

[Beverly27]

I would use cement board under the tray.

Thanks foxhole. What would the benefit of using the cement board be, rather than the marine ply? There are so many different opinions out there!

 

[old duffer]

Perhaps you should be made aware that marine ply does not mean its waterproof lol
If you are not using cement boards or foam boards then you are wasting time, money and will be having problems with leakage and rotting in the near future.

 

[freddiemercurystwin]

Perhaps you should be made aware that marine ply does not mean its waterproof lol
If you are not using cement boards or foam boards then you are wasting time, money and will be having problems with leakage and rotting in the near future.

Why add the lol comment? That's a bit arrogant isn't it, the site is for helping DIYer's not belittling them. Whilst cement board should be used, plywood underneath could also last indefinitely.

 

[martygturner]

Perhaps you should be made aware that marine ply does not mean its waterproof lol
If you are not using cement boards or foam boards then you are wasting time, money and will be having problems with leakage and rotting in the near future.

Err really...plenty of boats built out of the stuff. - why would you have leakage and rotting ... its not a given unless the tray moves. Marine ply does not flex in comparison to either cement or tile backer boards and ties in the joists to a single unit increasing rigidity of the subfloor. As other have said seal the wall gap with a good waterproof sealant and add a shower band. Personally I have never liked sticking a tray down, good bed of sand and cement.

 

[Beverly27]

Why add the lol comment? That's a bit arrogant isn't it, the site is for helping DIYer's not belittling them. Whilst cement board should be used, plywood underneath could also last indefinitely.

Thank you for this freddiemercurystwin. I share your view and it’s lovely to know that others are supportive.

 

[Diver Fred]

Diver Fred, thanks. I already have an Abacus No More Leaks kit that does the same job as the Classi seal, so thanks for reaffirming that I'm taking the correct approach. Thanks for the sealing advice. What's the purpose of doing this?

The PVA is a waterproofing solution in this case. It just add adds an extra degree of water resistance to the marine ply - particularly the edges. Once dry then lay the stiff motor mix on top to bed the shower tray on.
Use 18mm min - better 22mm ply.

 

[Beverly27]

Err really...plenty of boats built out of the stuff. - why would you have leakage and rotting ... its not a given unless the tray moves. Marine ply does not flex in comparison to either cement or tile backer boards and ties in the joists to a single unit increasing rigidity of the subfloor. As other have said seal the wall gap with a good waterproof sealant and add a shower band. Personally I have never liked sticking a tray down, good bed of sand and cement.

Thank you too martygturner. I didn’t know about the structural stability of marine ply, and given that the tray will likely crack if it flexes, then perhaps marine ply is the best option. I’m going to implement the other steps you’ve suggested, and will use a 5/1 sand/cement mixture to mount the tray on.

 

[Beverly27]

The PVA is a waterproofing solution in this case. It just add adds an extra degree of water resistance to the marine ply - particularly the edges. Once dry then lay the stiff motor mix on top to bed the shower tray on.
Use 18mm min - better 22mm ply.

Understood Diver Fred, thanks. I was going to use the 12mm marine ply I have, because this will be level with the rest of the subfloor, but I see you suggest a minimum of 18mm. Is using the 12mm, rather than 18mm, likely to be problematic in your view?

 

[foxhole]

Understood Diver Fred, thanks. I was going to use the 12mm marine ply I have, because this will be level with the rest of the subfloor, but I see you suggest a minimum of 18mm. Is using the 12mm, rather than 18mm, likely to be problematic in your view?

12mm is absolute minimum when over-boarding with ply, with your set up you need 22mm minimum .