Is 18mm plywood enough for a cold water tank support base? And what type of ply?

[DIYNotIan]

Hi

I've got some upcoming plumbing work and everything is being drained down for it, so I'm taking the opportunity to replace the boards under the tank in the loft because they have signs of woodworm... I'm hoping to use 18mm ply because it is available at B&Q where they will cut it to size for free, which will save time, hassle, and will mean I can fit it in the car! But is 18mm thick enough?

Here's some relevant info:

- The existing base is T&G floorboards and they're 20mm
- The tank spans three joists
- The joists are 150mm x 37mm. Joist centres are average 400mm apart.
- There are no walls beneath the tank. The nearest wall is a solid internal wall, which is 500mm away from the tank, to the side.
- The tank is 40 gallon / 182L actual (60 / 273 nominal)

If 18mm will be enough, it seems B&Q offer two types of ply and I wondered which is best please?

https://www.diy.com/departments/hardwood-plywood-board-l-2-44m-w-1-22m-t-18mm/5059340257433_BQ.prd

https://www.diy.com/departments/natural-softwood-plywood-board-l-2-44m-w-1-22m-t-18mm/945875_BQ.prd

Many thanks for sticking with me through this bathroom refit and all it involves!!!

PS - if this is more suited to a different sub forum please move it to where ever is best. Cheers.

 

[Mottie]

If you're only changing the board beneath the tank, it shouldn't matter where it is as presumably it's been there for years. 18mm should be fine.

I recently removed the raised platform that my cold water storage tank stood on (tank was removed a few years ago when we converted to unvented hot water) and the frame was made of 4"x2" but the platform was a piece of 12mm board.

 

[mattylad]

If your worried then make it double thick.

 

[DP]

Marine ply would be my choice or ply that will not delaminate if it gets wet

 

[DIYNotIan]

If you're only changing the board beneath the tank, it shouldn't matter where it is as presumably it's been there for years.

Yes, around 20 years or so at a guess. And yes, only changing the board.

the raised platform that my cold water storage tank stood on

Just for the record, my boards are directly on the joists. There's no platform or cross joists beneath. Not sure if I'd made that clear so thought best to mention it in case it changes anything.
(PS - love the avatar photo!)

If your worried then make it double thick.

Do you mean use wood that's twice as thick? Or put another 18mm sheet on top of the first? The trouble with going thicker - aside from costing twice as much - is that the pipes would then no longer align and there would be the additional work/spend when it came for the plumber to reconnect it. If it has to be it has to be... But if 18mm would be enough as some have suggested...

Marine ply would be my choice or ply that will not delaminate if it gets wet

Presumably both of those I've linked don't fit that bill? If not, I'll have another look for marine ply but the difficulty is finding somewhere with a cutting service, or who will deliver without charging £££. If I used the B&Q stuff, could I paint it or use some kind of oil to waterproof it?


Thank you all for the replies and help. Much appreciated

 

[Terrywookfit]

Use ordinary ply.
The fookin ceiling will have fallen in before it de laminates.

 

[Mottie]

I'm taking the opportunity to replace the boards under the tank in the loft because they have signs of woodworm...

Are you sure? Surely if you have woodworm in the boards and they are resting on the joists the joists could be suspect too? Why not just treat what you already have as well as the joists with some woodworm treatment? I think you may be overthinking things.

 

[DIYNotIan]

Are you sure? Surely if you have woodworm in the boards and they are resting on the joists the joists could be suspect too? Why not just treat what you already have as well as the joists with some woodworm treatment? I think you may be overthinking things.

Yes, I'm pretty sure. Only the boards have sign of active worm, and perhaps that's not active anyhow but I'd rather not take the risk. Any other worm holes in the loft are historic. We had a firm come and check all the loft timbers when we moved in and that's what he concluded too. And that's from someone who had something to sell

I treated what I could of the current boards but there's no way to get under the tank without moving the tank. And once I've gone that far I just thought it made sense to replace the boards with new. Good point RE the joists though. I will check them when they're up and give them a couple of coats of killer while I can access them, again to be on the safe side.

Use ordinary ply.

That does make my life easier Is the hardwood version the better of the two from B&Q? If I did decide to waterproof it before installing, anyone have a recommendation on what to use?


Huge thanks to all

 

[Mottie]

If you have to waterproof it, you'd have a more serious problem to worry about than just the board getting wet - you'd surely see water ****ing through your ceiling! Are the joists and ceiling waterproofed?

 

[DIYNotIan]

If you have to waterproof it, you'd have a more serious problem to worry about than just the board getting wet - you'd surely see water ****ing through your ceiling! Are the joists and ceiling waterproofed?

Fair point, and tbh I'm not espousing the need for waterproof/resistant boards, just re-iterating what I've read and been told elsewhere.

Having thought it over though, whilst your point is obviously good and valid for a major leak, what about a slow drip type leak? If it were slow enough for long enough, and the boards were soaking up the water, they could get quite wet/saturated before the water travelled far enough to be noticeable through the ceiling. Not necessarily what I think, just sharing my thoughts. And perhaps what I'm saying is an argument as to why chipboard would be a bad idea, but that the plyboards at the B and the Q would be a happy medium...

All in all, I think the 18mm will be fine, and probably fine just in it's off the shelf state. I made the thread wondering if all the answers would be a definite "NO!". So I'm feeling sufficiently re-assured now.

Thanks for all the help

 

[DP]

difficulty is finding somewhere with a cutting

B&Q will cut the panel to your size

 

[Alec_t]

If I used the B&Q stuff, could I paint it or use some kind of oil to waterproof it?

How about just put a waterproof sheet over the platform?

 

[DIYNotIan]

B&Q will cut the panel to your size

Yep, that's why I want to use them I did make that point in the first post, but can't say I blame you for not having read through all of it!

How about just put a waterproof sheet over the platform?

Good thinking, and it did cross my mind. But we do get mice in the loft. Even had a stoat once! And while I will be looking to try to stop them getting in at some point, it may well be an ongoing battle. In my experience, mice seem to love nibbling on tarps and the like...