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

[Oxbow16]

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.

[MikeSharp01]

Is the tank steel or plastic? If there are signs of woodworm you need to get the whole roof checked unless you have evidence that it has already been treated. 18mm will spread the load across 400mm without deflection even for a plastic tank and 150mm joists is deep for loft joists but 37mm is quite narrow. Can you lift the tank? If so you could put some cross members under the ply to spread the load across more rafters but given it is supported now and as long as there is no evidence of bulging in the room below replacing the boards with ply should work OK.   

[Oxbow16]

Hi @MikeSharp01

 

Thanks for the reply and for sharing your thoughts.  

 

The tank is plastic. 

 

There are no other signs of active woodworm up there.  We did get a company to come and have a look a couple of years ago and they were happy with what they saw.  Any holes aside from those in the tank base they said were historic and nowt to worry about.  I've kept an eye on things when I've been up there and think the other timbers look fine.  On top of that, the roof needs work at some point soon, at which point it will have much better ventilation which would make it less of a haven for any critters.  Perhaps the base I'm changing would then be fine too.  But seeing as everything is being drained down and it's easy enough to put a new base in, I thought it was worth it.  

 

Lifting the tank - no, unfortunately not.  On the one side, there is 430mm clearance between the tank top and rafter above, but on the other side that clearance is only 140mm.  But RE spreading the weight...  Perhaps I gave the wrong info?  I'm not sure... But what I meant is that the tank itself *JUST* spans three joists. The board will span 4 joists, possibly 5 (depending on what length I can get up there through the hatch). So does that make things even better or have I missed the point?

 

No evidence of bulge I don't think, but now you come to mention it I'm heading up to take another look!  

 

Thanks again

 

 

 

 

 

 

 

[SteamyTea]

4 minutes ago, Oxbow16 said:

But what I meant is that the tank itself *JUST* spans three joists

The closer the mass is to the joist  the better.

Think of it as a torque, the longer the spanner, the more you have to move the free end. That free end is the deflection.

 

https://www.engineeringtoolbox.com/beam-stress-deflection-d_1312.html

 

[Oxbow16]

@SteamyTea

 

I'll come back to that when the morning is further away and I've had x more coffees.  Perhaps not appropriate given your name, but I think you catch my drift?   

[SteamyTea]

6 minutes ago, Oxbow16 said:

x more coffees

Drinking coffee here.

 

[saveasteading]

10 hours ago, Oxbow16 said:

two types of ply and I wondered which is best please?  

Interesting question. 

 

They both have plenty of layers so strength the same.

Subject to woodworm and rot?...perhaps the softwood will attract British worms more than the tropical wood?

 

But the tropical aspect decides it for me. Another tree down in the Amazon or Madagascar, with a lovely certificate appearing along its trail?

Even if genuinely 'sustainable' it increases the demand for such wood. So the better choice is the softwood one, to me.

I might give it a swipe of 5in-one wood treatment esp on the cut edges, in case you get plumbing drips at some stage.

[Temp]

182L is going to weight 230kg max. Call it 180kg of water plus another 50kg max for the tank?  About same as three people? 

 

Think 18mm ply is fine but it does vary in quality a lot from the DIY sheds. I'd be inclined to buy a full sheet of WBB cut it in half and use both bits one on top of the other.

Edited November 18, 2021 by Temp

[SteamyTea]

12 minutes ago, Temp said:

I'd be inclined to buy a full sheet of WBB cut it in half and use both bits one on top of the other.

Sometimes, the obvious is so simple.

[Oxbow16]

Thanks for the further replies.  

 

@saveasteading -Good point RE the eco credentials and something that does matter to me, so I'll bear that in mind.  I do have a reasonably small carbon footprint though day to day, so if the hardwood trumped the soft on other counts then I might just close my eyes, clench my buttocks, and do the wrong think environmentally on this occasion...

 

@Temp - WBB?  What does that stand for or should it read WBP?  Trouble with that is sourcing it at this stage, and finding someone who can either cut it on site so that I can transport it, or who does delivery.  

 

Also, sticking one bit on t'other....  That would depend on whether it resulted in the plumbing work having to be redone due to the small change in height...  Something I'd rather avoid if possible...  

 

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.
 

Cheers for all the help

Edited November 18, 2021 by Oxbow16

[Temp]

6 hours ago, Oxbow16 said:

 WBB?  What does that stand for or should it read WBP?

 

Yes sorry it should be WBP. In case you get condensation on the outside of the tank. Most timber merchants have a panel saw and some will cut free.