Is 18mm plywood enough for loft tank support base, & what type of ply?

[Plumb Line]

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?

Hardwood Plywood Board (L)2.44m (W)1.22m (T)18mm | DIY at B&Q

Hardwood Plywood Board (L)2.44m (W)1.22m (T)18mm - B&Q for all your home and garden supplies and advice on all the latest DIY trends

www.diy.com

Natural Softwood Plywood Board (L)2.44m (W)1.22m (T)18mm | DIY at B&Q

This Plywood Sheet is Furniture manufacture, construction, pre-fabricated components, base for laminating

www.diy.com

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.

 

[Roy_66]

Make sure whatever you use its waterproof.

What timbers do you have at right angles to the joists?

It's really a skilled job to spread the load over several joists. Four joists would be a better number. A really good job would end up with a nice even load on each of the four.

Silly question: is there lots of other stuff up there?

Cheers,

Roy (amateur with no relevant qualifications)

 

[Sparkgap]

Ideally marine ply. Weight of tank is one thing but also allow for weight of bod fitting it/changing ballvalve etc on the joists! From the description with the wall 500mm away (what about the other direction?) you'll be okay for support.

 

[WC1]

18mm is fine, seen tanks on all sorts of junk. Marine ply is good if you want belt an braces. Why not pick up some more boards while you are there, for you and future engineers to stand on while working on the tank?

 

[Last Plumber]

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?

Hardwood Plywood Board (L)2.44m (W)1.22m (T)18mm | DIY at B&Q

Hardwood Plywood Board (L)2.44m (W)1.22m (T)18mm - B&Q for all your home and garden supplies and advice on all the latest DIY trends

www.diy.com

Natural Softwood Plywood Board (L)2.44m (W)1.22m (T)18mm | DIY at B&Q

This Plywood Sheet is Furniture manufacture, construction, pre-fabricated components, base for laminating

www.diy.com

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.

The water weighs 182Kg plus the tank and innerds. That's over 28.6 stone. (400.4 lb).

Its a fair weight and needs to be supported correctly. Have you looked at what the joists rest in or on?
Do you think they will take that weight?
Can you not add more support if necessary from something like a Purlin?

Ply is stronger than floor board so I think you're considering the right material for the tank base to sit on.

 

[Plumb Line]

Thank you very much for the replies. I'll try to address each point raised....

Make sure whatever you use its waterproof.

Ideally marine ply.

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? Or sheet it in a plastic membrane type thing? Mind you, the mice would probably make a meal of that last option...

What timbers do you have at right angles to the joists?

None. Sorry if I wasn't clear, but the boards are and will be directly on the joists. The height is restricted so there's no option to put right angled cross joists beneath.

Four joists would be a better number.

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 at least, possibly 5 (depending on what length I can get up there through the hatch). So does that make things better?

Silly question: is there lots of other stuff up there?

I think my questions are sillier No, nothing else up there aside from the F&E tank which is a little further along on the same platform (but a few joists away).

with the wall 500mm away (what about the other direction?)

Have you looked at what the joists rest in or on?

The wall that's 500mm away is an internal solid wall. Seems the joists rest directly on that. But then resting on the joists is a large purlin which is supporting the roof. It's the same on the other side, and I'll add a photo of that. The purlin on the other side is 1000mm away. However, the wall (external) on that side is lower, as the ceiling slopes down to it. So the joists don't sit on the wall on that side, but join the rafters on the far side of the chunky purlin. I think I've got my terms right! But let me know if none of that makes sense!

Here's those photos:

Why not pick up some more boards while you are there, for you and future engineers to stand on while working on the tank?

Good thinking. I have got loft boarding on my radar. There are some boards up there already which are enough for when working on the tank. It's a long loft though and I intend to put a "walk [crawl] way" from one end to the other. Perhaps I'll cover that in insulation, but in such a way that the insulation can be moved aside to uncover the boards when access is needed.


Cheers everyone

 

[Ric2013]

Assuming the hardwood version is Weather and Boil Proof (often marked WBP), then it should be fine. Someone did once show me a document that showed that by law you should be using marine grade, but I have yet to see an installation of a cistern on anything better than hardwood WBP. And you see a lot on chipboard! Ideally get the ply cut so it is at least 200mm wider and longer than the cistern, though bear in mind you need to get it into the loft. 18mm should be fine.

As far as your joists are concerned, I'd argue that what worked before will work again. 150mmx37mm joists are what I have in my first floor and they span 10'. If two heavy people walk around, there's your 30st. And that's a moving load, not a static one. The concern with joists tends being overloaded tends to be that they will bend a very long time before they break (George Orwell commented that the Wigan miners preferred wooden pit props to steel ones for this very reason), so sudden failure is very unlikely.

You do need to try to get a foot clearance above the access ports or lid of your new cistern if you want to be able to clean it (and to comply with the water regulations).

 

[Last Plumber]

I can't see everything but is it not better to span the purlins with timbers (if they are only 3' apart) and drop legs onto more timbers spanning the joist for a little added strength in the centre?

Like I say, I can't see it and don't know the height you have above the tank.

The purlins are normally set into the gables and will take more weight than ceiling joists.

 

[Ric2013]

I can't see everything but is it not better to span the purlins with timbers (if they are only 3' apart) and drop legs onto more timbers spanning the joist for a little added strength in the centre?

Like I say, I can't see it and don't know the height you have above the tank.

The purlins are normally set into the gables and will take more weight than ceiling joists.

Excellent idea, but the OP claims the height is a restriction.

None. Sorry if I wasn't clear, but the boards are and will be directly on the joists. The height is restricted so there's no option to put right angled cross joists beneath.

 

[Plumb Line]

Assuming the hardwood version is Weather and Boil Proof (often marked WBP), then it should be fine.

I don't think it is, but I'll give the branch a call and check for sure. I'll report back. If it isn't, could I still use it if I treat it with something, and any recommendations on what to use if so?

From the above, I presume the hardwood is better than the softwood version?

Ideally get the ply cut so it is at least 200mm wider and longer than the cistern

Longer should be fine. In fact, I'm going to try to get a piece that is long enough to allow for 400mm beyond the tank on each side, which will take it to the next joist on both sides (giving a 5 joist span in total).

Wider... In my current plan the board would only be 650mm wide. The base of the tank is 500mm, so only 75mm each side. The loft hatch is indeed what's determining that width. If I go ahead with the 5 joist span length and the 650mm width, that can all be one single piece of board. Option B would be to go wider and use two separate pieces, with the join beneath the tank. Not sure if that then reduces the strength compared to a single piece though?


Thanks for all this great help

EDIT - crossed posts. Yes, there's restricted height so the tank can't go any higher I'm afraid.