So who knows about treating plywood?

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Lathe User

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My objective - I need to support 670 kgs of wet sand for 10 years between 300 mm centres on a 8 x 4 sheet. I am going to cover the top of the board with a waterproof membrane.
The obvious choice is marine ply, rot resistant but it’s become hellish expensive and I need four sheets. Added to which I need to pay even more to get some with structural strength, which I am told is poor anyway.
The next choice for me is hardwood WPB ply - structurally much better and cheaper but while it’s sold as an exterior ply the more honest sellers say it needs treatment but conveintently omit to say with what!
My usual waterproof approach is to paint timber edges with a couple of coats of Exterior PVA.
After that I am out of affordable ideas.
Practical suggestions please .
 

Jameshow

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MDO / Buffalo board?

Why the ten years requirement?

Line it with Dpm?

Cheers James
 

baldkev

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A diagram would definately help. Whats your intended thickness? I bought 12mm marine ply for £36+vat a sheet locally..... so laminating is possible... or of course, get something like far eastern ply, cut to shape and epoxy the hell out of it before installation
 

Cabinetman

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If it’s the same hardwood water and boil proof ply I’ve used in the past, it just isn’t! Doesn’t resist anything at all.
Guessing here. I am presuming that you will be shovelling the sand off at times? Sort of a bunker system? Four different types of aggregate ?
The surface - if that’s what you will be doing- will need to be sheathed in galvanised tin possibly. Can’t think of much else that would take that punishment. Ian
 

Jones

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You can get ordinary ply tanalised .You'll need a merchant with a treatment vessel and wbp ply so the glue holds up. It will be a lot cheaper than marine. I used some on an outdoor job over 15 years ago and it's still good I'm not sure untreated marine ply would have lasted that well.
 

Lathe User

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Good evening,
Diagram - well it’s nothing more than a glorified bench or in this case 2 each 16 foot long and 30 inch wide some 30 inch high.
The wet sand is retained there by a 6 inch surround on top of the bench. The sheet, acting as a bench top is supported underneath by 2 inch widesteel sections every 300 mm centres,
10 years - well I am 69 now so by the time I reach 80 ….
Thickness? Ideally 6 mm so I get the biggest depth of wet sand but 12 mm is fine.
 

baldkev

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So its for growing something in? I cant quite figure out why you need wet sand 😆
 

Jameshow

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Get a catering fabricator to weld up a SS liner?

Otherwise roofing felt with the corners folded.

Cheers James
 

eezageeza

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Propagating bench, or bench for an alpine house ?

If so, I made some maybe 20 years ago with ordinary timber, painted with whatever preservative B&Q had in stock at the time, and lined with builders polythene. Still holding up!
 

Woody2Shoes

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My objective - I need to support 670 kgs of wet sand for 10 years between 300 mm centres on a 8 x 4 sheet. I am going to cover the top of the board with a waterproof membrane.
The obvious choice is marine ply, rot resistant but it’s become hellish expensive and I need four sheets. Added to which I need to pay even more to get some with structural strength, which I am told is poor anyway.
The next choice for me is hardwood WPB ply - structurally much better and cheaper but while it’s sold as an exterior ply the more honest sellers say it needs treatment but conveintently omit to say with what!
My usual waterproof approach is to paint timber edges with a couple of coats of Exterior PVA.
After that I am out of affordable ideas.
Practical suggestions please .
I would say that ply of any kind sounds like the wrong material unless it can be kept bone dry. Even 'durable' timber species will deteriorate if kept wet for ten years - whether treated with (toxic) coatings or not. I'd choose aluminium, steel and/or masonry instead. Fascinated to know what you're making.
 

TomGW

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I’m intrigued. No just plain curious/nosy.
670kg of wet sand is a very precise amount and will only remain a constant weight if the moisture content is consistent.
 

Lathe User

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😁 I am impressed by you guys, resourceful or what.
Yes, I am slowly building an Alpine House ( otherwise known by my wife as my obsession). For the uninitiated that’s a green house but with windows along both sides of the ridge beam and also all along each side plus doors both endS. Why so many widows? Well Alpines need lots of ventilation, cool conditions and definitely not drowning in our wet autumns/ winters. Which means the plants stay in pots but are surrounded by wet sand so they can draw just the moisture they need. (. Baldkev).
In the. 1950s they built their beds in brickwork and used old air raid shelter iron sheets for raised floors. Later they used paving slabs and brick supports. I bought some pallet racking. Cut down to less than a metre in height and I am in the process of cutting and rewelding the arms. They are rated at better than 1000 kgs. As for the 670 kgs TOM - well it’s the cubic capacity created by a 150 mm high plunge surround on a 8 foot board multiplied by the rated weight of wet sand 58 kgs.
Jamesshow I had a good look at SS catering benches but they are not cheap second hand being very much in demand and metal sheets of any kind in the sizes I need carry a premium if you’re not in the trade never mind transport.
Jones - I hadn’t known about tanalised plywood, unfortunately, so far, it seems only available in some locations but thank you for the suggestion.
woody# shoes- I share your concerns but having a membrane on top means the boards won’t be in permanent contact with the wet sand but the concrete lintels, now that’s an idea.
Essageesa - My idea for a membrane was also a roll of DPM I had left from a house extension but using timber as a base makes me wonder if a part load of recovered floorboards might not do the job, coated in preservative. You say yours has lasted 20 yrs ! That’s encouraging.
Oh the Alpine house is constructed in Canadian western cedar and the detailed frames etc are in recycled teak from the 1940s all coated in 2 coats of Sikkens.
 

Jones

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That's an impressive amount of detail. You might consider fiberglass roofing from a builders merchant. Typically you use 2.4x0.6m sheets of OSB ,roll out the matting on top and then apply resin with a paint roller, you can buy preformed upstands and corners or just use a timber angle fillet to make the corners. Once it's half cured you roll on a top coat with a fresh roller the whole process takes a few hours you should not leave it longer or the bond between top coat and matting won't be strong.
 

TomGW

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You are obviously under way with your project but I can’t help think that you are probably ‘reinventing the wheel’. What you are describing sounds very like mushroom growing racks. Modern racks look like pallet racks with aluminium trays. I know that many local growers have gone out of the industry and lots of these racks have been scrapped or reused at plant nurseries. It may be worth checking gumtree etc.
 
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