Manky plywood

[Deluks]

I picked up some cheap plywood several months ago, 4'x4' sheets, 20 of, 18mm and laminated with 6mm mdf which has been painted with a heavy duty floor paint. Got the lot for about £25 . They'd been used as a stage floor in a theatre and were sound. I was gonna use 'em to board out the loft*, but then decided a full conversion in the future would be money better spent, so put the boards in the garden, on top of a load of other timber I have stored, and underneath a heavy duty tarp.
I checked 'em the other day for the first time in weeks, and several of them have gotten wet with all this rain we've been having, the tarp had lifted in one corner. So now several of them are damp (a couple are soaked through) and are going a bit green on the corners. I'm hoping that rot doesn't set in and spoil the lot, so with that in mind, what can I do right now to save them getting completely wrecked? Getting them back under cover obviously, and I've given them an airing on dry days but should I paint them with and kind of treatment to stop them going bad?

*if not the loft then I'll use them for something else but they will get used for something.

 

[JohnD]

I don't know if this is considered good practic, but I treat my loft flooring and outdoor timbers with one of those water-based, water-repellent stains, as used for fences. They are cheap, go on quickly and easily with a big brush, and dry quickly (in dry weather, but will wash off if it rains heavily in the first few hours) and you can apply a couple of coats in a day.

They will not protect timber from rotting if it is in contact with the ground, or if there are knots, splits or shakes which allow rain to penetrate through the surface.

*if not the loft then I'll use them for something else but they will get used for something.

November the 5th is coming

 

[Deluks]

I've got some of that stuff, cuprinol shed n fence. The rafters in my lean to shed wouldn't look out of place in a country cottage I'm not sure whether the chemicals within make it suitable for (potentially) indoor use. Plus I've only got a little bit left.

What about this stuff? 12 litres sounds about right. Again though, there's the chemicals issue.

http://www.screwfix.com/app/sfd/cat/pro.jsp?cId=101091&ts=82367&id=12662#

 

[JohnD]

That's a preservative, not a water repellent. So it will delay rot and worm, but won't stop the ply and MDF swelling from damp. I find permethrin smells quite strongly.

The fence stains I had in mind aren't preservative, they just leave a waxy stain which repels water but allows it to breathe. They are water based and quite cheap (esp if you use B&Q or Wilkinson own-brands). I don't think they contain any significant chemicals.

Cuprinol shed and fence is a spirit based preservative with some waxy stain added, looks like good stuff. I used some recently as it was on offer at Wilkies (Cuprinol Green is usually about £20 for 5 litres)

 

[Thermo]

Cuprinol shed and fence is a spirit based preservative with some waxy stain added, looks like good stuff

that definetly will stink the house out for days if not weeks. when ive done fences with it you can smell it in the garden for days afterwards, and thats in the fresh air!

 

[JohnD]

Then I'm still drawn to the water-repellent stain. Seems to me we're trying to protect it from damp. Just the sort of thing water-based fence stains do. Also cheap and don't smell.

 

[big-all]

if any mdf has got wet for more than a passing spray its likly to be shot

your ply dependant on type may be fine you will only know if its shot bylooking at it and or loading it up

 

[David P]

I hate to be a misery but there isnt much point in slapping any waterproofing on the boards you have. the reason is that the moisture content of the air alone is like to cause the ply to delaminate and the mdf to swell. So even if you cut away the bits that have already got wet you will probably find that the boards will be different thicknesses when you come to try butting them together, not to mention the strength of the ply being seriously compromised.

if you want to use the boards for anything at all get them inside before the weather starts to get wet.

oops too late.

Have you considered that you may need to strengthen your roof joists to take the not insubstantial weight of 24mm of ply and mdf? mdf being almost completely useless as a rigid floor but absolutely fabulous at adding loads of functionless weight.

 

[Deluks]

Not planning on doing this for some time so the smell of preservative probably wouldn't be a problem anyway. I would be doing the full monty loft conversion so they would be taken into account when working out loadings. (20kg each) The mdf layer has been routered with a v-groove to give a 2' square tile effect, and painted thick with some pretty hardwearing black paint/stuff. As it was used for a stage floor it had to be very tough, and still looks ok. Although it would do as a floor it will probably get carpeted over anyway, so not too fussed about the finish. If I manage to squeeze a bathroom in there then I'll lay the boards mdf side down and tile onto the ply side.

There are only about a quarter of the boards affected, I was really only enquiring as to potential damage to the whole lot, like how fast can rot spread if the damp ones started to go bad.
There is the exact square metre-age to do my entire loft space, that's what made me put them aside for that purpose, I had planned on using them as garage roof, before thinking loft storage>full conversion.

This is becoming a bit of a hot topic, all about some damp plywood

 

[big-all]

do not under any circumstance use normal mdf anywhere near water ESPECIALY a floor

green mdf if in occasional contact with water but still needs full sealing for protection and not for flooring