Subfloor Query

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16 Jul 2020
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Apologies if I'm in the wrong place ...

I'd be grateful if anyone can advise about the following as I feel i'm going in circles with what should be a very simple issue.... (I'm neither a woodworker or builder).

OK, I'm in West Cork, in a small 2 bed bungalow, 1930s or. I've decided to replace all the joists, boards and plates with new timber (and I have a friend who can help). Now that the floor is up I'm standing in an approx one and a half foot drop to ground level - and when I say ground, that's exactly what I'm standing on: stone, clay, rock, but nothing sinister. A few of the air vents needed clearing out, and I've done that, and am happy that I'll have good cross-ventilation going forward.

My main problem, though, is the subfloor material. I hoped to buy MR P5 [green] Norbord or similar, but I can't get this anywhere in Ireland. Maybe right up north, but that's a 600 mile round trip for 25 8x2 sheets....

My 18 mm options, then, are

OSB (grade 3?)
Marine Ply (though really expensive)

The void beneath the subfloor won't get wet, and I don't even believe it'll be damp, but I want to put reclaimed parquet on top of the subfloor, and as there will be considerable effort and money involved I'd like the subfloor to be as resistant to any damp buildup as possible.

I'm not concerned about insulation, nor strength, nor appearance, nor squeaks, just the best chance I might have of resisting some damp-ish air in the winter months blowing through the vents and creating problems....

Has anyone had the same issue to contend with? In the absence of P5 MR Norbord, is Marine Ply the only and best option...... Or would WBP be a safe bet? I had a man in a local hardware shop tell me about MR MDF, but that didn't sound quite right for my job, and I wondered about the suitability of MDF and fixings.

Would be really pleased if anyone has advice or ideas, or better still, experience of dealing with a similar problem (west of Ireland can't be all that different to many places 'across the water').

cheers :shock:
Norbord........that's a chipboard flooring, is it? Your issue is just availability of material, then, is it?

OSB just isn't suitable. Marine ply is unnecessary. WBP ply is the only one from your list I'd consider......but I'd be looking for some t&g flooring ply, or a source of chipboard flooring instead. And for the pennies extra, I'd be going for 22mm.
As long as there is a good cross airflow beneath the joists damp shouldn't be a problem. I'd check the vent positions and make sure there isn't any 'dead' space and add vents if necessary. Ply should be fine if you can't get anything else, fit noggings to pick up the board edges including the perimeter, can seal around the edges with expanding foam too.
many thanks Mike.... appreciate it.

yes, the Norbord is supposed to be v good (though this is all hearsay, and I really can't say, other than repeat what I've seen written on other forums).... in any case, I can't get it, so i'm a bit stuck on that one.

do you think WBP would be a sound decision, or do you have any other suggestions (18 or 22 is fine, it's more best material for the job that's my issue)....

you say a 'source of chipboard flooring instead'.... would there be anything in this line you'd think might work?

would t & g make for a better 'seal'?

many thxs!
thanks WSB for the comments.... yes, you're right about dead space... there's one little bit at the end of the hall that leads to a small bedroom, which at some stage had the floor ripped up and concrete poured.... it's working fine as far as i can see, no cracks or anything, but it's cut off a very useful vent that used to dry out the end of the hallway, and i need to see about that.............. considered putting a floor vent in at that point, when i've the new flooring in place (one of those vents with a sliding open and shut thing on it, rather than a completely open grill), but someone said i'd be taking all sorts of risks.... without a barrier/membrane, anything could come up the vent (i wasn't concerned about a bit of cold air; can stick on another jumper)...

will bear in mind your suggestion about foam around the edges though! thanks again for your ideas.
T&G joints get glued, and virtually eliminate squeaking. That's their point. Any MR chipboard flooring would work in your circumstances, which are absolutely bog standard.
OK, I see what you mean.... sounds like I'm going to have to dig deeper for moisture resistant boards (a lot claim to be MR, but only ones that have the P5 grade are properly so...... like hen's teeth in these parts though!). cheers!
Having trawled before laying my woodblock floors , none of the pros I saw writing about it would lay direct on chipboard, either they rip up and lay on >22mm T&G plywood or (more common) put down a layer on top of the existing chipboard or flooring boards of (usually) 6mm WBP ply gun-stapled down to the chipboard with divergent staples, laying the ply crossways to the chipboard.

Mike might say I am wrong or wasteful or both, but in my epically slow renovation I always put a DPM underneath any flooring facing down to more damp.
thanks for this Jake........... getting a bit late for me to be picking your brains, so by all means leave responding till another time, but let me ask you this anyhow: when you say you put a DPM underneath the flooring facing down to more damp, did you have the same sort of suspended floor set-up as myself? and if so, did you just lay the dpm onto a 'cleared' earthen area, or was there any particular additional prep involved?