Subflooring subflooring...
August 26, 2013 7:26 PM   Subscribe

I'm installing cheap laminate flooring (I know, I hate it) over lousy floorboards that I won't replace, and I'd like to get a few years (of sock-footed padding like a cat) out of it. The floorboard plywood sheets are gapped and uneven at the gaps and the builder didn't use spiral or ring-shank nails (but I'm adding screws). Can I just fasten thin plywood subfloor over plywood?

Due to uneven expansion rates it's always recommended that one surface "float" (unfastened) above another, but I've experimented with screwing down even 7mm plywood to the floorboards and it's a surprisingly durable surface that doesn't buckle even with summer temps ranging from 50s F to 80s in the same week, 50s and 70s in the same day/night. Will one sheet of plywood eventually tear away from the other? It doesn't seem likely, but it's such a DIY sacred cow. I have very little moisture (wafting up from the basement) but plenty of ambient humidity, and I'm rolling down a primer-style vapor barrier (no plastic sheeting, thank you) that lessens the likelihood of humidity between the two layers of plywood. (Thanks.)
posted by Shane to Home & Garden (5 answers total) 1 user marked this as a favorite
 
Nah, this seems totally fine to me. You could glue and screw the thin layer of plywood. It's not going anywhere.
posted by humboldt32 at 1:00 AM on August 27, 2013


Maybe someone with more flooring experience can answer to this better, but from a physics standpoint I would think that screwing in another sheet of plywood would cover the gaps (which may not have any significant impact on the integrity of the laminate) but would either replicate the unevenness and/or squeak fiercely if you don't shim it a lot to get it level but still supported. And then depending on the amount of unevenness a shimmed-to-level 7 mm sheet of plywood is probably not substantial enough.

I think self-leveling mortar would be a better solution--this is exactly what it's designed for.
posted by drlith at 3:40 AM on August 27, 2013


I have done this without the vapor barrier on top of the original subfloor. I ripped up 25 year old carpet and padding and made sure that all of the staples and nails were pulled. I covered the entire subfloor with 3/8" sheathing grade plywood held down with a large amount of construction adhesive and deck screws that went through the old subfloor and into all of the floor joists. It worked wonders and covered up a ton of gaps in the old subfloor and made the new floor very easy to lay. It does not squeak or shake at all now. Just make sure to have a plug in drill to do it. It will make it go a lot quicker since I would chew through batteries driving in all of the deck screws.
posted by Nackt at 4:06 AM on August 27, 2013 [1 favorite]


Just to aid in your research, the sheets of plywood fastened to the joists are not floorboards; they are the subfloor. If you fasten down another layer of thin material such as lauan plywood, that's underlayment, not subfloor.

The uneven expansion rates (which are not at all an imaginary sacred cow) are mostly an issue when you're dealing with solid wood boards. Plywood and other engineered sheet goods like MDF, chipboard, etc. are much more stable across varying moisture contents. Still, I wouldn't recommend butting the sheets of underlayment tight against each other; tiny gaps to allow for just a bit of expansion without having the sheets press against each other are probably a good idea.
posted by jon1270 at 4:25 AM on August 27, 2013 [1 favorite]


I'm not an expert but that's what we did in a similar situation. After screwing down the new subfloor (quarter-inch plywood) normally, we walked all over it and any place that still squeaked we kept driving new screws into that area until it stopped squeaking.
posted by winston at 5:51 AM on August 27, 2013


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