Should I add a plywood underlayment between my chipboard subfloor and new wood floorboards I want to install?

Question

Question

I’d like to remove carpet in a bedroom, and install a 14mm thick engineered oak floor instead. (The floor boards will have a multi-ply core, like these ones).

Between the chipboard subfloor and the new floor boards, should I use a rigid underlayment (e.g. plywood sheet)? If so, how thick should the plywood sheets be?

Or, is some foam underlayment (rather than plywood) enough?

More context

• The house was built 12 years ago. The floors seem flat.

• Under the carpet I’m removing, the subfloor is made of chipboard, which is 3/4" thick. I don’t know the layout of the joists. Here's a picture

• The chipboard subfloor panels are in good condition and are generally level, except that some panels are 2mm higher than the next panel. Some nail heads protrude from the subfloor by 1mm.

• The floor subfloor doesn’t currently creak in the bedroom I want to install the new floor in. But it does creak in another part of the house.

• I haven’t yet decided whether the new floor boards will attach using a click system, or use tongue-and-grooves and nails into the subfloor.

• At my local floorboard store, I was told that a rigid underlayment wasn’t needed, and a 3mm flexible underlayment would be enough. But I’m reading different things in different articles online.

• If I didn't add an underlayment at all (i.e. if I added the new floor boards directly on top of the subfloor), the new floor would be exactly flush with the floor of the corridor leading to it. So any millimetre of underlayment thickness creates a difference in height between the room and the corridor.

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Thanks a lot for your advice!

JF

Answer 1

You are asking this question without having picked out the exact floor you want to install. Which means the answer you get will correlate directly with how good of a question it is (not very good yet).

First there are almost no engineered flooring systems that require nails anymore. The ones that are still out there are very expensive unless you are getting "cabin grade". The point of me mentioning this is is you are spending that much, just get wood. Even with the cheapest oak you can stain it darker, greyer, whatever to get a modern look.

The "good" engineered hardwood is pure wood (usually plywood backing) and thick and almost all are pound in tongue and groove. --Hint -- this is the only kind you should buy unless trying to do a cheap flip. So with this type the plywood isn't going to help the install. I have thrown down 1/2" ply before installing engineered but that is to combat an already noisy floor usually due to thin subfloor, wide joists, or OSB/chipboard.

Now you could just install it to be preemptive to a future issue but that is more of a call based on the build of the house, what you will be doing and just a general feel. Knowing the thickness of chipboard and joist layout would help. But giving advice on should I install a plywood layer??? Well almost all the times it helps but in your case will it be worth it?

The thing that doesn't make sense here are your height calculations. I don't understand how padding+carpet=engineered flooring with no underlayment. These should be close to a 1/2" off. As far as it meeting up with rest of the house... Transitions in a doorway are really easy and no big deal. If we are talking about 1-2 doors not even an issue. If you are going to misalign heights under tons of thresholds, that isn't good planning.

Big thing here to keep in mind is that if you aren't putting down 1/2" plywood or more then the ply layer is useless for your intention.

Answer 2

The proper answer to your questions depend on what floor you decide to use. If you choose the nailed down, no padded underlayment is needed. If the choice is a floating floor,(click system) it is best to look at the manufactures recommendations.

In either case any nails, staples tacks, etc. should be hammered flush with the floor.

If you decide to go with a floating floor and use a thin padding it will cover any unevenness between the subfloor panels if those are just 2mm.

Bottom line is pick your final flooring before acting on any additional subflooring.