How do I float 3/4" plywood sheets over above grade concrete without nailing, screwing or gluing to the slab?

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Hello mistersneak,

Thank you for your question and welcome to the community!

I personally wouldn't recommend to install plywood directly over concrete for your engineered floor. I would only opt for this if you first seal the concrete floor....if it is rated below grade. Knowing what the manufacturer of your bamboo states for install will determine if it can be glued or nailed down.

The best wood-based foundation over concrete for glue, nail, or even floating systems would be to create floor members via studs under plywood. This is really needed for below grade concrete floors. The diagram below shows more of this in further detail.

The materials in the diagram above are really the best bet if you still want to use wood as a substrate to go between the concrete and your bamboo. 4 MIL or thicker plastic works great for a vapor barrier and exterior grade plywood is really your best bet if you wish to do this type of setup.

I would strongly...

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Thanks for the feedback!

It's a 35 year old 1st floor slab. It’s had Parquet hardwood for 10 years with no problems. The slab is very dry - the Polyfilm Test shows no moisture.

SUBFLOOR METHODS “RSEARCHED”
Floating – Mainly DRIcore. Eliminated because Armstrong said it would void the warranty.

Glue down – Plywood system over Bostik's Best and sleeper system in mastic. Sleepers eliminated due to added height. Glue down was "eliminated" in general because nailing it down seems sufficient (see original post).

Nail down – Plywood over vapor barriers vs. plywood over Delta FL (and Delta look-a-likes).

* Armstrong (I’m going with Bruce stuff) is not prescriptive on the issue of subfloor - their main guidance is the usual "Dry, Flat, Structurally Sound" etc. phrasing which covers a multitude of sins.

* NOFMA recommends plywood-on-slab or sleeper sub-flooring systems for concrete. Either is for 3/4" hardwood flooring up to 4" wide. They state to...

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Suggestions for installing a solid wood, "floating" floor over concrete. June 6, 2001

Question
I have purchased solid Indonesian hardwood flooring (18mm kiln dried T&G pre-finished planks designed for false floors) to install over an existing concrete slab. The instructions I have are for secret nailing to 38 mm joists, which would end up too high. I wish to glue it as a floating floor. Ideally, I'd glue the T&G, as is the practice with the laminate floating flooring. I have been told, however, that this can be joined in such a manner because it is more stable and that I need to cover the entire concrete slab with a flexible adhesive and glue each plank to the concrete, similar to tiling. I am in Australia, if that causes different conditions.

Forum Responses
You need a membrane between the concrete subfloor and wood floor. I suggest laying out felt paper over the concrete, sealing the joints with an asphalt adhesive and then laying the wood floor...

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Floating wood floors over concrete

Suggestions for installing a solid wood, floating floor over concrete. June 6, 2001

Question

I have purchased solid Indonesian hardwood flooring (18mm kiln dried TG pre-finished planks designed for false floors) to install over an existing concrete slab. The instructions I have are for secret nailing to 38 mm joists, which would end up too high. I wish to glue it as a floating floor. Ideally, I’d glue the TG, as is the practice with the laminate floating flooring.

I have been told, however, that this can be joined in such a manner because it is more stable and that I need to cover the entire concrete slab with a flexible adhesive and glue each plank to the concrete, similar to tiling. I am in Australia, if that causes different conditions.

Forum Responses

You need a membrane between the concrete subfloor and wood floor. I suggest laying out felt paper over the concrete, sealing the joints with an asphalt...

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Alternatively, screw rows of pressure-treated 2- by 4-inch studs at 16-inch intervals over the polyethylene sheeting. Ensure the stud rows are set perpendicular to the direction you intend to install the floor planks. Nail sheets of 3/4-inch pressure-treated plywood on top of the studs, and nail the wood flooring planks on top of the plywood. This reduces the amount of wood in contact with the concrete.

Another method is to use a plywood sub-floor, which involves gluing wood flooring planks to the slab. Remove all the baseboards using a pry bar, and remove any paint from the floor using a floor grinder. Clear the floor of dust and debris, and use a V-notch trowel to apply a liquid moisture barrier. Let the barrier set for 24 hours. Plan out your flooring layout, making sure to lay the planks parallel to the longest wall in the room. Use a V-notch trowel to apply the urethane wood adhesive onto the floor, and only spread enough for a few rows of flooring at a time. Use a...

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If you love solid wood floors and your home rests on a concrete slab, you're not out of luck. But it's considerably more difficult to install a 3/4-in. wood floor over concrete than over a wood subfloor. You'll first have to install 3/4-in. thick wood underlayment. The combination of the underlayment and flooring raises the height of the floor 1-1/2 in., so be sure the extra height doesn't create problems around cabinets, in doorways or at transitions to other types of flooring.

Also, be absolutely sure that the concrete doesn't get damp because of exterior water problems.

Lay 6-mil polyethylene sheeting over the concrete to minimize moisture migration from the concrete up into the wood flooring. Then fasten 3/4-in. treated plywood to the concrete with concrete screws spaced every 16 in. Alternatively, screw rows of treated 1x4s to the concrete 16 in. on center. Lay the 1x4s perpendicular to the direction you intend to install the floor planks.

Then nail the...

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