Flooring Correct
Can engineered hardwood float over concrete?
Yes, and floating is the usual method. Once it is chosen, two things underneath do the work: the underlayment, which separates wood from concrete and carries the vapor retarder, and the gap left at every wall, which gives the connected plank field somewhere to go when it expands. Close that gap and the floor lifts.
Start where the slab question leaves off
Floating is one of three ways wood goes over concrete, and which of them is even available to you is settled by the slab and by the product before preference gets a vote. <a href="/answers/can-hardwood-be-installed-on-a-concrete-slab/">Our page on hardwood over a concrete slab walks the moisture tests by name, the flatness prep, and the glue-down and built-subfloor alternatives</a>, and engineered is the construction that makes any of it work for the reasons the engineered hardwood page sets out. This page picks up after all of that. The slab has been tested, floating is the chosen method, and what is left is the part of the assembly nobody looks at. The plank field is only half of it. The underlayment beneath it and the gap around it are the other half.
What the underlayment is actually doing down there
It is easy to read underlayment as padding, something soft that makes a floor feel better underfoot. Over concrete it is doing four jobs at once. It breaks the contact between wood and slab, so the planks never sit against the material that is moving vapor upward day and night. It carries the vapor retarder, either built into the roll or laid as a separate sheet beneath it, and that layer is what holds the moisture back rather than the click joints above it. It bridges the fine variation left in a surface that was poured flat rather than machined flat. And it gives the field something to slide on, which matters more than it sounds, because a floating floor has to stay free to move as one piece. That is why the underlayment gets chosen against the slab and the specific line, not by how it feels in the hand.
The gap at the wall is structure, not a tolerance
A floating field is one connected object. Every plank is locked to its neighbors, so the whole room expands and contracts together instead of board by board, and it has to have somewhere to do that. The perimeter gap is that somewhere. It is not a sloppiness allowance and it is not optional. Baseboard and shoe moulding cover it, fastened to the wall rather than down through the floor, so the trim hides the gap without taking it away. The same clearance belongs anywhere the field runs into something that cannot move.
- Every outside wall in the room
- Both sides of a doorway, and around the jamb legs
- The toe kick of a cabinet run or a built-in
- A hearth, a column, a stair stringer, a pipe collar
How that gap gets closed after the fact
This is the failure people actually live through, and it usually arrives months after the crew has gone. Shoe moulding gets nailed down through the plank instead of into the baseboard, and the floor is now fastened at its edge. A bead of caulk gets run into the perimeter to stop a draft, and it cures into a bond. A threshold gets glued to the flooring on both sides of a doorway instead of anchored to what is underneath it. A heavy fixed object lands on top of the field, an island, a built-in bookcase wall, a stone hearth surround, and pins it in one place while the rest of the room is still trying to move. Every one of those turns a free field into a trapped one, and a trapped field relieves the pressure the only way it can. It lifts. A plank stands proud at a seam, a ridge rises somewhere across the middle of the room, or a lock joint pulls apart and will not go back.
The two questions worth asking before the order goes out
Was the slab actually tested for moisture, or was it assumed dry because nothing looks wrong today. And is the specific line you are choosing rated by its manufacturer to float over concrete, and with which underlayment and retarder under it, because not every engineered product is rated for it and the ones that are do not all want the same layer beneath them. Both answers live on the spec sheet, and both are worth asking for by name at the counter rather than taking on faith.
None of it gets settled over the phone. The free in-home measure is where the slab gets tested instead of assumed and where the underlayment and vapor retarder get matched to the line you are leaning toward, and our engineered hardwood page covers the constructions on the showroom floor. If the room is below grade, the basement version of this question has its own page. If it also carries in-floor heat, the method changes again. Call 636-332-0654.
More on floating engineered hardwood over concrete
Which lines are rated for a slab, what the vapor retarder is actually holding back, and where the gap belongs besides the walls.
636-332-0654Not automatically. Most do, since floating is the installation method built for a slab, but a handful of thinner or lighter-duty lines are only rated for a wood subfloor. The spec sheet for the specific line settles it, and it is worth asking to see that page before you commit to a product for a slab room.
The plank field is protected from the slab by the underlayment and the retarder together, not by the click-lock joints alone. Skip the retarder and the underlayment absorbs moisture it was never built to handle, and that tends to show up later as swelling at a seam or a musty smell rather than anything visible on day one.
Yes, and the ones people miss are inside the room. A floating field needs clearance wherever it meets something that will not move with it: doorway jambs, the toe kick under a cabinet run, a hearth, a column, a stair stringer, the collar around a pipe. One tight spot is enough to pin the whole field, because the field only moves as a single connected piece.
Get your slab tested before you order
Tell us it's going over concrete and roughly which rooms. We test the slab in person, match the underlayment and vapor retarder to the line you're leaning toward, and quote from that.
Free estimate
636-332-0654