The subfloor is the solid layer under your finished floor. In a house upstairs, it is usually wood panels on the joists. In a basement, it is often the concrete slab itself, or panels laid on top of it. Your new floor is only as good as what is under it. A wet, cold or bumpy slab can ruin a good floor. This guide explains, in plain words, how to check the slab, when to add a subfloor panel, and when to level the floor first.
It is the base your floor rests on. It has to be solid, flat and dry. The finished floor (vinyl, laminate, tile or carpet) goes on top, sometimes with a thin underlay in between. The underlay is not the subfloor. It is a thin layer for comfort, sound or a smoother feel. FloorBox carries both: Membranes & Underlayments for the thin layers, and Plywood & Cement Board for wood and cement panels.
Yes, it can be. Many floors can go straight onto a slab if it is flat and dry enough. The question is how damp the concrete is. Concrete can look dry on top and still let moisture come up from below.
Test it before you buy the floor. Two common tests are sold at FloorBox:
Each flooring brand sets its own limit. For example, the Torlys guide for its click vinyl asks for 80% relative humidity or less, read with a probe in the slab. Check the guide for the floor you pick. More tools are in Moisture Testing Tools.
Panels lift the floor off the concrete. This helps when you want a warmer, drier feel underfoot. The Barricade Air Plus Subfloor Panel 2' x 2' x 3/4" is OSB wood on a high-density plastic base. Its page says it helps protect against the moisture that causes mold and mildew, and that it works under any finished floor. The panels fit together with tongue and groove edges, so they go down like a puzzle.
Want more warmth? The Barricade Insulated Subfloor Panel 2' x 2' x 1" has foam under the wood. Its page gives an R-value of 2.7, which is how well it holds heat in, and says that is twice the insulating strength of Air Plus. Both panels are about 2 ft square. Air Plus covers 3.75 sq ft per panel and Insulated covers 4 sq ft. All Barricade products are on the Barricade page.
Panels add height. The Air Plus is 3/4 in. thick and the Insulated is 1 in. thick. Check doors and stairs before you start.
Most floors need a flat base. The Torlys guide for its click vinyl asks for a floor flat within 3/16 in. over 10 ft. Small dips can be filled with a floor patch. A larger area can be covered with a self-leveling underlayment. This is a liquid mix that you pour, and it spreads to make a flat surface. The FloorBox Levelers page explains this choice and says many systems need a primer first. Rollers, pins and rakes for the pour are in Self-Leveling Tools. Our guide What Is Self-Levelling Underlayment? explains how the pour works.
Tile and grout can crack when the floor under them moves. The Schluter DITRA Uncoupling Membrane is a thin plastic sheet that goes between the subfloor and the tile. Its page says it helps prevent tile and grout cracks, and that small spaces under it let moisture and vapour escape from the floor below. It can go over concrete, plywood and OSB. It is only 1/8 in. thick, so it adds very little height. Similar products are in Uncoupling & Underfloor Heating Membranes.
It depends on the floor. The FloorBox Vinyl page says most vinyl floors do not need one if the base is smooth and level. It also says an underlay helps if the subfloor is uneven, and can add quiet and warmth. Many click planks already have a pad attached. Separate pads for vinyl are in Vinyl Underlayment.
