How To Install Subfloor

Subflooring is a key structural component. and today I will share with you this informative article that I prepared after doing deep searches and of course as always with my experience in this field by giving the pieces of information and answering the questions as What thickness subfloor is recommended? Can I put plywood over subfloor? Is OSB or plywood better for subfloor? and more related other questions just keep reading and focus.

Installing a Subfloor

The type of subfloor you choose depends on moisture conditions and the flooring you will install. Once you have the moisture issue figured out, prepare a subsurface suited to your flooring material.

  • If you will install carpeting or carpet tile, the subsurface need not be very smooth or very firm. You may need only to patch large cracks or holes in concrete, or you can install onto a plywood subsurface. Small imperfections will not show through noticeably.
  • If you want to install resilient flooring like sheet goods or vinyl tiles, the subsurface must be very smooth, but it need not be extremely firm. Use the existing concrete floor only if it is very smooth, the material will telegraph every dip and bump. It can be very difficult to patch a damaged concrete surface to this level of evenness, so you may need to apply a layer of the concrete backer board or a plywood subfloor as you see later.
  • For ceramic or stone tile, the subsurface does not need to be very smooth, but it must be very firm. The thin-set mortar that you set the tiles in will fill imperfections, but if the surface on which the tiles are set flexes at all, the tiles or grout joints could crack. Usually, these tiles are set onto the concrete surface or onto a layer of concrete backer board applied directly to the concrete. If you need to install a new subsurface that is insulated because the concrete gets moist, then you will need to install first a sheet of air-gap underlayment, then a layer of 3⁄4-in. plywood, and then top it with a layer of 1⁄2-in. concrete backer board.

Removing and cutting trim

Butting new flooring up against existing wall trim will certainly lead to a sloppy-looking result. Before you install any substrate and finish floor material, clear the walls of base trim and other obstacles.

If your baseboard has a base shoe at the bottom, you could just remove that and replace it after the floor is installed.

However, that could make the baseboard look a good deal narrower, depending on your installation. If you will reuse the existing baseboard, use two taping knives and a flat pry bar to keep from damaging the trim. If the trim doesn’t look great, it will look out of place next to your new floor, so plan to replace it with new trim.

Don’t Feed the Mold To grow, mold needs not only moisture but also anything organic. It’s not picky and will thrive on things like glue, wood, and drywall paper (as well as soap residue left on the surface of grout). So if your concrete is certain to get moist on a regular basis, avoid applying anything that mold likes to munch on. That includes mastic adhesive (often called “organic mastic”). The concrete backer board is free of organic mastic, as is the thin-set mortar.

Applying concrete backer board

Applying concrete backer board to a basement floor smooths out imperfections and helps consolidate the floor, it also offers a certain amount of protection against moisture infiltration.

It’s a good first step if you will install resilient flooring, and it is often done in preparation for ceramic or stone tile as well. Simply applying directly onto the concrete means you will raise the height of the floor 1⁄2 in., which may be a consideration if your ceilings are low.

  • Plan the layout. Chisel away any protrusions and sweep the floor thoroughly. Measure the room’s width and length, and plan for an installation that avoids any strips of backer board narrower than 6 in. or so. If the room includes cabinetry, measure to avoid narrow strips there as well.
  • Mix a batch of thin-set mortar. Use a notched trowel to apply an even coat to the floor.
  • You can take this opportunity to fill in small holes. Use long, sweeping strokes to form an even surface. Lower the first sheet into the adhesive.
  • If the wall it snugs up against is less than straight, you may need to adjust its position slightly so its outer edge forms a straight line. Position two or more sheets in the thinset, check for alignment and nudge them into position if needed, and then drive screws.
  • The thinset will provide most of the holding power, but you need to drive screws to ensure the sheets lie flat. Using a masonry bit sized to match the masonry screws you will use, drill a series of holes spaced 16 in. apart or so.
  • Drill additional holes where the sheet needs to be held down. The holes should be about 1⁄4 in. deeper than the length of the masonry screws you will drive; wrap a piece of tape around the bit to use as a depth guide. Periodically dip the bit in water to keep it from overheating. If the bit starts to smoke, take a break to let it cool down. Drive masonry screws into the holes so their heads are slightly below flush with the surrounding surface.
  • Wherever possible, position a sheet against an obstruction to mark for cutting. Elsewhere, use a tape measure. Here, a straightedge is used to mark for a notch cut. As much as possible, avoid small pieces, which can make the surface less than even. To cut backer board, you will need to slice deep enough to cut through the embedded mesh.
  • Then bend the piece back and finally cut through the mesh on the other side. Make at least some of the cuts with the sheet lying on the floor, so you can press hard. On a cutout like the one at left, you will need to cut with several passes on the short side until you slice all the way through; then cut along the long side. After bending the cutout back, slice through the mesh on the other side or cut from the front.

note: In small areas where a subfloor slopes up or has other problems, it is often best not to cover with backer board, this area can be filled with mortar, either now or when you set the tiles.

If you have a lot of backer board to cut, consider buying a special backer board cutting knife, which cuts deeply with ease. However, if you have a dozen or fewer sheets to cut, a knife works fine. This work dulls blades quickly, so replace the blade often

WHAT CAN GO WRONG Don’t try to make backerboard fit tightly. Make your cuts about 1⁄4 in. short, so the pieces will fit easily.

Can I put plywood or OSB over subfloor?

In order to install nailed down tongue-and-groove strip flooring, sheet goods, or resilient tiles, it is often best to install a plywood (or OSB) subfloor. It is possible to simply attach plywood (preferably pressure treated) directly to the floor, but that invites moisture damage, and attaching to concrete is a difficult undertaking. It’s better to install a subfloor along with an air-gap underlayment. This can be done by first laying out the membrane and setting plywood on top, or by purchasing 2-ft.-square OSB tiles that have a similar membrane affixed to their undersides.

Air-gap underlayment

  • This type of dimpled underlayment comes in large rolls that cover over 150 sq. ft. The type shown here has a top layer of foam; other types are hard plastic on both sides. It is roughly 5⁄8 in. thick and has insulating properties. Buy the special tape meant for the rolls. To install, roll out the sheets and place them side by side. Make any cuts with a utility knife. Cutting does not need to be precise; within 1⁄2 in. of the wall is close enough.
  • Fasten sheets together first with 12-in.-long pieces of tape across the seam every 24 in.
  • The sheets may not lie flat at first. Push gently with your knees as you apply tape along the seam to seal the sheets together. Be sure to press the tape firmly, so it forms a moisture-tight seal; otherwise, vapor could damage the plywood underlayment you will put on top.
  • To further encourage the underlayment to lie flat, slide a fairly heavy board along the surface. Engineered hardwood, laminate, or other floating-type floors can be installed directly on top of this product. If you want to install flooring that gets nailed down, install an OSB or double-plywood subfloor first for a nail surface.

OSB ON TOP OF AIR-GAP UNDERLAYMENT To install OSB or plywood underlayment on top of the air-gap underlayment, cut 3⁄4-in. sheets so their seams are offset by at least 2 in. The sheets should basically lie flat, but you will need to drive occasional screws at places where the sheets rise up. Drill masonry holes using a bit of the right size for your screws, then drive screws long enough to penetrate the concrete by at least 1 1⁄2 in.

INTERLOCKING UNDERLAYMENT PANELS An alternative to the rolled underlayment sheets is to install tongue-and-groove underlayment panels that have a layer of air-gap underlayment laminated to their undersides. Attach together by slipping tongues into grooves and cut using a circular saw.

PLASTIC SHEETING If you are confident that your floor will only occasionally have minor moisture, here’s a minimalist approach that has worked for many people over the years: Simply apply a double layer of 5-mil or thicker plastic sheeting to the floor. If you snap clear layout lines on the concrete, you can see them through the plastic.

Doubled plywood

If you will drive staples or nails to attach flooring, 3⁄4 in. of nailing, the thickness may not be enough and you don’t want your staples to penetrate the subflooring. First, lay air-gap underlayment. (Here we show a black version.) Lay one layer of 1⁄2-in. plywood on top of the underlayment with joints offset. Then lay another layer of 1⁄2-in. plywood with joints offset from each other and from the layer below. You will need to drive occasional 3⁄4-in.-long screws to keep the sheets from rising up here and there.

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