Proven Methods to Prepare a concrete slab for wood flooring

concrete slabs are a common substrate in single-story buildings and on the first floor of multiple-story ones. in much of the south and west, slab-on-grade construction is standard. in the north, most homes have basements and owners often want to finish them.

can concrete slabs bring their own problems to flooring installation?

of course yes, first, because they’re installed on the ground unless detailed properly they can be a direct conduit for moisture. And unless you’re there to see the slab installed, there’s no way to know for sure if it’s properly detailed. And unless expertly placed and finished, concrete slabs are notorious for being out of flat.

Can you put hardwood floors on a slab? installing a wood floor over a concrete slab may require building some sort of wooden subfloor assembly first.

Which way do you lay wood flooring on concrete?

There are a number of ways to install wood floors over concrete (including direct glue-down, floating subfloors, and the screed method), but in all cases, the slab should be clean, flat, dry, and structurally sound.

Before You Start

the slab should be clean, flat, dry, and structurally sound. The concrete should be flat within 3⁄16 in. over 10 ft., or 1⁄8 in. over 6 ft. if it’s not, the remedy is to apply a self-leveling compound such as Bostik SL 150 high-compressive strength, self-leveling, cement-based underlayment to level low spots and to grind down the high spots.

humidity affects the cure rate of moisture-cured adhesives to a greater degree than temperature—the higher the humidity, the faster the cure. Under normal conditions, light foot traffic is acceptable after 8 to 10 hours and normal traffic after 24 hours.

Install the Vapor Retarder First

concrete slabs are like large rock sponges. They have the potential to supply large amounts of moisture that can damage wood flooring.

Be sure to perform proper moisture testing, although testing indicates only that the concrete is within the correct moisture level range at that particular time.

Because of concrete’s potential to supply a tremendous amount of moisture to wood flooring, I use a vapor retarder with a lower perm rating than I do over a wood subfloor.

installing a vapor retarder with a perm rating 0.13 or lower between the concrete and wood flooring is imperative to avoid future problems. there are three common ways to do this.

You can use a minimum 6-mil construction grade polyethylene film. The plastic should meet Astm d-1745 (the information should be on the package) and have a high tensile, tear, and puncture resistance.

in the past, it was common to apply two layers of #15 asphalt-saturated paper, adhering both layers with an appropriate adhesive (usually asphalt mastic).

Because of concerns about VOCs, this method is now rarely used. normal sheet vinyl may also be an effective vapor retarder, but you cannot use cheap adhesive to install it (cheap adhesive generally fails when subjected to moisture).

finally, a urethane membrane such as Bostik mvP4, or other chemical system accepted by the flooring manufacturer, can give good results. I prefer this method.

Direct Glue-Down

Once the slab is level and a vapor retarder such as mvP4 urethane applied, it’s possible to lay some wood flooring directly on top of concrete. strip flooring under 1⁄2 in.

thick, solid parquet flooring and engineered wood flooring can be glued to concrete, though it can be a challenge to force and hold the pieces of solid wood flooring together while the adhesive cures.

even though some adhesive manufacturers warrant it, I don’t recommend solid (3⁄4 -in.-thick) strip flooring and plank flooring for direct glue-down to concrete. many boards are not perfectly straight and the tongue and grooves don’t fit tightly.

This is relatively easy to work with when you’re using a flooring nailer that both helps to force the boards straight and then mechanically fastens them to the subfloor.

however, there are no mechanical fasteners used when directly gluing flooring to concrete, and it’s extremely difficult to draw imperfect flooring tightly together and then hold it there while the glue sets.

however, the practice is becoming more common in order to save money when installing over concrete. gluing flooring directly to the concrete slab is the most economical method.

in addition to direct glue-down, there are several other ways to install wood floors over concrete. They all involve creating some sort of framing or subfloor between the concrete and the finish flooring.

The most cost-effective of these methods is to create a floating subfloor composed of two layers of plywood. All other methods require the expense of gluing or fastening the subfloor and any framing to the concrete slab.

Screed Method

Screeds are generally pieces of 2×4 nailed, screwed, or glued to the concrete slab. Since it is difficult to obtain perfectly straight lumber, pieces shorter than 48 in. are used to be sure they conform to the concrete.

Standard 3⁄4-in. strip flooring can be installed directly over screeds, and thinner engineered flooring can be installed over screeds if there’s a 3⁄4-in. plywood subfloor installed first.

Using Screeds or Sleepers

nailing hardwood flooring directly to screeds (or “sleepers”) installed over a vapor retarder is a common method of installing over concrete slabs. screeds or sleepers are usually made from kiln-dried, pressure-treated 2×4s laid flat and fastened to the concrete with masonry nails or screws.

tongue-and-groove solid board flooring at least ¾ in. thick and less than 4 in. wide may be installed directly over screeds spaced 8 in. to 10 in. on center. no subfloor is needed, though an alternative is to space the screeds at 16 in. and add a layer of 3⁄4-in. subfloor.

Because boards are typically less than 3⁄4 in. thick, engineered wood flooring generally is not recommended for installation directly over screeds. for many years it was common practice to lay screeds directly on their flat face into hot (poured) or cold (cutback) asphalt mastic.

The end joints were staggered so the ends lapped at least 4 in. As you might imagine, the smell of the asphalt drove this method from favor, and urethane adhesives have been substituted instead.

I generally do not use screeds. They’re more expensive and labor-intensive, and adhesives have improved so much that most floors are directly glued to the concrete or installed on a floating plywood subfloor. (today, screeds mainly reside under athletic floors.)

About the only time, I use them is if the concrete slab is in such bad shape that other methods won’t work, or if insulation is required under the floor.

Plywood Subfloor on Slab

Plywood nailed or screwed directly to a concrete slab is an economical alternative to screeds. Additional layers may be necessary to accommodate the length of fasteners.

Mechanically Fastened Plywood Subfloor

one way to install a plywood subfloor over concrete is simply to nail or screw it into place. The minimum recommended plywood thickness is 5⁄8 in., and it must be thick enough to accept the length of fastener required by the wood flooring being installed.

The plywood needs a designation of exposure 1 to ensure it’s manufactured with exterior adhesives. if pressure-treated plywood is used, it must be kiln-dried and not have an elevated moisture content (check with a moisture meter if you’re unsure).

fasten the plywood with either powder-actuated fasteners or concrete screws every 6 in. along the edges and every 12 in. in the field (at a minimum).

And fasten the plywood working out from the center to ensure it will lie flat. stagger the sheets of plywood, spaced 1⁄8 in. from other sheets and ¾ in. from vertical obstructions.

Glued-down Plywood Subfloor

plywood to a concrete slab creates an incredibly strong bond and minimizes the number of holes through the vapor retarder.

The minimum recommended thickness for the plywood is 5⁄8 in., and it should be designated as exposure 1.

You may need to use thicker plywood or multiple sheets so it’s thick enough to accommodate the flooring fasteners you intend to use. The plywood is generally cut in half, either lengthwise or in width, and its bottom is scored in about a 1-ft. grid pattern using a circular saw adjusted to cut to approximately half the plywood’s thickness.

These measures ensure the plywood is flexible enough to conform to minor irregularities in the slab. stagger the sheets of plywood, spaced 1⁄8 in. from other sheets and 3⁄4 in. from vertical obstructions.

I use Bostik’s Best moisture cure urethane adhesive for all such installations, applying the adhesive over Bostik’s MVP moisture retarder.

holding the plywood down firmly enough to make full contact with the adhesive while it cures is the biggest challenge. You can weigh it down (i often stack the hardwood flooring on the plywood) or shoot in concrete fasteners at strategic locations.

Floating wood Subfloor

A floating wood subfloor, which is the preferred method used by most wood flooring professionals, is made from two layers of minimum 3⁄8-in., exposure 1 plywood.

floating floors are not fastened to the concrete, so there are no holes in the vapor retarder and no glue to mess around with. The first layer lies in line with the walls. stagger both layers of plywood, spaced 1⁄8 in. from other sheets and ¾ in. from vertical obstructions.

lay the second layer at an angle (45 or 90°) to the first layer. (Plywood is generally only placed on a 45° angle when it needs to span multiple rooms. installing on an angle helps to avoid joints in the doorways, which might lead to problems with the finish floor.)

fasten the layers together on a 12-in. grid pattern in the field and every 6 in. around the perimeter. my floating subfloors generally consist of two layers of 1⁄2-in. plywood glued with construction adhesive and screwed together.

Floating Wood Subfloor

Two layers of plywood comprise a floating subfloor. The layers are fastened only to each other, and “float” above the slab. This method is the author’s preferred installation over concrete.

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