Sunday, March 29, 2020

What’s a Subfloor?

By Crystal Hosking, Hosking Hardwood Flooring

The Foundation of a House

One of the most important factors of a house’s structural integrity comes from how strong the flooring system is. Essential elements include strong foundation footings, properly spaced support columns and appropriate sized and spaced floor joists.

The consequences of a poorly structured flooring system could definitely affect how flooring choices in the future perform as well as the structural integrity of your home. Movement of the floor system, sagging subfloors, unbalanced heights, separation of flooring and squeaky subfloors can all develop from inadequate floor foundations.

Types of Subfloors

Once you have a stable flooring foundation, your subfloor creates a base for your actual flooring. Whether you’re building a new home or renovating an older one, it’s important to know the differences and benefits of standard subfloor types. Knowing what your subfloor is will help guide you to what type of flooring you can use as well as how much work will be involved with installing a new floor.

**Keep in mind that all manufacturers have their own installation instructions. The information provided below is for generic purposes only and we always recommend consulting the specific installation instructions for the particular brand you’re installing prior to installation to avoid voiding that manufacturer’s warranty.**

Plywood Subfloor

Plywood has been the most popular material used for subflooring for homes built within the last 40 years. Plywood is typically made of cross layers of birch or pine. The cross layers of plywood offer a myriad of benefits. They reduce splitting, reduce expansion and contraction and reduce warping. Overall, a plywood subfloor’s cross layers will increase the collective dimensional stability of the foundation. Plywood subflooring is typically either 5/8 IN. or 3/4 IN. thick and comes in sections of 4 FT. x 8 FT. sheets. CDX plywood sheets have tongue and grooves, allowing for the sections to be connected to each other for a strong connection. It’s recommended, when installing, to apply a thick bead of subfloor caulking adhesive to the flooring joists before using 2-1/2 IN. deck screws (every 8 IN.) to attach the CDX plywood subfloor.

Plywood subfloor

Plywood subfloor

As long as it’s securely attached, flat and not flexing, plywood is the perfect subfloor to install virtually any type of flooring over (solid, engineered, click lock floating or laminate). If you’re installing a floor type that is thinner than 1/2 IN. thick, you may want to consider strengthening the subfloor with an additional layer of 3/8 IN. or 1/2 IN. thick plywood.

If you’re stapling or nailing down, it’s important to use a layer of 15 lb. black felt paper between the plywood subfloor and the new flooring to prevent transfer of anything that may travel up through the subfloor. For floating applications, you will need to use the manufacturer’s recommended foam underlayment.

Plank Subfloor

Sometimes, in older homes, homeowners will tear up old flooring and find that their subfloor is wood planks. These planks are usually 3/4 IN. thick and anywhere from 4 IN. to 8 IN. wide. Typically the planks are made from pine boards which are nailed to the flooring joists. When installing new flooring over plank subflooring, it’s important to make sure each plank is securely fastened to the flooring joists and, if not, re-secure them prior to starting the new install.

Plank subfloor

Plank subfloor

Once planks are deemed secure and flat, you can pretty much install virtually any flooring type over this subfloor (with a few caveats). When installing solid flooring, it’s important to install the length of the boards perpendicular to the subfloor planks, otherwise you run the risk of having the new flooring pull apart with expansion and contraction of the plank subflooring. If running the new solid board in the same direction as the subfloor planks is important in your situation (running the boards parallel to the longest wall will make the room appear larger), you can install a layer of plywood (3/8 IN. or 1/2 IN. thick) over the plank subfloor (glued and screwed down) to increase strength. The same goes for installation of engineered flooring when it’s stapled or nailed down to the plank subfloor.

If you plank on floating an engineered floor (or a laminate), using a click lock floating floor, it’s important to make sure your plank subflooring is flat. If there are discrepancies, you will need to add a layer of 3/8 IN. or 1/2 IN. thick plywood (glued and screwed down) prior to installation. With floating installations, as with solid/engineered staple or nail down installs, you will want to install the boards perpendicular to the planks in the subfloor. However, if you add that extra layer of plywood, you can install in any direction you’d like.

Oriented Strand Board (OSB)

OSB is a surface made from wood chips glued together. Usually OSB subfloors are 3/4 IN. thick. These subfloors are typically both glued and screwed down to flooring joists, as this will make the OSB more stable and will eliminate squeaking.

OSB subfloor

OSB subfloor

Solid hardwood, engineered hardwood, click lock floating floors and laminate can all be installed over OSB subfloors. It’s always recommended to install flooring planks perpendicular to the flooring joists for added stability. When stapling or nailing floor boards down, as always, 15 lb. black felt paper is recommended as an underlayment and a foam underlayment is recommended for floating applications.

If the OSB subfloor seems weak or flexible in some spots, it’s recommended to add a layer (glued and screwed down) of 3/8 IN. or 1/2 IN. plywood to the subfloor for added stability.

Concrete Slab Subfloor

Concrete slabs are usually poured everywhere from 4 IN. to 6 IN. thick. Keep in mind that when concrete floors are first poured, it could take 3 months or more for the water in the mix to evaporate and for the subfloor to dry out. It’s important to perform a moisture test on any concrete slab subfloor prior to installing hardwood flooring. Additionally, most manufacturers will have guidelines as to the limit of how uneven a concrete floor can be for installation of their product. Installers need to make sure to refer to these guidelines as warranties will be void if the differential across the subfloor is too high. Old, uneven concrete subfloors can be fixed easily by using a self leveler. Self leveling agents dry within 1 to 8 hours, depending on the thickness of the pour. If any adhesive residue from previous flooring has been left on the concrete, it needs to be cleaned off with a scraper.

Concrete slab subfloor

Concrete slab subfloor

3/4 IN. thick solid hardwood flooring¬†cannot be installed over concrete slab subfloors; there is just way too much moisture transferring up from the concrete and the thick solid can’t handle it. The results would be cupping, buckling and warping of the boards. If solid hardwood flooring is an absolute necessity over a concrete subfloor, a sleeper system can be built over the concrete slab or you can glue down and screw two layers of 1/2 IN. thick plywood into the concrete. Both options are quite labor intensive and will add significant height to your subfloor. With the majority of hardwood installs over concrete, the homeowner goes with engineered hardwood flooring.

Engineered flooring was designed specifically to go over concrete slabs and can be installed either by direct glue down or by floating. If gluing engineered down, it’s important to make sure the concrete subfloor is at less than 4% moisture year round. It’s also recommended to use a water resistant adhesive with included moisture vapor protection. When floating an engineered floor, click lock floating floor or a laminate over concrete, it’s important to use the recommended foam underlayment with an attached vapor barrier. For extra protection against moisture transfer, we also recommend using an additional 6mil plastic sheet over the concrete.

Particle Board

Particle board

Particle board

Particle board is a material usually found when old carpeting is ripped up. It is not structurally appropriate for installing new flooring over and is basically a glorified cushion. Particle board has excellent potential to absorb extra moisture and fall apart, becoming burdensome to any type of flooring installed directly over it. Additionally, particle board does not hold staples or nails well. Hardwood flooring adhesives have the potential to eat right through it over time. It is highly recommended (and usually required by manufacturers) to take up any particle board prior to installing new hardwood flooring. If absolutely necessary, engineered, click lock floating or laminate flooring can be floated over particle board but it’s definitely not recommend because of moisture and structural integrity problems. Additionally, with most manufacturers of hardwood flooring and laminate flooring, if you install over particle board, you’ll be voiding any warranty offered with the new flooring.

Installation Over Existing Flooring 

Sometimes, homeowners looking to save time and money inquire about installing new flooring directly over their old flooring. This option is typically not recommended. Several problems can emerge affecting the overall room appearance, door clearance (both inside doors and doors leading to the exterior of the home), appliance space and transitions to adjacent rooms. Installing new flooring directly over old flooring is also going to significantly add weight to the subfloor and floor joists, potentially compromising the structural integrity of the entire system. If installation over existing hardwood flooring is absolutely necessary, installing perpendicular to the existing hardwood planks will help to prevent gapping.

Installing new flooring over existing ceramic tile is also not necessarily recommended, but if absolutely necessary, engineered flooring, click lock and laminate flooring can be floated over existing ceramic tile as long as the tiles are still adhered securely to the subfloor. Of course, an underlayment would be required and it’s very important to make sure the tile floor is flat, with nothing protruding from the surface.

Fixing Old Subfloors

With use, everything has potential to wear out and become less pristine than it once was when first installed. Subfloors are no different. Prior to installing new flooring, it’s important to always check how your subfloor is holding up and fix any trouble spots. Plywood and OSB can become weak and flexible in spots. Plank subfloors can shift with expansion and contraction and become loose and/or unattached from the floor joists. Concrete subfloors can become uneven over time. Taking the time to fix these problems prior to installation of new flooring can prevent headaches down the road.

If you notice squeaky subfloors under flooring that you don’t plan on replacing anytime soon, there are solutions that don’t involve taking up your flooring. Squeaks are typically caused when the subfloor isn’t joined securely enough to the floor joists. Repair kits are available, enabling homeowners to use snap screws to secure loose subflooring through hardwood or carpet. Additionally, joist bracket systems are available if you have access to the flooring joists (from the room below) which will tighten the connection of the joists with the subfloor.

You may also be interested in: How to Fix a Squeaky Floor, Homeowner Expectations for a Hardwood Flooring Purchase


Tags: Installation, Subfloors

Comments: 5

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  • Kristen Caldwell

    If I just had a contractor install new plywood for a subfloor over water damaged, weakened and flacking 30 year old plywood should I be looking for a new contractor?

  • Hi Kristen,

    Unfortunately, since we’re not there to see the extent of the damage to the old plywood subfloor, we can’t judge it to be okay or not okay. If you walk across the new plywood and feel any flex, you may want to consider bringing in a different contractor for a second opinion.

  • ryan

    Hello, very informative website. Thanks

    Question: I am looking to install a new hardwood floor. Entails removal of tile from plywood therefore I must remove the thinset under the tile that is adhered to the plywood. After removing the thinset, what flooring installation method would you recommend over the plywood… glue down, nail down or floating? Also, is it possible to use a foam mat as an underlayment for the nail down option?

  • Bruce Gavin

    Please consider a comment on subfloors comprised of 2×6 T&G.

    My 1960 vintage house subfloors are entirely 2×6 with a joist span of 48 inches.

  • Hi Ryan,

    Over plywood, you can use any of those install methods. Make sure there’s no flex in the plywood when you walk across it. If there’s any movement, make sure to resecure to the joists with deck screws and in some situations, you may want to add another layer to plywood to add structural stability. You can’t use a foam underlayment if you plan on nailing down — those are only for floating installs. If you want to nail down, we recommend the 15lb black felt paper between the plywood and the new flooring. Or, if you’re looking for a little cushion or for better sound insulation, you can first staple/nail down a cork underlayment and then nail the flooring down. Just make sure when nailing the flooring that you are using nails long enough to go through the hardwood and the cork and down into the plywood.

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