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A Guide to Subfloors Used Under Wood Flooring

By Catherine Liewen
December 2007/January 2008
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Ask any seasoned wood flooring contractor, and he'll tell you that your final installation is only as good as the subfloor you're installing over. Before you have your bundles or boxes shipped to the job site and start racking out the floor, you should take the time to carefully inspect the subfloor and fix any problems that could affect your installation. The industry mantra is that all subfloors should be clean, dry and flat before wood flooring goes down. Following these simple rules and understanding what you are dealing with will help lead to a successful installation.

Here are the common subfloors you'll deal with on the job and checklists to help you make sure the subfloors will be ready for your wood flooring installation. The following are general guidelines from NWFA's Installation Guidelines. For specific information, always follow the manufacturer's recommendations for the product you're installing.

Plywood and OSB Over Joists

Plywood and OSB subfloors over joists are most common in newer homes and new construction, typically in homes with basements or crawlspaces.

1) Suitable Wood Flooring: Any.

2) Thickness: Usually manufactured in 4-by-8-foot sheets, plywood panels and OSB are acceptable subfloors for hardwood flooring as long as they are the proper thickness. Keep in mind that for mechanically fastened floors, OSB doesn't hold fasteners as well as plywood, so you need to use a thicker material to get the same holding power. Generally, the minimum thickness for plywood is 5/8 inch; for OSB it's 23/32 inch. However, as joist spacing increases, the subfloors need to be thicker. For joist spacing of more than 16 inches up to 19.2 inches on center, the minimum thickness for both plywood and OSB is 3/4 inch. For joists spaced more than 19.2 inches on center, the minimum thickness for plywood is 7/8 inch and for OSB, 1 inch.

3) Check Before Installation: Once you determine the subfloor is the proper thickness, check that the subfloor is installed properly, with no issues that could affect your installation. Subfloor panels and fasteners should be properly placed according the panel manufacturer's specifications. If there are protruding fasteners, squeaks or loose panels, refasten the subfloor to the joists. The subfloor should also be clean and free of contaminants such as paint, sealer or drywall compound.

Inspect the subfloor from below and note the direction of the joists. With the above specs, the flooring should be installed perpendicular to the floor joists. The flooring can be laid parallel to floor joists, but you may need to add a 1/2-inch layer of plywood diagonally or perpendicularly to the joists, or install a brace between the joists.

4) Flatness: Ensure the subfloor is flat to the recommended tolerances for the flooring you are installing. For installations using mechanical fasteners 1 1/2 inches and longer, subfloors should be within 1/4 inch in 10 feet or 3/16 inch in 6 feet. For gluedown installations and those using mechanical fasteners less than 1 1/2 inches, the subfloor should be within 3/16 inch in 10 feet or 1/8 in 6 feet. The subfloor can be flattened by sanding down the high areas or by installing shims such as roofing shingles or various thicknesses of plywood.

5) Moisture: Another critical part of the subfloor inspection process is checking the moisture content (MC) of the subfloor and joists. A moisture meter should be your best friend on the job. Whether you are using a pin or pinless model, make sure it is calibrated correctly for the species you are testing. (For more on moisture meters, see "Measuring Moisture" in the October/November 2006 issue of Hardwood Floors.) Take several readings—a minimum of 20 per 1,000 square feet—and clearly document them. Many installers write the MC reading and date right on the subfloor, log them in the job-site notes and take digital photos for further documentation. If problems arise later, you may need to submit proof that you followed the correct procedures. For solid strip flooring, there should be no more than 4 percentage points' difference in MC between the subfloor and the flooring; solid plank flooring wider than 3 inches requires no more than 2 percentage points' difference.

Although moisture may be in check at the time of installation, it could slowly seep in through the wood subfloor and eventually into the wood flooring above. To help prevent this, an approved moisture-vapor retarder should be placed between the subfloor and the wood flooring. Approved vapor retarders include 15-pound asphalt felt paper, asphalt-laminated paper and 15-pound kraft paper. A vapor retarder material with a perm rating of 0.7 or less, such as polyfilm, can trap moisture in the wood subfloor and should not be used. Some manufacturers have their own recommendations for vapor barriers depending on which product you are installing, so always check with the flooring manufacturer first.

Particleboard

Particleboard is frequently discovered as a subfloor layer over plywood when carpeting or vinyl are ripped out.

1) Suitable Wood Flooring: Floating floors. Particleboard has no holding power for fasteners. Other wood floors may be installed if the particleboard is ripped out, but if the original subfloor is not thick enough, at least a 1/2-inch layer of plywood or 3/4-inch OSB must be added once the particleboard is removed. The other option is to install a layer of plywood or OSB over the top of the particleboard (although that may create unacceptable height differences with the adjoining rooms; always check).

Concrete Slabs

Concrete slabs are common in homes without basements, typically in the Southern and Southwestern regions of the United States. Below-grade areas of the home where the soil is 3 inches above the floor, such as walk-out basements, also have slabs.

1) Suitable Wood Flooring: There are several options for installing wood flooring over concrete. Always use the product and installation method recommended by the manufacturer.

• Directly Over Concrete: Floating floors, engineered floors, some solid strip and plank, and some parquet. Solid wood flooring is never recommended below grade, but some solid wood flooring manufacturers now recommend their products for glue-down over slabs. A vapor retarder is recommended any time you install a solid 3/4 –inch wood flooring over concrete.

• Over Concrete with a Wood Subfloor: Any.

• Over Concrete with Sleepers: Solid strip or plank can be laid directly over a sleeper system, but engineered wood flooring less than 3/4 inch thick or solid plank wider than 4 inches requires a wood subfloor over the sleepers. For obvious reasons, parquet cannot be installed over sleepers unless an additional layer of plywood or OSB is added.

2) Weight: There are different types and weights of concrete slabs, and not all are acceptable for glue-down installation of hardwood flooring. The slab must be a minimum of 3,000 psi; lightweight concrete is not acceptable unless you are installing a floating floor or a wood subfloor over the slab. A quick test is to drag a nail or other sharp object across the slab—if it leaves an impression, there's a good chance it's lightweight concrete.

3) Surface: As with all subfloors, inspect the surface of the slab for areas that can affect the flooring installation. The slab should be clean and free of contaminants such as sealers, waxes, drywall or paint. Slick slabs can prevent adhesive transfer, so areas may need to be abraded according to the adhesive manufacturer's recommendations. Also check for hollow spots, cracks and loose areas. You may need to grind down the slab or repair these problem areas.

4) Flatness: The slab should also be flat to the wood flooring manufacturer's specifications, usually 1/8 inch in a 10-foot radius. Mechanical methods for flattening the slab include grinding, bead-blasting or abrading. There are many self-leveling concrete compound products on the market, as well as patching compounds. The condition of the slab and products you are working with determine which methods to use. Always check with the flooring adhesive manufacturer to determine what is the best method to flatten the slab with the products you're using.

5) Moisture: As with plywood and OSB subfloors, moisture testing is one of the most important parts of your installation. Concrete is a porous material and can retain moisture and emit moisture vapor, which could end up in the wood floor above unless the proper precautions are taken. (For more on concrete subfloors, see "Concrete Facts" in the August/September 2007 issue of Hardwood Floors and "How to Get a Concrete Subfloor Ready for Wood Floors" in the February/March 2003 issue.) In new construction, the slab should be given at least 60 days to cure. No matter if it's new or old, or on or above grade, the slab should be thoroughly tested for moisture before any installation begins. Common concrete moisture testing methods include:

• Calcium-chloride test (ASTM F1869): measures the amount of moisture emitted from a 1,000-square-foot sample of slab surface in 60 to 72 hours.

• Concrete moisture meter: uses electrical impedance and electrical resistance to measure moisture in the slab.

• Phenolphthalein test: applied every 200 square feet in a job site.

• Relative humidity (RH) tests: measures RH by using either the Insitu Method (ASTM F-2170) or the RH Hood Method (ASTM F2420).

For solid wood products, a moisture barrier or vapor retarder is typically recommended. These include 6 to 8 mil polyethylene film applied over asphalt mastic, 15-pound felt applied over mastic  and various other liquid sealants on the market specifically for concrete. Always check with the flooring, adhesive and moisture barrier manufacturers before installation to ensure that all products and methods you are using are compatible.

6) Additional subfloors: Depending on the product you're installing, you may need to install a wood subfloor over the slab. There are several methods, including:

• Floating subfloor: There are two common methods for this. The first is to lay two layers of 3 8 -inch plywood over the slab without fastening or gluing them to the slab. One layer is laid, and the second layer is stapled or glued perpendicularly or diagonally over the top. The second method is to loose-lay one layer of 3/4 -inch plywood cut into 16-inch planks perpendicularly or diagonally to the direction of the flooring. The panels should be staggered every 2 feet and spaced 1/8 inch between ends.

• Glue-down subfloor: 5/8 -inch plywood panels are glued in a staggered joint pattern directly to the slab.

• Nail-down subfloor: 5/8 -inch or thicker plywood panels are fastened every 12 inches on center and every 6 inches along the border.

• Sleeper (screed) system – Common over radiant heat, this system has 2-by-4s spaced 12 inches apart. Solid strip or plank can be laid directly over a sleeper system, but engineered wood flooring less than 3/4 inch thick or solid plank wider than 4 inches requires a wood subfloor over the sleepers.

Whichever method you use for an additional subfloor, allow 3/4 -inch expansion space at the walls and vertical obstructions.

Older Subfloors

When you're walking into a remodeling situation, you never know what you will encounter on the job site. Removing existing floor covering can reveal many unpleasant surprises such as rotten boards, water damage, severe dips or humps, old adhesive or even mold. That is why it's important to do a thorough inspection from below from the basement or crawlspace. Common older subfloors include:

• Solid Wood Subfloor. This type of subfloor is common on older homes. As with plywood subfloors, if there is movement or squeaks, refasten the subfloor to the joists and set protruding fasteners into the subfloor. The boards should be no wider than 6 inches and installed on a 45-degree angle. If the subfloor is not acceptable, it can often be overlaid with 1/2 -inch plywood. Some wood flooring may not hold sufficiently if installed directly over solid wood subfloors. Engineered or solid strip flooring less than 1/2 -inch thick and parquet flooring may need an additional 3/8 -inch layer of plywood installed over these types of subfloors. Always check with the flooring manufacturer when in doubt.

• Existing Floor Covering. Whether it's vinyl, linoleum or an old wood floor, your best bet may be to install the new floor directly over the old one. You should never remove an old floor covering if you suspect asbestos was used. Older homes built before the 1950s often feature the infamous 9-by-9 tiles that contain asbestos. There were also vinyl tiles manufactured into the 1970s that contained asbestos. If you can't determine the age of the floor or are in doubt, for the safety of the crew and the occupants, have a sample tested before you rip out the floor. Any floor containing asbestos requires a professional abatement crew to remove the old floor.

Even if you know older floor covering doesn't contain dangerous contaminants, you may still want to leave it in place. For mechanically fastened floors, you can leave vinyl floor covering in place, and it will even act as a vapor barrier. However, for glue-down applications, you may need to remove vinyl or other floor covering, because they could affect the bonding of the adhesive, or they themselves may no longer be wellbonded with the subfloor. Always check with the adhesive manufacturer for its recommendation. If you leave existing floor covering in place, always check the height differences with floor coverings in adjoining rooms.

No matter which subfloor you are dealing with, you need to let everyone involved know what is needed for a successful wood flooring installation. Your customer may not see what is beneath the beautiful wood floor, but what you do below determines how the floor will look above. By following all the steps to prepare your subfloor, you create a solid foundation for your wood flooring.

Sources for this article included Rusty Swindoll, NWFA; Mickey Moore, NOFMA: The Wood Flooring Manufacturers Association; and Kevin Mullany, Benchmark Wood Floors Inc.


Catherine Liewen is a former managing editor/art director of Hardwood Floors.



        Subfloor preparation                Plywood and OSB Over Joists    Particleboard    Concrete Slabs    Sleeper    glue-down    nail-down       

 Comments:

Could using Vantech for subfloor cause you to have squeaks? The house I am working on is new construction, but the builder only uses vantech and I am having a lot of problems with squeaks thoughout the house. It is as if the subflooring is not holding the nails.
Marshall Cheek  owner  10/4/2011 12:46:27 PM

We currently have carpet in our living/dining room. Also, in the entry ceramic tile was installed, first plywood and them the tile. We now want to install hadwood flooring throughout. I believe the tiles under the carpet are 9X9. In order for us not to disturb the tiles, we are assuming they are asbestos, we are wondering where it is suitable to put plywood on top of the tackless and continue with plywood throughout and then install 3/4 hard solid flooring. My question is should it be a special plywood and if so how thick. We have lived in the house 34 years and have always had carpeting.
Marlene Guardino  Hard wood flooring installation  11/20/2011 10:34:06 PM

I have a concrete slab foundation and live in an area where the soil is black clay. Alot of slab movement occurs in this area. I have done the drilled pier method for stabilizing the slab and it has been 5+ years since that was done. I have since monitored the movement and there are no indications of slab move ment like there was prior to the slab stabilization. W now want to install wood flooring ove the concrete slab. Need advise, my research has been that I want to use asphalt felt paper for my moisture barrier, then lay T&G OSB over that and not secure it to the slab so it can float so to say, the use 3/8 inch engineered wood flooring (probably glued to the OSB perpendicular to the joints on OSB. What are your recomendations. Your input would be greatly appreciated. Thanks, James
James Morison  homeowner  10/27/2012 9:58:51 AM

Hey guys, I am planning to put hardwood down in our family room (at grade) which is currently concrete slab covered with carpet. One of your suggestions for concrete slab is to glue plywood directly to the concrete slab which you could then fasten the hardwood to with nails/staples. Two questions, wouldn't it be necessary to put down a moisture barrier between the plywood and the concrete slab? If this is the case how should I secure the plywood to the floor. Second, would I be able to get away with half inch plywood instead of 3/4's as I am concerned with the final height of the new flooring and the transition into our adjoining hallway? Any direction would be appreciated. Thanks! Jeff
Jeff  hardwood over concrete  4/7/2013 10:01:17 AM

I wish to install Solid Wood flooring with Profiles on the edges onto Plywood. There should no visible screws or nails on the face of the floor, what is the best was to install or fix the solid wood floors on plywood. There is a metal frame on which these panels have to be placed.
Sailesh Hemnani  Partner  5/16/2013 7:18:18 AM

Help! Our house has laminate flooring that was put down 14 years ago when the house was built. We have starting pulling up the old flooring and it was installed with no vapor barrier and was installed with u shaped 1 inch staples. There are staples every few inches and they aren't coming up with the flooring! There are hundreds of them in every room and they stick up about one quarter inch above the plywood sub flooring. We are having about 1400 sf installed - my wife andI both work and it would take weeks to get them all up but we don't wan to compromise our new floor. If we hammer them down - could we then have the SVC silicone vapor barrier and per finished 3/4" click and lock oak flooring installed over the flattened staples? Thank you for any advice.
Dan Holloway  Homeowner  5/18/2013 10:00:21 PM

installing a floating floor over osb in a 20 year old house(removed carpet) the first floor sits over a basement and i noticed light from the basement between some of the boards. should i seal the cracks to keep moisture from the basement out??
Mike Adams  home owner  5/26/2013 5:44:55 PM

I am looking to install some sort of hardwood(either solid or engineered). I have a crawlspace. The subfloor is about 2 1/2 - 3 feet above grade. I've layed 6mil black plastic over the ground 2 or 3 weeks ago and taped the seems. Didn't do anything to seal around the edges. Not atheny water sitting except a few drops from condensation from the air duct. I've checked the moisture content of the subfloor with a General brand meter. I am getting 11-12 % throughout the subfloor where I pull up the carpet. What are my options for the type of hardwood I can install without it cupping or warping. Also, what about underlayment. Roofing felt? I've got mixed answers, and would like to know my options. I don't want to put the time and money into something that will fail. And I don't want carpet. Any advice is appreciated. Thanks
Benny  Subfloor has higher moisture content  6/4/2013 11:57:31 PM

Taking up particle board which leave 1/2 plywood, can I put 3/4 nail down hardwoods on top od that?
folsom    11/7/2013 7:53:34 PM

Folsom, this article details the steps you would want to take after removing particleboard and before putting down solid wood floors: http://hardwoodfloorsmag.com/articles/article.aspx?articleid=148&zoneid=2
Kim Wahlgren  Editor  11/8/2013 2:26:24 PM

I read somewhere that having an excessively dry subfloor (5 - 6 % MC) can affect a solid hardwood floor (7% MC) after it is installed. The house is new construction and the builder has installed and turned on a furnace (heat pump). Is this correct and what can be done to prevent any problems when the A/C gets turned on in the summer?
Suzy Weaver  Owner  1/15/2014 1:38:12 PM

We are getting ready to install hardwood floors in our upstairs. We have plywood subflooring and it's old and squeeks a lot. We are adding more screws to help eliminate some of the noise. We are thinking of using a 1/4" exterior plywood as an underlayment instead of the paper or foam types we have seen. What I'm not sure of, will this provide the needed moisture or vapor barrier, or do we need something else as well? Also, i understand the plywood should have a 1/4" gap around the edges near the walls for expansion, do we use this gap around all sides of each board, or can the boards be butted up against each other throughout the room?
Stacy  Using 1/4 plywood for underlayment  2/23/2014 7:11:13 PM

We have a 30 yr old house and have pulled up all of the floor coverings (carpet and lino). The sub floor is 1/2" plywood on top of the joists and a layer of 1/2" OSB on top of the plywood. Going to be putting in 3/4" nail down hard wood with 2" cleats and was wondering if the subfloor was sufficient. Was told by "others" that it was but another opinion wont hurt. Thanks.
Adam    4/24/2014 12:25:57 PM

I am remodeling my 102 year old house, and replacing some of the flooring. I am installing a 3/4" oak hardwood floor, but the subfloor is 3/4x6" planks that are running in the same direction as the hardwood will run. Can I lay a 1/8" or 1/4" layer of plywood over it to level it off and cover the gaps between planks?
Todd  Remodel  6/18/2014 5:19:22 PM

Old tile was removed from my kitchen. The original tile was very thick and was laid on top of a plywood subfloor. The floor needed to be raised to accommodate the hardwoods so the contractor glued, nailed, and screwed another layer of plywood subfloor to the original floor. I returned home to find the workmen had left some of the wood outdoors during a rain storm and then installed the damp wood. It is not soaking wet but definitely damp to the touch. Is there a danger of mold? Will it dry out? Is moisture testing enough of a guarantee that there will not be mold growth?
Eve  Remodel  7/15/2014 3:20:49 PM


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