Everything You Need To Know About Subfloors (And How It Will Effect Your New Flooring Install)

By Jeff Hosking – Hosking Hardwood Flooring Inc.
© 2004-2023, Copyright protected. All Rights Reserved

What you walk on makes a difference!

So, you bought a new house and want to make it your own. New flooring is the perfect way to personalize your new home, so you start tearing up the old carpets -- but what do you find underneath? And how will it affect your renovations? The subfloor is the layer of material underneath the flooring or carpet in your home and there are a few different subfloor types: plywood, planks, OSB and concrete. The type of subflooring in your home dictates what type of flooring you can install over it as well as what methods of installation are appropriate for your situation. It's important to always consult the installation instructions of the flooring manufacturer to make sure your subfloor meets the requirements for their specific flooring product. The manufacturers' installation instructions will also offer recommended installation methods for your new flooring.

How does the subfloor fit into your home as a whole? The flooring system (joists and subfloor) is one of the most important parts of any building. In a structure, the floor system will carry the weight of interior walls, roof, furniture, appliances, finished flooring, people, etc. The structural integrity and type of subfloor will ultimately dictate how much weight the floor can carry and how structurally sound your building is. Homes that are built with weak foundational footings, improperly spaced support columns, clumsily spaced floor joists or inadequately nailed sub floors are building design failures. These design failures sequentially lead to performance or structural breakdowns of the floor system which, in turn, cause excessive movement, sagging, unequal heights, separation, or squeaking within the floor system.

Types of Subfloors:

Plywood Subflooring

Without a doubt, CDX tongue and grooved plywood has been the most commonly used exterior structural subfloor material used in homes. This type of plywood subfloor is made from thin sheets of (usually southern pine) veneer that are cross-laminated and glued together forming 4' x 8' sheets that are either 5/8" or 3/4" thick. Because the edges of this CDX plywood are tongue and grooved, they interlock to each other and provide a strong, secure base for any finished flooring. When installing new plywood subfloors -- it's highly recommend to apply a thick bead of subfloor adhesive caulking to the top of the floor joists before laying each sheet. Then, screw the CDX sheets to the floor joists using 2-1/2" long deck screws screwed every 8 inches. This will create a strong, stable hold and help prevent squeaky floors in the future, which occur when plywood subfloors become loosened from the joists over time.

The great news is that any hardwood flooring option or engineered variation can be installed over 5/8" or 3/4" thick CDX plywood subfloors. This includes Solid Wood Flooring, Engineered Wood Flooring, Floating Wood Floors, Laminate Flooring, Vinyl/Wood Hybrid Flooring and Luxury Vinyl Planks.

Installation Tips for Flooring Over Plywood Subfloors:

Although it is standard practice to install all hardwood flooring no matter the thickness, across the floor joists to support, stabilize and strengthen the entire floor structure, sometimes that board direction is not preferred and exceptions can be made. If the existing sub floor is 3/4" thick, solid and flat with no deflection between the joists when walked on and the floor joists are spaced 16 inches or closer -- you may be able to run the hardwood flooring parallel with the floor joists to obtain a desired look. For example, running the floorboards parallel to the longest wall of the room will make the room appear larger than it is.

Engineered Wood Flooring, Solid Wood Flooring, Laminates or Vinyl Planks that are less than 1/2" thick can be installed over a plywood subfloor. However, if you have a weak subfloor that flexes up and down when walked on, it's recommended to add an additional layer of 3/8" or 1/2" thick plywood underlayment, glued and screwed down over the weaker subfloor. This will add stability to the weaker plywood subfloor and ensure longevity of your new flooring install. Installing these thinner wood flooring options over an already weak or thin subfloor may cause the whole floor system to flex up and down when walked on. 

Solid Hardwood Flooring or Engineered Wood Flooring can typically be nailed or stapled into plywood subfloors. Floating Wood Flooring, Laminate Flooring and Click Lock Vinyl Planks can be floated over plywood subfloors using a recommended underlayment. Glue down Vinyl Planks can be glued down over plywood subfloors using the manufacturer's recommended adhesive.

Even though you may think your homes environment is dry, interior relative humidity moisture levels and ground water tables can change frequently from month to month, We recommend laying 15 lb. black felt Hardwood flooring underlayment paper over the wood subfloor overlapping the seams 4-6 inches to help retard moisture transfer that may migrate up from damp basements through the subfloor and affect the stability of the Hardwood floor.

Plank Subflooring

Plank subfloors are usually made up of 3/4" thick x 4-8" wide southern yellow pine boards. Installation of plank subflooring usually consists of nailing these boards to the floor joists. Since this type of sub flooring is usually found in older homes and can loosen up over time due to the boards expanding and contracting, it is very important to remember to re-secure these planks to the floor joists using 2-1/2" deck screws prior to installing any floor covering. Loose or damaged boards will affect the performance of the finished flooring.

Installation Tips for Flooring Over Plywood Subfloors:

3/4" thick Solid Wood Flooring must be installed perpendicular to the the plank subfloor boards (at a 90 degree angle). If the solid hardwood flooring is installed parallel with the subfloor planks, the new solid wood floors could pull apart leaving open gaps or the new flooring could develop waviness later if the subfloor planks expand/contract or the edges cup upwards. If you desire to run the wood flooring parallel with the subfloor boards you must install an additional layer of 3/8" or 1/2" thick plywood underlayment glued and screwed down over the plank subfloor.

Thinner Solid Wood Flooring, Thinner Engineered Wood Floors or Floating Wood Floors that are less than 1/2" thick can be installed over a wood plank subfloor as long as the 3/4" plank subfloor is flat. Again, these thinner hardwood flooring  options must be installed crossing perpendicular to the subfloor planks (90 degree angle). If there is any deflection in the subfloor when walked on it would be advisable to add an additional layer of 3/8" or 1/2" thick plywood underlayment glued and screwed down over the wood plank subfloor to add stability to weak subfloors. Adding this extra layer of plywood, would also allow you to run your new wood flooring in any direction desired.

9/16" or thicker Floating Engineered Wood Floors can be installed directly on plank subfloor. The hardwood flooring must be installed perpendicular to the subfloor planks (at a 90 degree angle). If there is any deflection in the wood plank subfloor when walked on it would be advisable to add an additional layer of 3/8" or 1/2" thick plywood underlayment glued and screwed down over the plank subfloor to add stability to weak subfloors. Doing this will also allow you to run your new wood flooring in any direction desired.

Laminate Flooring, which can only be installed using the floating method, can be installed over a 3/4" thick plank subfloor (over the recommended foam pad) as long as the plank subfloor is flat. The flooring must be installed crossing the subfloor boards (at a 90 degree angle). If the laminate flooring is installed parallel to the direction of the plank subfloor the laminate flooring could develop distortion later. If there is any deflection in the subfloor when walked on it would be advisable to add an additional layer of 3/8" or 1/2" thick plywood underlayment glued and screwed down over plank subfloor to add stability prior to installing any flooring.

We recommend laying 15 lb. black felt Hardwood flooring underlayment paper over the subfloor overlapping the seams 4-6 inches to help retard moisture transfer.

OSB (Oriented Strand Board) Subflooring

OSB Subflooring looks like, and is, a bunch of wood chips glued together. Installation usually consists of gluing and nailing the OSB sheets to the floor joists.

 3/4" Thick Solid Wood Flooring can be installed directly over 3/4" thick OSB subfloor. It is standard practice to install the hardwood floors at a 90 degree angle across the floor joists to stabilize and strengthen the whole floor structure. If the existing subfloor is 3/4" thick, solid and flat with no deflection when walked on you, can run the wood flooring parallel with the floor joists to obtain a desired look. If the OSB sub flooring flexes up and down when walked on then you should add an additional layer of 3/8" or 1/2" thick plywood underlayment glued and screwed down to add stability. OR, install 2x6 blocking between the floor joists for added stability. Running the floorboards along the longest dimension of the room will make the room appear larger than it is.

When installing engineered, solid or floating floors that are less than 1/2" thick we highly recommended you add an additional layer of 3/8" or 1/2" thick plywood underlayment. The additional underlayment should be glued and screwed down over 3/4 inch thick OSB subfloor to add stability. As an alternative you could install 2x6 blocking between the floor joists for added stability. Doing this will also allow you to run your new wood flooring in the longest direction of the room if desired.

Floating Engineered Wood Floors can simply be installed directly on OSB subfloor. The hardwood flooring must be installed crossing the floor joists (at a 90 degree angle). If there is any deflection in the subfloor when walked on it would be advisable to add an additional layer of 3/8" or 1/2" thick plywood underlayment glued and screwed down over the OSB subfloor to add stability. OR, install 2x6 blocking between the floor joists for added stability. Doing this will also allow you to run your new wood flooring in any direction desired.

Laminate Flooring - Thinner laminate type flooring can be installed over OSB subfloor (over the recommended foam pad). The flooring must be installed crossing the floor joists (at a 90 degree angle). If the laminate flooring is installed parallel to the direction of the floor joists the Laminate flooring could develop distortion later. (If there is any deflection in the subfloor when walked on it would be advisable to add an additional layer of 3/8 or 1/2 thick plywood underlayment glued and screwed down over OSB subfloor to add stability OR install 2x6 blocking between the floor joists for added stability.)

We recommend laying 15 lb. black felt Hardwood flooring underlayment paper over the subfloor overlapping the seams 4-6 inches to help retard moisture transfer.

Concrete Slabs

Concrete slabs usually consist of a 4-6" thick, 3,500-5,550 lb strength concrete pour. Water used within the mix of newly poured slabs can take up to 3 months or more to dry out. Moisture testing should be performed prior to any installation of hardwood flooring. One method is to tape the edges of several 2' x 3' sheets of plastic down in several areas of the slab, wait approximately 48 hours to see if moisture develops under the plastic. If signs of moisture appear, DO NOT install any wood flooring until the excess moisture level is corrected. Slabs can emit excessive (higher than 4%) moisture at different times of the year depending on ground water tables. Slab floors that have a 4-6 mil plastic installed prior to the pour will better retard moisture transfer later.

 3/4" Thick Solid Wood Flooring cannot be nailed or directly glued down over a concrete slab. Moisture is a never ending concern with slabs and the use of a 3/4" solid wood floor could cause the floors failure. If a 3/4" thick hardwood floor is desired, a sleeper system can be built on top of the slab using 1/2" or 3/4" thick x 3" wide strips of wood spaced 12 inches apart with 3/4" cdx plywood screwed to the top then the flooring installed. OR, glue and screw two layers of 1/2" plywood cross layering diagonally over each other directly over the slab making sure the seams do not line up. We recommend to lay a 4-6 mil plastic down on the slab first to retard any minor moisture transfer from the concrete.

If you plan on using a glue-down engineered wood floor, we recommend making sure the slab has a moisture content of less than 4% throughout the year, and to use an adhesive that is water resistant.

Floating Engineered Wood Floors were designed for installing over concrete and really work the best. These hardwood floors can simply be installed using their 1/8" thick padding directly on the concrete slab over a 4-6 mil plastic.

Laminate Flooring can simply be installed using their 1/8" thick padding directly on the concrete slab over a 4-6 mil plastic.

Concrete slabs need to be flat. Use a self-leveling cement type floor leveler to fill any depressions within the slab and allow to dry before installing any flooring.

Installing New Flooring Over Old Flooring

This is a question we get asked enough to post information on the subject. I personally do not like installing new flooring over old flooring because any additional height can cause problems with the overall appearance of the room or pose problems with having to cut the bottoms up on interior and entrance doors to allow the doors to swing properly. There can also be clearance problems with appliances and it can create unevenness where other rooms connect and adding additional layers of new flooring places unnecessary weight to the floor system.  And if the old flooring should ever become loose it could affect the new flooring installed over it. One of the benefits of removing the old flooring is it allows you the chance to fix any loose subflooring or squeaks by re-securing the main subfloor to the floor joists prior to installing the new flooring. With all that being said though the answer is YES you can install New Wood Flooring over the old. BUT…

New Hardwood Flooring Over Old Hardwood Flooring: 3/4" thick or thinner Solid or Engineered hardwood flooring must be installed across the old wood floor boards (at a 90 degree angle). If the hardwood flooring is installed (nailed down) parallel to the direction of the old wood flooring the new wood floors could pull apart leaving open gaps or develop waviness later if the old wood flooring under it expands or contracts. If you desire to run the wood flooring parallel with the old wood floor boards you must install a layer of 3/8" thick plywood underlayment screwed down over the old wood flooring which acts as a separation and will allow you to run your new wood flooring in any direction desired. We recommend laying 15 lb. black felt Hardwood flooring underlayment paper over the old flooring overlapping the seams 4-6 inches to help retard moisture transfer. To reduce any added height problems you can use a thinner 3/8" - 9/16" thick engineered wood floor or 5/16 thick solid hardwood floor. You can also use a floating engineered floor which can usually be installed over the older floor in any direction. All floating wood floors require a suitable underlayment pad. 

Laminate Flooring, which are floating floors, can be installed over most any other floor covering except carpet. All laminate floors require a suitable underlayment pad.

Porcelain or Ceramic Tile: If the Ceramic Tile is well secured and flat then you could install a Laminate or Engineered Floating wood floor over it using the recommended underlayment pad.  Due to the smooth texture of tile and the need for the wood adhesive to stick to it, it is not recommended to direct glue down wood flooring over tile.

Sheet Vinyl: Because Vinyl sheet flooring is a thin flexible material, the outline of the ceramic tiles will telegraph through and be visibly seen. I would recommend removing the tile and adding a plywood underlayment to the existing main subfloor to obtain the desired height then install the sheet vinyl. Sheet vinyl can be installed over an old hardwood floor but a layer of 1/4 inch or thicker underlayment has to be installed over the old hardwood flooring first.

Particle Board - Underlayment

Particle board underlayment has the appearance of oatmeal but is actually made up of tiny wood particles glued and pressed into 4" x 8" sheets. It is NOT suitable to use as a structural subfloor material.

I have only included Particle Board in this article because a lot of people see it when they remove old carpet and think it is a structural subfloor material like plywood or OSB. Particle board is NOT a structural subfloor material and is only manufactured to be used as a inexpensive underlayment filler sheet on top of the main structural subfloor material to raise the height of carpet to a desired level. Particle board can absorb excessive moisture is very brittle and does not hold nails very well. If this type of underlayment is present, it will have to be removed when planning to install a nail or glue down hardwood floor.

If desired, Particle Board Underlayment can be left in place if a Floating engineered wood floor or a Floating Laminate floor system is going to be installed over it. BUT the additional height of both the particle board and the new flooring may pose a problem with doors and appliances.

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Prior to installing any finished flooring, it is important to remember:

• Re-secure any loose wood subfloor planks, OSB, or plywood subfloors to the floor joists using 2-1/2" deck screws.
• Loose or damaged subfloors can cause squeaking and affect the performance of the finished flooring that is installed over it.
• Concrete slabs should be flat, use a leveling compound to fill any depression or voids.

Remember: Even the best wood flooring will react to the presence of moisture. In the dry winter heating months, moisture leaves the wood causing the floor to contract, which can leave unsightly gaps between each plank. In the summer months when the humidity is higher the wood will absorb excess moisture and expand and the gaps will disappear. If there is too much moisture it may cause the wood planks to cup, or buckle. This is why it is important when installing a wood floor to acclimate the wood to the home from 4 to 7 days or more prior to installation and to leave the proper expansion gap around the perimeter and at all fixed objects. It is also important to keep the home's relative humidity level at between 30 - 50% or what the manufacturer recommends. Doing this will help minimize any movement within the wood flooring later.

Frequently asked questions about

Hardwood Flooring FAQ
Laminate Flooring FAQ

Before you purchase your flooring read our other Articles:

All About Solid Wood Flooring
All About Engineered Wood Flooring
All About Floating Engineered Wood Floors
All About Installing Hardwood Flooring Over Radiant Heat

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