All About Installing Hardwood Floors Over Various Subfloors
By Jeff Hosking – Hosking Hardwood Flooring Inc.
© 2004, Copyright protected. All Rights Reserved
One of the most important parts of any buildings structure is its floor system. The floor system carries the weight of interior walls, roof, furniture, appliances, people, etc. How the floor structure is designed and built will determine how much weight the floor can carry. Too many times over the years I have seen homes that were built that have had weak foundation footings, Improper spaced support columns, undersized and/or improperly spaced floor joists or weak improperly nailed sub flooring. These design failures lead to multiple performance or structural failures of the floor system causing excessive movement, sagging, unequal heights, separation, or squeaking within the floor system.
Below we have photographed the four main types of subfloors usually found in a home. We explain what they are and what type of flooring would be best used with them: 3/4" thick Solid Wood Flooring, Engineered Wood Flooring, Floating Wood floors or Laminate Flooring.
Prior to installing any flooring is important to remember to 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 will affect the performance of the finished flooring. Concrete slabs should be flat, use a leveling compound to fill any depression or voids.
For more information regarding the differences in the types of wood flooring please see other articles I have written.
Plywood Sub flooring
Plywood is made from thin sheets of (usually southern pine) veneer that are cross-laminated and glued together with a hot-press, forming 4 foot x 8 foot sheets. Installation usually consists of gluing and nailing the sheets to the floor joists. CDX Grade is the most commonly used exterior structural subfloor throughout the industry.
3/4" Thick Solid Wood Flooring and most all other wood flooring including Engineered or Floating engineered wood flooring and Laminate floors can be installed directly over a 5/8" or 3/4" CDX plywood sub floor. It is standard practice to install the hardwood flooring across the floor joists to stabilize and strengthen the whole floor structure by cross layering the flooring to the floor joists and the sub flooring. If the existing sub floor is 3/4" thick, solid and flat with no deflection when walked on, you may be able to run the hardwood flooring parallel with the floor joists to obtain a desired look. Running the floorboards along the longest dimension of the room will make the room appear larger than it is. We recommend laying 15 lb. black (roofers) felt paper over the subfloor to help retard moisture transfer.
Engineered or Solid Wood Flooring that is less than 1/2" thick, can be installed over a plywood subfloor by adding an additional layer of 3/8" or 1/2" thick plywood underlayment, glued and screwed down over 5/8" or 3/4" thick subfloor will add stability to a weak subfloor.
Laminate Flooring - can be installed over a plywood subfloor. It would be recommended to add an additional layer of 3/8 or 1/2 inch thick plywood underlayment glued and screwed down over a 5/8" or 3/4" thick subfloor to add stability to a weak subfloor.
Plank Sub flooring
Plank subfloors are usually made up of 3/4 inch thick x 4-8 inch wide southern yellow pine boards. Installation 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 it is very important to remember to re-secure these planks to the floor joists using 2-1/2 inch deck screws prior to installing any floor covering, as loose or damaged boards will affect the performance of the finished flooring.
3/4" thick Solid wood flooring must be installed crossing the subfloor boards (at a 90 degree angle). If the hardwood flooring is installed parallel to the direction of the plank subfloor the wood floors could develop distortion later. 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. We recommend laying 15 lb. black (roofers) felt paper over the subfloor to help retard moisture transfer.
Engineered, Solid Wood or Floating Wood Flooring 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. The hardwood flooring must be installed crossing the subfloor boards (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. Doing this will also allow you to run your new wood flooring in any direction desired. We recommend laying 15 lb. black (roofers) felt paper over the subfloor to help retard moisture transfer.
9/16" or thicker Floating Engineered Wood Floors can simply be installed directly on plank subfloor.The hardwood flooring must be installed crossing the subfloor boards (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 - Thinner laminate type flooring can be installed over a 3/4" thick plank subfloor 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.
OSB ( Oriented Stran Board ) Sub flooring
OSB Sub flooring 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 diagonally across the floor joists to stabilize and strengthen the whole floor structure by cross layering the wood flooring to the floor joists and the sub flooring. 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 moves 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. We recommend laying 15 lb. black (roofers) felt paper over the subfloor to help retard moisture transfer.
When installing engineered, solid or floating floors that are less than 1/2 inch thick we highly recommended you add an additional layer of 3/8" or 1/2" thick plywood underlayment. The addtional 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. We recommend laying 15 lb. black (roofers) felt paper over the subfloor to help retard moisture transfer.
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 inch 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. 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. )
Concrete Slabs
Concrete slabs usually consist of a 4-6 inch 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 foot x 3 foot 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 inch 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 - These floors can simply be installed using their 1/8" thick padding directly on the concrete slab over a 4-6 mil plastic.
*Particle Board - Underlayment
Particle board underlayment Looks like oatmeal and consists of tiny wood particles glued and pressed into 4 x 8 foot 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 cheap 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 floating laminate floor system is going to be installed over it. BUT the additional height may pose a problem with doors and appliances.
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