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Wood, because of its cellular makeup, is hydroscopic. Wood has the ability to absorb and release moisture, depending on the relative humidity of the home. Hardwood flooring is milled and dried out to approximately 19% percent moisture content. Contrary to popular literature, the hardwood flooring arrives into your home having an approximately 19% moisture content, and when it is acclimating to the home’s relative moisture content, it will either absorb or release 6-9% of the home’s relative humidity. The average home’s Relative Humidity should be around 30-50%.

When the relative humidity of the home falls below 30% for an extended period of time, it can cause the wood boards to dry out to less than the 6-9% of moisture content the new hardwood flooring had acclimated inside the house prior to installation! Dry environment can cause the boards to contract, causing open gaps between the boards. These gaps can close when humidity returns the following season. Cupping of the boards can result during extended periods of dry weather.

 The same thing can occur when the relative humidity of the home rises above 50% for any extended period time.

ACCLIMATION OF HARDWOOD FLOORING the recommended period is 3-7 days or more prior to installation. Acclimation must take place in conditioned space in the dwelling rooms so the new hardwood flooring either absorbs or release moisture relative to the humidity of the home. Unconditioned space such as garage or sheds is unsuitable space for acclimation.



1. Maintain humidity levels between 30-50% in the home.

2. Make sure the new hardwood flooring is properly acclimated in the home for 3-7 days or more. For damp basements or concrete slabs, a 15lb. felt hardwood flooring underlayment paper is recommended to act as barrier from moisture rising up through the subfloor. During installation of the 15lb. black felt paper, overlap the seams by 4-6 inches. For floating engineered floors or even Laminate flooring an underlayment pad with a moisture retarder film attached is recommended.

3. During the wet or dry seasons, you may need to run air conditioning or de-humidifiers.

4. You may need to run humidifiers when the humidity levels drop below the 30-50% range.

Hardwood Flooring Installation Methods


Nail Down

For ¾” solid wood flooring 2” nailing cleats are used to fasten the flooring to the subfloor. Depending on different installation types and wood thickness, different cleat sizes are used. For thinner solid and engineered flooring, air compression nailer gun is used for fastening application.


Staple down

A modified pneumatic flooring nailer that is used to drive 1-1/2" - 2" Staples versus cleats to attach tongue and groove engineered or solid wood flooring to the subfloor. A pneumatic nailing gun is used to drive the staple into the wood flooring and subfloor.  Wood flooring 5/8" or thinner would most likely require a smaller pull trigger pneumatic nailing gun and smaller sized staples. Read the manufacturers installation manual to assure you have the right staple gun and right size staples for your particular flooring.

For engineered hardwood or solid hardwood tongue and groove or click and lock, an air compression pneumatic nailing gun is used to fasten 1’1/2” – 2” staples into the wood flooring and the subfloor.  For wood flooring 5/8” or thinner, smaller sized staples are used for the application.

Glue Down

An alternative installation method for installing tongue and groove engineered wood flooring and the thinner 5/16” thick solid wood flooring is the glue down installation. The proper wood flooring adhesives, like the MS Plus Advanced, is evenly troweled across the plywood subfloor to adhere the wood flooring to the subfloor. Engineered hardwood floors can be glued down. However, solid hardwood cannot be glued and instead must be nailed down.

Click Lock

An alternative to nail down and glue down installation for engineered hardwood is the tongue and groove floating floor system where the boards click and lock into each other without the messy glue. Click and lock installation is faster and easier than other methods.


Similar to the click and lock installation method, the engineered wood planks are adhered together with beads of glue and the click and lock floats on an underlayment pad which is placed between the wood flooring and the subfloor. The underlayment provides a cushion and buffers noise when walked on. It also blocks any moisture arising from below. 

Please consult the manufacturers installation instructions to see which method is recommended for your flooring.


Wear Layers on Engineered Flooring

If you are going to choose engineered floor construction over solid floor construction here is what you need to know:

The construction of an engineered floor is made up of an inner core consisting of anywhere from 3 to 12 layers of plywood which are glued and pressed together. A layer of choice wood specie such as maple, birch, walnut, etc., is fused on top. This is what is referred as wear layer.

The advantages of engineered wood floor over solid wood floor include greater dimensional stability due to its engineered ply layers. Engineered floors can be installed in more areas of the home, such as the basement (below grade) or over concrete slab and over radiant heat. The engineered cross layers will reduce moisture related expansion and contraction.

Functions of Wear Layer

Wear layer thicknesses from 2.5mm to 4mm and thicker depending on the product. 

The thickness of the wear layer will determine how many time an engineered floor can be sanded and refinished. Professional refinishers will sand on average .75 to 1mm of the wear layer to remove scratches and dents. Finally, a coat of finish such as water based polyurethane is applied to the hardwood to protect and restore the hardwood to its original sheen.

By comparison, a solid ¾” tongue and groove solid hardwood floor can be sanded and refinished approximately ten times in its lifetime.

As a rule of thumb:

  • .5 to 1mm wear layer cannot be sanded. It can only be recoated with a layer of finish such as water based or oil based polyurethane or urethane.
  • 2mm wear layer can be sanded and recoated 1 or 2 times.
  • 3mm wear layer can be sanded and recoated 2 or 3 times.
  • 4-6mm wear layer can be sanded and recoated 3 to 6 times.

Installing Hardwood Floors Over Different Subfloors

Plywood Subflooring

Plywood and OSB come in 4’ x 8’ sheets that are either 5/8” or ¾” thick. Plywood should be fastened with 2-1/2” long deck screws every 8 inches along the floor joists below. To prevent squeaks and noise later on, apply adhesive caulking on top of the floor joists before laying the plywood sheet.

¾” Thick Solid Wood floors can be installed directly over 5/8” or ¾” plywood subfloors. As well, engineered hardwood and laminate floors can also be installed over 5/8” or ¾” plywood. 

Laying the boards along the long length of the room will make the room appear larger.

If the subfloor is old or weak and flexes when walked on, it is recommended to add an additional layer of plywood 3/8” or ½” thick using glue adhesives and screwed down to add dimensional stability over the weak subfloor – this adage applies to all hardwood and laminate floorings whether the installation method is glue down, nail down or floating floor system.

Laminate flooring can only be installed using the floating method over plywood subfloor with a recommended underlayment pad between the laminate flooring and the subfloor.

If the subfloor is old or weak and flexes when walked on, it is recommended to add an additional layer of plywood 3/8” or ½” thick using glue adhesives and screwed down to add dimensional stability over the weak subfloor. As well, humidity levels fluctuate constantly and ground water table (The level below which the ground is completely saturated with water) so depending where you live, it is recommended to lay 15lb. felt underlayment paper over the subfloor to contain the moisture transfer which may rise up from damp crawlspace and basements and possibly affect the dimensional stability of the floor system.


Plank Subflooring

Plank subfloors are usually found in older homes and the nails on the plank boards will loosen from the boards expanding and contracting over time. Using 2-1/2” deck screws re-secure the planks to the floor joists below, as loose or damaged boards will affect the dimensional stability of the finished floor.

 3/4" Thick Solid Wood Flooring must be installed perpendicular to the plank subfloor boards (at a 90 degree angle). Otherwise, if it is installed parallel to the direction of the plank subfloor, the newly installed hardwood flooring could gap and pull apart if the plank subfloor expands or contracts. If you want the floors to run parallel to the subfloor, make sure to add an additional layer 3/8” or ½” thick plywood underlayment over the plank subfloor.

Less than 1/2" thick Engineered, Solid Wood or Floating can be installed over a wood plank subfloor as long as the 3/4" plank subfloor is flat. The hardwood flooring must be installed perpendicular to the subfloor boards (90 degree angle).  If the subfloor shows any deflection when walked on, it is recommended to add an additional layer of 3/8” or ½” plywood underlayment over the plank subfloor. The added dimensional stability allows you to run your new wood flooring in any direction desired.

9/16" (0.5625”) or thicker Floating Engineered Wood Floors can be installed on plank subfloor. The hardwood flooring must be installed parallel to the subfloor boards (at a 90 degree angle). If there is any deflection in the wood plank subfloor when walked on it is recommended to add an additional layer of 3/8" or 1/2" thick plywood underlayment over the plank subfloor to add dimensional stability to weak subfloors. The added dimensional stability allow you to run your new wood flooring in any direction desired.


Concrete Slabs

Concrete slabs are usually 4-6” thick and the moisture content of concrete never dries out. It will “sweat” for the life of the slab, producing moisture vapors and depending on the time of the year, emit higher than normal (more than 4%) moisture vapors depending on the ground water table. Moisture testing must be performed. Tape 2’ x 3’ sheets of plastic over couple of areas of the slab. Wait 24-48 hrs and see if the plastic collected any moisture. Plastic 6 mil thick moisture barrier, moisture resistant adhesive glue, 15 lb. felt underlayment paper and plywood are some of the options used to correct the excessive moisture levels. 

3/4" Thick Solid Wood Flooring first lay 6 mil plastic vapor barrier over the concrete slab. Then lay 2 layers of  ½” thick plywood over the plastic vapor barrier. For glued down engineered wood, it is recommended 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.


Particle Board – Underlayment

Particle boards are unsuitable for nail down or glue down hardwood installations. They are brittle – nail fastened will loosen over time and cause movement and noise. For hardwood nail down or glue down, particle boards must be removed prior to installation.