Sunday, 8 March 2009

Installing Hardwood Flooring Over Concrete and Wood Sub Floors

Expansion & Contraction of Hardwood Flooring

"The flooring shop said to leave 10mm for solid wood floor expansion at the the boards are lifting off the floor." Ill informed advice like this often leads to floor problems. Many flooring sales people unfamiliar with solid wood flooring assume expansion is the same as laminate flooring. In reality that 10mm is not enough for solid wood flooring.

What Is An Expansion Area? Why Do We Need It?

Expansion areas are spaces left around the perimeter of rooms, against fixed objects such as columns, thresholds, hearths, skirting boards, and other stationary items built or secured into the framing structure of the home. Even though Hardwood flooring used is no longer living and breathing it still reacts to moisture changes in the environment. The cells in the solid wood will take on or absorb moisture when the relative humidity is high, or when exposed to water. Expansion takes place, and the hardwood grows, for lack of a clearer term, across the grain (width) of the plank (see below, not all will react this way). Conversely when air moisture levels decrease, moisture content evaporates, shrinking of the solid wood will occur.

When we say spaces, they are defined as installing the hardwood flooring up to and away from the fixed objects. For example; "we recommend at least 15mm small to medium area-18mm expansion on larger areas at the skirting board or wall if new skirting to be fitted."

Hardwood flooring can react to the presence of moisture. In the dry winter heating months, moisture can leave the wood flooring causing the floor to contract slightly in size, which can leave slight gaps between each floor board. In the summer months when the humidity is higher, the hardwood flooring will expand and the gaps will disappear. If there is too much moisture in the subfloor it will cause the wood planks to cup, or buckle. Nearly all problems related to hardwood flooring are due to improper installation and sub floor preparation. This is why it is important when installing hardwood flooring that you follow the correct recommended installation methods by the manufacturer.

Direct Glue Installations

A direct glue installation requires the use of a flooring adhesive applied directly onto the subfloor (never directly apply to the boards). This method can be used on concrete and wooden subfloors. When done correctly this is the most stable method for fitting solid wood flooring as the elastic flooring adhesive allows the floor to expand and contract with no problems.

If you are laying over a concrete subfloor you will first need checked the concrete moisture, if the floor has a moisture content of over 4% then either wait until the sub floor dries to meet this level or use an appropriate moisture barrier, we recommend Sika Mb Primer to ensure no damp rises up into your new floor.

Also note that new concrete slabs require a minimum of 60 days drying time before covering them with a wood floor.

All Concrete sub-floors must be dry, smooth, level and free of structural defects. If the concrete sub floor is uneven we recommend using sika latex self leveling compound to level the subfloor. The concrete must also be free of contaminants i.e. paint, oil, wax grease, dirt and curing compounds (the reason for this is that you need the self leveling compound to bond to the sub floor).These may be removed chemically or mechanically as your local hardware store and they will sell you the correct floor cleaner, but do not use solvent-based strippers under any circumstances. The use of residual solvents can create problems with the bonding of flooring adhesives. It is important to ensure a proper bond between the adhesives and concrete and wood panels.

If you have a wooden subfloor you will need to lay a plywood base over the existing floor boards (we recommend using 4mm -6mm exterior grade plywood and screw down every 15cm along the edges and at 20cm intervals throughout the rest of the board using the 1 inch deck screws) before installation, this will then give you a smooth and level surface for you to install you hardwood flooring onto.

You will need to leave a 15mm expansion gap around the perimeter of the room for large areas over 6 meters in length we recommend 18mm expansion gap. To maintain this gap during installation spacers are provided in the kits. After you have installed the hardwood floor you can cover this expansion gap with scotia or new skirting boards The 15mm expansion gap also needs to be left in doorways where the hardwood floor follows through from one room to another we sell on this website a full range of door profiles available to match your floor.

Nailed Down Installations

Nailed down installation is the traditional way to install hardwood floors and can be done either directly onto wooden subfloor or on a concrete subfloor with the use of battens. When fixing battens onto concrete you will need a power-actuated nailer to drive through the battens and fix directly into the concrete, firstly lay out polyethylene moisture sheet then plywood over the battens to create a new sub floor. We recommend 12mm exterior grade plywood and 2inch x 4inch battens. Lay battens around perimeter at ½ inch from the wall, leave ¼ inch gap at each end. Position the first batten so its centre is 16 inches from the outside edge of the perimeter batten. Lay out the remaining battens across the area 16 inch intervals. Always check you battens are level using a level gauge you may need to use cedar shims to level out dips in the concrete, slide the shims under the battens until level. (If in doubt we recommend consulting a professional fitter for this method).

Wooden Subfloor

Hardwood floors can be nailed into existing wooden subfloors such as floorboards or plywood but not into chipboard floors as these are not strong enough to hold the nails. The floorboards must be in good condition and level if not it is advisable to lay 6mm exterior grade plywood down first.

To avoid creaking, check over the subfloor and securely fasten any loose floorboards. For added soundproofing it is also advisable to use a standard foam underlay.

When you nail down a solid wood floor you use a Porta Nailer to drive a nail at a 30 degree angle through the tongue and into the subfloor. Porta Nailers can be hired from any good hire shop but are expensive to buy.

Again you will need to leave a 15mm expansion gap around the perimeter of the room. To maintain this gap during installation spacers are provided in the installation kits.

After you have installed the floor you can cover this expansion gap with scotia or new skirting boards.The 15mm expansion gap also needs to be left in doorways where the floor follows through from one room to another; we sell full range of door profiles available to match your floor on this website.


When ordering your new floor we recommend you allow for some wastage due to cuts a professional fitter would add on an extra 2%-5% of flooring needed to finish a job. We recommend non professional fitter should add on 5%-10%.
for futher information visit our website where you can also buy the wood flooring online at

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