Before Installation Begins
Successful hardwood installation depends on site conditions being within acceptable parameters before any boards go down. Two variables require measurement before work begins: subfloor moisture content and the moisture content of the wood itself.
Subfloor moisture testing
Concrete subfloors require testing before any hardwood installation, whether nail-down, glue-down, or floating. The standard methods used in Canada include the calcium chloride test (ASTM F1869) and in-situ relative humidity probes (ASTM F2170). The NWFA specifies maximum acceptable moisture emission and RH levels for different installation types — these thresholds vary by adhesive system and product specification, so the flooring manufacturer's technical data sheet should always be checked alongside the NWFA guidelines.
Wood subfloors (plywood or OSB) are tested with a pin-type or pinless moisture meter. The NWFA recommends that the moisture difference between the subfloor and the hardwood being installed be no greater than 4% for strip flooring (boards under 3 inches wide) and no greater than 2% for wider planks.
Acclimatization
Hardwood flooring should be delivered to the installation site and stored in the room (or an adjacent space with similar temperature and humidity) for a minimum of 3–5 days before installation. This allows the wood to equilibrate to the building's ambient conditions and reduces post-installation movement. The HVAC system should be running during this period at normal occupancy settings.
In new construction projects completed in winter, the building envelope may not yet be controlling indoor humidity effectively. Installing hardwood into a structure with low winter RH (below 30%) and then having the space humidified to normal occupancy levels during spring and summer can cause the boards to expand significantly. Confirming that the building is at a stable operating humidity before installation avoids this problem.
Nail-Down Installation
Nail-down is the traditional installation method and remains the most common for solid hardwood over wood subfloors. A pneumatic flooring nailer drives cleats or staples through the tongue of each board at a consistent angle, fastening the board to the subfloor without surface fasteners visible on the face.
Requirements for nail-down installation include:
- A wood subfloor (plywood or OSB) of sufficient thickness — the NWFA recommends a minimum of ¾ inch for 2¼ inch strip flooring, with additional thickness for wider planks.
- Subfloor moisture content within acceptable range.
- Joists running perpendicular to the flooring direction, or a diagonal subfloor that accommodates perpendicular installation.
- A moisture barrier or rosin paper layer between subfloor and flooring.
Nail spacing is typically 8–10 inches on centre for standard strip flooring. The first and last few rows must be face-nailed because the nailer cannot operate near walls, and face-nail holes are subsequently filled with a matching wood filler.
Glue-Down Installation
Glue-down installation bonds hardwood directly to a concrete or wood subfloor using a flooring-specific adhesive. It is the required method for most solid hardwood installations over concrete on grade or below grade, and for many engineered hardwood products.
Adhesive selection
Flooring adhesives vary significantly in their moisture tolerance, open time, and flexibility characteristics. Urethane-based adhesives are most commonly specified for solid hardwood glue-down over concrete, as they accommodate minor moisture vapour transmission and provide some flexibility to handle seasonal wood movement. The adhesive manufacturer's technical specifications should always be matched to the wood species, board width, and subfloor conditions.
Moisture vapour barriers or moisture-tolerant adhesives are required when concrete RH levels are within range but not at the lowest threshold — the product technical data sheet defines the applicable limits.
Coverage and layout
Adhesive is applied with a notched trowel, with trowel notch size specified by the adhesive manufacturer for the product and board size combination. Coverage rate and open time must be managed in sections — applying adhesive over a larger area than can be covered before the adhesive skins over results in reduced bond strength.
Floating Installation
Floating installation does not attach the floor to the subfloor; instead, the boards are connected to one another at their edges and the entire floor floats as a unit on top of an underlayment. It is most common for engineered hardwood but is also applicable for some solid products in specific circumstances.
The absence of a mechanical or adhesive bond to the subfloor means that floating floors can move slightly underfoot, which some homeowners notice as a hollow sound when walking. This characteristic is inherent to the method and is minimized by proper underlayment selection and ensuring the subfloor is flat within manufacturer tolerances (typically 3/16 inch over 10 feet).
| Method | Suitable subfloor | Grade level | Typical use case |
|---|---|---|---|
| Nail-down | Plywood, OSB | Above grade, on grade | Solid hardwood over wood frame construction |
| Glue-down | Concrete, plywood | All grades | Solid or engineered over concrete slab |
| Floating | Most flat subfloors | All grades | Engineered hardwood, renovation projects |
Layout Planning
Starting wall and board direction are typically determined by the longest uninterrupted run in the room, which is usually run parallel to the longest wall or to the dominant light source from windows. Running boards perpendicular to floor joists is structurally preferred for nail-down installation.
A dry layout — arranging boards across the floor without fastening — allows the installer to identify problem areas: short cuts at doorways, awkward transitions, and colour or grain mismatches between boards from different bundles. Blending boards from multiple bundles during installation distributes any colour variation evenly across the floor.
Expansion gaps (typically ½ inch minimum along all fixed vertical surfaces) are required around the perimeter of every installation to allow for seasonal wood movement. These gaps are subsequently covered by base moulding or quarter-round. Skipping or reducing expansion gaps is a documented cause of floor buckling, particularly in Canadian homes where seasonal humidity swings are significant.
Transitions and Finishing Details
Where hardwood meets a different flooring material (tile, carpet, different hardwood), a transition moulding provides a finished edge and accommodates height differences. The type of transition (T-moulding, reducer, end cap, or threshold) depends on the height differential and whether the adjacent floor is at the same level or different level.
Stair nosings, where hardwood continues onto stairs, require a nosing profile that overhangs the riser and is secured to both the tread and the riser. These are available as matching species profiles from most flooring suppliers.
Technical references: NWFA Installation Guidelines; ASTM F1869 and ASTM F2170 for concrete moisture testing standards.