Do I need to use an adhesive for the sub layer when I am nailing the flooring down?

When planning to nail down a timber floor, it’s important to understand how the boards, underlay, and subfloor work together. In a typical nail-down installation, cleats or staples hold the boards firmly to the subfloor, and no adhesive is required between the floor and the underlay or subfloor. However, certain situations benefit from adhesive to prevent squeaks and improve stability.

Standard nail-down installations

The most common method for solid or engineered timber flooring uses angled cleats or staples along the tongue of each board. This mechanical fixing provides a tight fit and allows the wood to expand and contract with seasonal changes.

In this arrangement:

- Adhesive is not normally used under the boards because glue can restrict the movement of timber and may void the manufacturer’s warranty.

- A vapour retarder such as rosin paper or a specialist underlay is laid over the subfloor to reduce moisture transfer.

- Fasteners are driven every 100–150 mm (4–6 inches) along each board and 25 mm (1 inch) from the ends to meet the National Wood Flooring Association (NWFA) nailing schedule.

- Provided the subfloor is flat and the boards are of standard width (<5 inches), nails or cleats alone are normally sufficient.

When is adhesive useful?

Although not mandatory, using adhesive with nail-down flooring – often called glue-assist – can address specific problems. Seasonal expansion and contraction can loosen fasteners and cause boards to move or squeak. A trade guide notes that adding a glue-assist can prevent noise because the adhesive helps the fasteners hold the flooring firmly to the subfloor. The same source recommends a glue-assist for all nail-down floors and especially for:

- Wide boards: Solid or engineered boards that are 5 inches (≈127 mm) or wider are more prone to movement. A bead of adhesive across the width of the board strengthens the connection and reduces squeaking.

- Thin engineered boards: Engineered boards under 12 mm thick have a slim plywood core and can benefit from additional bonding.

- Rooms with moisture swings: Adhesive helps stabilise floors in environments with large swings in humidity or temperature.

A glue-assist involves running a bead of urethane or modified-silane adhesive across the subfloor or on the back of the board just before nailing. It is not a full glue-down; the nails still provide the primary fixing, and the adhesive supplements them. Always follow the adhesive manufacturer’s instructions and maintain recommended gaps (e.g. 10–12 inches between serpentine beads).

Factors to consider

Before adding adhesive, consider the following:

- Timber type – Some species (e.g. hickory) move more than others. Wide or less stable species benefit from additional bonding.

- Subfloor condition – Adhesive works best on clean, dry plywood or concrete. Uneven or damp subfloors may need to be levelled or dried first.

- Board width – Wider boards may need glue-assist, while narrower boards (<5 inches) generally do not.

- Moisture and sound – Full glue-downs can act as moisture barriers and improve sound insulation; however, they are more labour-intensive and costly.

- Manufacturer’s instructions – Always follow the flooring manufacturer’s installation guide. Many require or recommend glue-assist for boards above a certain width or for particular products.

Sub-floor adhesives versus underlay adhesives

Construction adhesive is sometimes used between sub-floor layers (e.g., gluing plywood to joists) to reduce movement and squeaks. This differs from the glue-assist described above. A small amount of adhesive between layers can stiffen the floor and reduce squeaks, but it isn’t meant to bond the underlay to the top flooring. Do not glue a foam or cork underlay unless explicitly instructed by the product manufacturer; most nail-down systems rely on the underlay remaining free-floating.

You generally don’t need adhesive when nailing down standard-width wooden flooring. Nails or cleats secure the boards and allow for natural movement. However, glue-assist methods can be worthwhile if you are installing wide planks, thin engineered boards, or working in areas prone to moisture fluctuations. A glue-assist uses a bead of adhesive to supplement, not replace, the nails and can reduce squeaks and improve stability. As always, consult the flooring manufacturer’s guidelines and, if in doubt, speak to a professional installer.