Traditional Secret-Nail Fixing of Wood Floors
Jun 10, 2009 DIY tips, bonding, how to, wood flooring
Solid tongued and grooved strip and plank flooring may be installed by secret-nail fixing to suitable wood-based sub-floors such as floorboards, flooring grade chipboard, plywood, or OSB.
In most cases existing decorative hardwood floorcoverings should be removed before the installation of new hardwood flooring.
Environmental Conditions
- All wet trades must be thoroughly dry before flooring materials are delivered to site.
- Maintain suitable ambient humidity 40 to 60% RH. (Monitor with a hygrometer)
- If necessary employ a domestic humidifier or dehumidifier to maintain suitable humidity.
- Ambient temperature to be 18 to 22 Celsius during acclimatisation and installation.
Basic Requirements of the Sub-floor
- Wooden sub-floors must not be installed as a floating floor, but be securely fixed (e.g. floorboards to joist).
- Ensure all sub-floors are sound, free from decay, and of load-bearing strength.
- Wood based sub-floor materials must not be more than 2% higher in moisture content than the new wood flooring materials being installed.
Tip
- Flooring must not be exposed to artificial heat sources from the sub-floor. Hot water pipes beneath suspended wood floors can cause excess heat and should be thoroughly insulated. Floor temperature must not exceed 24 Celsius.
Evenness of sub-floor
- All wood based sub-floors must be even to within 2mm over any 2 lineal meter area.
- In order to achieve the necessary degree of evenness, traditional softwood floor boarded sub-floors typically require a WBP plywood covering to be securely fastened (preferably by screwing) to the floorboards before secret-nail fixing new wood flooring. (Plywood must also be of suitable moisture conditions. See ‘Basic Requirements of the sub-floor’ above.
- It is recommended that flooring grade chipboard, OSB & old floorboards are first covered with WBP plywood which is securely fastened to the sub-floor before secret-nail fixing new wood flooring, as this will hold nails well and help avoid squeaking.
Suspended wood based sub-floors at ‘ground level’
- Voids beneath suspended wood based sub-floor on ground level must be well ventilated to avoid high humidity in the sub-floor void.
- First lay a building paper moisture barrier (e.g. Sisalkraft Moistop), over the existing sub-floor overlapping joins by 100mm and taping with a waterproof tape.
- Then install a WBP ply layer, securely fixing through the building paper to the sub-floor.
Secret-Nail Fixing
- When fixing flooring solely by secret-nailing, we recommend that a building paper moisture barrier (e.g. Sisalkraft Moist Stop), is laid over the existing sub-floor overlapping joins by 100mm and taping with a waterproof tape.
- Boards of 150mm or wider may be secured with adhesive in addition to secret nail fixing.
- When using the combination of secret-nailing and adhesive, a moisture barrier building paper is first laid over the existing wood based sub-floor, followed by WBP ply. The WBP ply is securely fixed through the building paper into the wood based sub-floor. Hardwood flooring is then bonded to – and nailed through the ply base into the existing wood based sub-floor.
- Secret-nail fixing is achieved using a purpose made floor nailer (e.g. Portanailer, Primatech etc) which are used with purpose made serrated galvanised floor nails of varying length depending on the thickness of flooring being installed. (18-20mm flooring thickness = 50mm nail).
- Tongue and groove flooring is secret-nailed (also called ‘blind nailed’) fixed through the tongue at approx 45 degrees into the sub-floor at intervals of 200 to 250mm.
- Nails are placed 40 to 60mm to each end of each boards, with not less than two nail fixings per board.
- In addition to secret-nailing on the tongue side, starting rows one and two are face nailed or can alternatively bonded to sub-floor to secure the groove side that is not linked into an adjoining tongue
- If starting from a centre line using biscuit tongues, the two centre boards may be face nailed or preferably bonded to the sub-floor using a flexible adhesive.
Provision for expansion
- As a guide only allow a minimum 15mm expansion wherever the floor meets obstacles including perimeters walls, structural supports, hearths etc.
- Create additional expansion breaks in doorways using suitable profiles such as T-section thresholds / transition strips.
- Create additional expansion breaks in large floors.
- Where possible install flooring so that planks, strips run parallel to the longest walls, so the direction of greatest potential expansion (i.e. across the grain) does not coincide with the direction of greatest dimension, esp. in large floors.
- The precise combined provision for expansion must be judged by the installer taking into account environmental humidity, moisture content of wood at time of installation, timber specie and size of the floor.
For more information see our wood flooring site.
Tags: adhesives, Bona, bonding, DIY guide, secret nailing, Sika, solid wood flooring, underlays
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