Fig1 Plywood floor

Installation of Plywood Residential Flooring

LOADING SUPPORT AND THICKNESS OF PLYWOOD FLOORING.

Sub-floor beams and joists must be sized and installed in accordance with AS1684 "SAA National Timber Framing Code".

Table 1 sets out recommendations included in the Code for the minimum allowable thicknesses of F8, F11 and F14 stress grades of structural plywood over standard joist spacings in residential flooring applications. The minimum thicknesses are suitable for trafficable floors in residential applications with maximum occupancy requirements of uniformly distributed loads up to 3kPa and concentrated live loads of 2.7kN.

Live load requirements for floors are detailed in AS1170 SAA Loading Code Part 1: "Dead and live loads and load combinations".

NOTES

1. 15mm F11 or 14mm F14 provide equivalent stiffness to 19.5mm particleboard at 450mm joist centres. If the plywood is to be overlayed with large ceramic tiles use 17mm F11 or equivalent at 450mm joist centres, or use the bonded mortar bed system detailed for area flooring. This will minimise the likelihood of the tiles cracking.
2. The plywood must be installed as shown in Figure 1.
3. The plywood face veneers must not be thinner than any of the inner veneers.

FIXINGS

T&G structural plywood may be fixed to the sub-floor with hand or power driven fasteners, or a combination of mechanical fasteners and structural elastomeric adhesive for a more rigid, squeak free system. Table 2 specifies the recommended fasteners, while the adhesive installation method is described in the section on wet area flooring.

FLOOR INSTALLATION

The T&G structural plywood must be installed in accordance with the following recommendations:

1. The face grain of the panel and the tongue and grooved edges, must run parallel to span, i.e. perpendicular to the joists.
2. The end joints must be made on a structural sub-floor member, usually a joist.
3. Each panel must be continuous over more than one span.

NOTES:

1. If elastomeric glue is used in areas other than wet areas, the fastener spacing shown in Figure 1 may be doubled.
2. Structural elastomeric adhesive should be used where plywood is fixed to unseasoned floor joists.
3. It is good practice to use deformed shank nails where resilient floor coverings are to be fixed directly to the plywood, and/or the sub-floor members are seasoned softwood.
4. Plywood can be fixed within 10-12mm of its edges.

If square edge structural plywood is used instead of T&G, the side edges must be fixed to a common nogging running between the joists, the nogging of timber being not less than 75mm x 30mm set flush to the top of the joists.

SUB-FLOOR VENTILATION

It is critical that local government sub-floor ventilation requirements be adhered to strictly. If proper sub-floor ventilation cannot be achieved, preservative treated plywood should be used.

FINISHING

A light sanding of a T&G structural plywood floor will usually be required prior to the laying of flooring materials. The objective of the sanding is to remove construction dirt and to smooth out any ridges at joints between panels. The sanding is very important where the plywood is to be used as the underlay for thin resilient floor covering such as vinyl. If the moisture content of the plywood has been stabilised after installation and the floor is in an environment subject to minimal variation in humidity, the joints should be filled prior to laying thin resilient floor coverings. T&G structural plywood flooring requires no further underlays. It is designed to perform the dual functions of a structural flooring and an underlay. In residential buildings, 'standard' T&G plywood flooring is intended for use in floors to be covered by a wide range of floor coverings. Floors that may not require covering (e.g. rumpus rooms) can be made serviceable by applying a standard timber floor finish. Interior wood stains may be applied to the plywood prior to the floor finish to ensure uniformity of colour. If a permanent 'decorative' plywood floor is required, plywood with a single species face of an appropriate quality may be specially ordered. Decorative T&G structural plywood flooring can be clear finished in exactly the same way as strip timber flooring.

Reference: Plywood Association of Australia brochure "T&G Structural Plywood for Residential Flooring"

TABLE 1: Minimum Allowable Plywood Thickness For Residential Applications.

TABLE 2: Minimum Fastener Specification

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Timber Research Unit
Department of Architecture
University of Tasmania