Slab condition aspects for design of timber tops for concrete slabs

In Australia, concrete slab on ground accounts for about 75% of the primary floor surfaces in new domestic structures. Because of the blandness of the bare concrete surface, it is inevitably covered with other more attractive material, particularly in the living areas of the house. Covering the raw concrete surface with plywood offers several finishing options. The plywood can provide an underlay over which a tongued and grooved strip floor or other flooring surfaces can be easily laid; or the plywood can be sanded and surface treated to highlight its grain and provide the final finished surface.

Slab Condition

Moisture Content

Singularly, the most important factor determining the construction procedure to be followed when fixing plywood to a concrete slab is the moisture migration characteristics of the concrete slab. For a properly constructed slab, with good ventilation, drying from one face only and poured on top of a continuous vapour barrier, it could be expected that 75 to 80% of the moisture originally present will have been released to the atmosphere 4-6 months after laying. After the moisture release period has been completed the surface of the concrete slab will attain equilibrium conditions with the surrounds. To check if such a condition does exist can be done either by:

1. hygrometer test (requiring specialist apparatus); or
2. a sheet of glass or polyethylene film test.

This second test involves taping a 450mm square sheet of glass or clear polyethylene to the floor and leaving it for 24 hours. These pieces should be located on the slab near any possible problem or damp areas. Also, a number of locations should be tested over the slab.

If moisture appears on the glass or film, the slab is unsuitable for overlaying with plywood.

The long term (greater than 6 months) presence of excessive moisture in a concrete slab can be attributed to a number of reasons. They can be:

• broken, or no vapour barrier;
• thickening of the slab due to structural reasons;
• faulty plumbing; - watering gardens; - accidental flooding; and
• end ingress of moisture due to the slab being below ground level;

Physical Quality

If the final appearance of the plywood floor or plywood underlay/timber floor is to be acceptable it is essential the finish of the concrete slab is relatively flat, with few surface undulations. As a guide, the maximum depth of undulation over a straight edge length of 1500 mm, should be less than 3mm when the plywood is to be fixed directly to the slab. Should this limit be exceeded it is preferable to minimise the undulations by the application of a layer of lightweight proprietary cement.

Surface Preparation

When fixing the plywood directly to the concrete slab (particularly by gluing) it is highly desirable that the slab be free from all contaminants such as oil and grease. If not, it will be necessary for the concrete surface to be thoroughly cleaned using either an abrasive blast or a high pressure water wash. In some cases it may also be necessary for the concrete to be scrubbed using an industrial degreaser solution prior to high pressure water washing.

The plywood

For guaranteed compliance to Australian product, design and application standards, and guaranteed reliability and performance, the plywood should be branded with the PAA Product Certification stamp.

This stamp ensures the product has been manufactured under a JAS-ANZ accredited product certification scheme meeting the internationally recognised IS0 Type 5 system for product reliability to AS/NZS 2269 and has the permanent Type A marine bond. If the plywood is to be used as an underlay for some other flooring surface, e.g. strip flooring etc. then the C face will be satisfactory.

If, however, the plywood is to be featured it will be necessary to specify an A or S face.

Laying Plywood Sheets

If battens are used, the sheets must be arranged to span with the face grain perpendicular to the battens. Tongue and grooved plywood panels are recommended. Tongue and grooved plywood sheets should not be forced together by clamping. Hand pressure is adequate. The resultant gaps between sheets will be sufficient to accommodate normal moisture movement in residential floors. For large floor areas small perimeter gaps should be left between the flooring edge and the walls. The gap can be hidden by a skirting board nailed to the timber wall.

As a guide the perimeter gap should be: 0.25mm/m length or width of floor

Timber

A minimum 70 x 35mm seasoned timber battens, preservative treated, or otherwise depending on the moisture content of the slab, should be used. If the slab has been constructed to conform with the requirements outlined above, then moisture should pose no problems and on this basis untreated battens may be used.

If, however, the slab has been laid without the inclusion of a vapour barrier under it then where the:

1. vapour barrier is laid directly over the surface of the concrete slab, the battens need not be preservative treated;
2. vapour barrier is laid over the battens, the battens must be preservative treated.

NOTE: All timber (including the plywood) to be used in the floor construction should be allowed a few days on site for its moisture content to acclimatise with the surrounding environment.

Connectors

Tables 1 and 2 provide PAA recommendations for fixing the plywood to the slab. These recommendations are not exhaustive and further information can be obtained from the fastener or adhesive manufacturers. If preservative treated timber has to be used the question of fastener corrosion must be considered. To account for this possibility when using mechanical fasteners, ensure the nails are either stainless steel or zinc coated. Adhesives have been developed (e.g. H.B. Fullers Sturdi Bond) that can be satisfactorily used in conjunction with preservative treated timber.

Reference: Plywood Association of Australia brochure; "Timber tops for concrete slabs"

Table 1

Table 2

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Timber Research Unit
Department of Architecture
University of Tasmania