Sub-Flooring: Types and Preparation

The surface quality of a finished floor is only as good as that of the sub-floor beneath. There are various types of floor surface and a variety of ways to renew or merely reinforce these. Whether you renew worn areas or provide a base for a decorative surface, an even and properly prepared sub-floor is important.

Sub-floors consist of two types: Solid-concrete tiles or stone floor and Suspended-timber boards or sheets fixed to timber joists.

Solid floors

Solid floors provide an excellent base for any type of wood flooring but must be dry and level. Any problems of dampness must be cured, at source, before a new floor is laid. An uneven surface may need rescreeding.

Minor irregularities in level can be rectified by using a proprietary self-levelling compound.

Suspended floors

Suspended floors may present more problems than solid floors. Even a sound surface will tend to move, as wood expands and contracts with the fluctuation of the moisture content in the air. Normally, this movement can be offset by laying a covering floor skin, such as hardboard.


If the floorboards are in a poor state of repair they may need completely renewing. New tongued-and-grooved or square-edged boards can be laid, but a flooring grade of chipboard, 19mm thick, is worth considering. Chipboard consists of wood chips, bonded and consolidated with resin, under heat and pressure. The result is a hard-wearing surface which is quick to lay and can appear attractive when sealed with a clear polyurethane varnish which provides a hard, durable finish.

First, remove old, worn floorboards. Check the condition of joists and replace damaged sections as necessary. Measure the centres of the joists, as the width will determine the size of chipboard sheet you use. The edge of each chipboard section must fit along the joist centres.

To fix down the ends, you may have to insert cross pieces, or noggins, between the joists. These are angle nailed through the joists with 75mm wire nails. At the end of a wall, the nails are driven in at an angle through the noggins into the joists.

Careful measurement of a room is needed to establish how many whole and part sheets are needed to ensure the most economical floor coverage.

Before laying chipboard, any projecting screw heads or brads should be removed or knocked down and the top of the joists cleaned, so that the new board will fit evenly. At the borders of rooms, chip back plaster projections with a bolster and a club hammer.

Chipboard is fixed with 50mm countersunk screws at 300mm intervals along the board edges. At adjacent edges, stagger the position of the screws to even stress. Work round each section, partly driving in each screw. Go round again and finally screw down. This ensures that boards go down evenly.

Flooring-grade chipboard can also be laid over badly worn and uneven floorboards. Prepare the floorboards, using a rotary sanding machine if necessary and nail the chipboard to the flooring with 75mm wire nails.


Hardboard also provides a firm base over worn floorboards. The boards must be fixed securely and smoothed down to remove any ridges or irregularities. Before securing a hardboard surface to a floor, ensure that wiring or pipe runs remain accessible.

Flooring-grade hardboard 5mm-6mm thick, should be used. Where floors are subjected to humid or damp conditions, oil-tempered hardboard should be used. Standard hardboard can be tempered by sponging or brushing the mesh side of the surface with water to allow the boards to expand to their fullest extent.

Place the sheets back to back, and leave them for 48 hours before fixing. Boards should be fixed with 13mm hard-board nails at 150mm centres.

If there is likely to be any movement on the sub-floor, nail it down securely before fixing the hardboard. Make sure the joints between the hardboard sheets do not coincide with those between underlying floorboards.

Hardboard can be nailed down or screwed. If it is to be screw fixed, pre-drill for the screws but do not counter-sink.

Another method of fixing is to use a suitable grade of flooring adhesive to stick down the hardboard to the sub-floor which must be clean and free from dust or grease.

Before fixing hardboard to a concrete screed, again ensure that the surface is clean and free from dust and grease. Worn areas can be built up with a self-levelling compound or a filler consisting of a 1:3 cement: fine sand mix with one part of PVA adhesive, diluted with three parts of water. Trowel on the mixture and feather off the edges.

Apply adhesive to the backs of the hard-board sheets, taking care to cover edges and corners. It is not essential to cover the back of the sheet entirely. Once in position, the sheet should be weighted down until adhesion is complete.

On new concrete screeds, hardboard may be laid after allowing two to three weeks for drying out. Ensure that the surface is free from dust, apply adhesive and then position the hardboard.

Floorboards in good condition can be rubbed down and treated with polyurethane varnish, to provide an attractive floor finish. First, nail boards down securely and fill small gaps with papier mache filler.

10. November 2011 by admin
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