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Subfloors – Think Before You Go Overboard

Martin Cummins on overboard floors

A LONG established method of preparing a subfloor before laying resilient or textile floorcoverings is to overboard the area with a suitable grade of plywood or hardboard. The floorcoverings can then be loose-laid if required on a domestic project or bonded, more usually, if it’s a commercial project.

Usually there is a need to feather out any discrepancies from screw heads, joints between sheets etc, to give a continuous smooth floor. But more and more there is a desire to apply a continuous smoothing compound over the overboarded area and onto any adjacent areas.

So should this pose a problem? Do we need to be aware of any potential issues? The short answer to the above is YES.

The main causes for concern are on the quality of the overboarding, both in its integral nature and how it has been installed, and also to the changing nature of the subfloor that will occur with new floorcoverings.
Addressing the first concern is something that, as a contractor, you can help yourself to minimise. Always use a suitable grade of boarding.

I am not going into the practicalities of staggering of joints, countersinking of screws etc, as this is something that installers will be instructed on at any good training school. It is still an essential part of any flooring contractor’s training. So assuming you know the particulars of ‘fitting’ the overboarding, what else is there to be aware of?

If you choose a hardboard, make sure it is a minimum 6mm thick. If it has been conditioned then all well and good, but if not OR if you are unsure if it is conditioned, then it needs dampening down prior to fixing to ensure stability. Make sure you do this!

If suddenly applying a wet adhesive to hardboard that hasn’t been conditioned you will effectively be dampening it down which will result in it swelling and buckling off the floor … in addition to your nice newly laid floorcovering.
Even when loose laying floorcoverings over hardboard it is important to carry out this
process, otherwise you may get lumps and bumps across the floor.

If choosing plywood, look for a minimum quality of exterior or WBP grade. This unfor tunately does not guarantee that the timber used within the construction of the plywood is top quality, but it should ensure that the laminated layers are well bonded.

If just overboarding then a minimum 6mm thickness is required. However if you are trying to add stability and strength to the subfloor, look at thicknesses from 15 to 22mm. Make sure it is screwed and fixed at appropriate points, usually a minimum 300mm centres.

Interestingly, the supply of plywood into the UK is extremely inconsistent at present and, probably over the last few years. A recent manufacturers’ meeting brought this to the fore and a programme of research and assessment into plywood available in the UK is now taking place.

Have you had any issues with plywood? If so, we would very much like to hear about them. Afterall you are at the sharp end so will have hands-on experiences.

The type of issues we come across are inconsistency in layer thicknesses, voids within the individual layers and also separation of layers. Furthermore the species of timber and its strength appears to have little control.
So there may be a thin hardwood veneer which is ‘adhered’ to a thicker softer timber resulting in the plywood being very easy to mark and impress. Whether you feather or completely smooth over this, the same problem remains … a heavy point load will indent it. Similarly voids within a layer can have the same effect.

Poorly bonded interlayers can split the laminate, resulting in clicking of the plywood which can, in the worst case, result in the floorcovering coming loose with a layer of plywood.

Always consider changes in the flooring balance: Overboarding most often takes place on timber substrates. A timber subfloor that is in balance with the surroundings can ‘breathe’ to some degree. This means that moisture vapour (I am not referring to water in the liquid form here) can be absorbed by timber and then released again depending on the humidity of the environment. So assuming the present subfloor isn’t suffering any problems with twisting, warping or shrinkage, can it be safely assumed that we can overboard it? NO!!!!
The overboarding itself will not effect this balance, but the consideration is to do with the fact that the overboarding will be covered by a flooring material.

If this floor material is of low permeability, ie vinyl, rubber, backed-carpets etc, then the moisture vapour cannot get out of the substrate as readily as it can enter.
Rooms in houses generally have warm dryish environments, particularly in winter with central heating, so will try to ‘pull’ through moisture from the ground into this atmosphere. Any barriers will result in moisture accumulating underneath the barrier (think in the same way as epoxy dpms and wet subfloors).

You will need to ensure that there is adequate ventilation in the airspace below the timber and, if necessary, instruct that airbricks need to be introduced and that the airspace need to be cleaned out.

As construction techniques move on, particularly on ensuring acoustic and thermal gains as well as offering green benefits from materials used, then the use of timber and other subfloor materials will increase. This poses challenges for us all in the future … and hopefully more things for me to talk about in CFJ.

Martin Cummins is Ultra Floor technical sales manager
T: 01827 871871

This article has been reproduced from the Contract Flooring Journal website. You can find them at