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Should you overboard a subfloor? The answer is…

After putting together sections of the CFA guide with industry colleagues, Martin Cummins was asked by CFJ to revisit an old article on overboarding subfloors. Has much changed since he wrote the original piece? The answer is both yes and no.

A LONG-ESTABLISHED method for 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, more usually, bonded if it’s a commercial project.

This article isn’t referring to cementitious or calcium sulphate screed board installations, but is purely focusing on ’overboards’ that the flooring contractor may use during an installation.

The following is an overview and summary. Further information can be found in the CFA guide to Good Flooring – Section 4 Substrates.

Before any overboarding is considered, it’s necessary to take out significant ridges and high point height variations on the subfloor to ensure an approximate smooth finish. On timber substrates, this can be done by planing, sanding etc, while on solid screed or concrete more robust methods will be needed.

If any significant hollows or voids remain then these need to be infilled with a suitable repair mortar or similar product. Failure to do this this will create a depression point that the overboards will compress into when under load.

Other areas that must be addressed are the quality of the overboarding, both in its integral nature and how it has been installed, as well as the changing nature of the subfloor that will occur with new floorcoverings.

The first concern is something you as a contractor can easily overcome by always using a suitable grade of boarding. In my original article, I discussed discrepancies and qualities of plywood within the flooring sector.

This posed a major issue on many fronts, including surface consistency using different wood species, the thickness of the various layers, as well as the adhesion between them and the presence of voids within them. The CFA has since carried out consultancy and test work with the aim of creating an acceptable standard of plywood for the industry. Happily, this seems to have taken hold with a realisation that the flooring laid can only ever be as good as the subfloor below.

Furthermore, the CFA has produced a guidance document incorporating specifications for various aspects of the plywood to ensure it meets the needs of the flooring contractor. Following on from this, the guidance has been incorporated into the British Standard BS8203:2017 as an appendix (annex A (normative) Guidance on the selection and specification of suitable plywood for fabricated underlays). Several plywood suppliers have also adopted the standard to make the necessary plywood available commercially.

From a CFA perspective, this is exactly what we are trying to do: improve standards and improve installations. In this instance, it’s taken time and toil but hopefully has worked out to everyone’s benefit.

You’ll now see in the industry that we no longer advise ‘flooring grade ply’ or ‘water and boil proof (WBP) plywood’ but instead advise that plywood conforms to certain standards of manufacture and performance – as prescribed in the annex quoted above.

If you choose a hardboard, make sure it is a minimum of 5mm thick. If it’s been conditioned, then all well and good, but if not or if you are to do this yourself, then it must be dampened down prior to fixing to ensure stability. It’s recommended you use 1 litre of water per full sized (1.22 x 2.44m) panel.

Be aware: if you suddenly apply a wet adhesive to hardboard that hasn’t been conditioned you’ll effectively be dampening it down, which will result in it swelling and buckling off the floor. Obviously, this won’t be a welcome feature for your nice newly laid floorcovering. Even when loose laying floorcoverings over hardboard, it’s important to carry out this process, otherwise you may get lumps and bumps across the floor.

As an alternative, particularly when loose laying the floorcoverings, look to use a preconditioned oil-tempered hardboard. Always check with manufacturers that this won’t hinder adhesion if subsequently bonding products.

I won’t go into the practicalities of staggering joints, countersinking screws etc, as this is something installers will be instructed on at any good training school. It’s still an essential part of any flooring contractor’s training. The CFA Guide to Flooring will give you guidance on types of fixings, regularity, distance apart and more, with an example being that fixings should be a minimum 2.5x thickness of the overboarding.

Assuming you know the particulars of ‘fitting’ the overboarding, what else is there to be aware of? When it comes to utilising plywood as just overboarding, a minimum of 6mm (usually supplied as thickness quoted 5.5mm) is required. However, if you’re trying to add stability and strength to the subfloor, look at thicknesses from 15-22mm. Make sure it is screwed and fixed at appropriate points, usually a minimum of 300mm centres.

Often, discrepancies need to be ‘feathered’ out from screw heads, joints between sheets etc, to give a continuous smooth floor. Increasingly though, there is a desire to apply a continuous smoothing compound over the overboarded area and onto any adjacent areas. Fibre reinforced smoothing compounds are often considered a great option on timber subfloors owing to their ability to remain integral even with bounce and vibration in the subfloor.

If this is being done make sure the correct smoothing compound and where necessary the correct primer is used. The amount of fibre reinforcement can vary significantly between products, so don’t assume a product described as ‘fibre reinforced’ will automatically be suitable. The primer is key in assisting adhesion to the overboarding as well as controlling the drying of the smoothing compound application.

Always consider changes in the flooring balance: overboarding takes place most often on timber substrates. A timber subfloor that is in balance with the surroundings can ‘breathe’ to some degree. This means moisture vapour 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 won’t affect this balance, but the overboarding will be covered by a flooring material and this does have to be considered.

If this floor material is of low permeability, (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 and fairly dry environments, particularly in winter with central heating in use, so they’ll try to ‘pull’ moisture from the ground into this atmosphere. Any barriers will result in the moisture accumulating underneath the barrier in a similar way to epoxy DPMs and wet subfloors.

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

Construction techniques are moving on, with particular emphasis on ensuring acoustic and thermal gains, as well as offering greater green benefits from the materials used.

The use of timber and other subfloor materials will also continue to increase,
which poses challenges for us all in the future… and hopefully more things for me to talk about in CFJ.

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