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Questions That Lead To More Questions

ONE of the most frequent calls made to our technical helpline is on whether products can be laid over a timber floor. Technical datasheets would be the size of a small booklet if they were to include all scenarios and possibilities, so often many make basic statements such as ‘suitable with flooring grade plywood’ or ‘timber floors must be over boarded.’

So the calls come through and the ‘can of worms’ that this question opens up can sometimes lead to quite a lengthy conversation with the customer, usually telling them there needs to be a great deal of thought and consideration before blindly applying levelling compounds and adhesives. I would like to take this opportunity to go through the basic questions that I presently ask of a customer when this vague question is raised…and of course my responses to them.

Q: What is it you are referring to as timber?….is it a previous finished decorative timber floor, tongue and grooved floorboards or some type of boarded floor?

A: A plywood floor mechanically fixed and secure and of flooring grade (now that’s another topic!!) would generally be fine. A flooring grade chipboard if mechanically fixed can be overlaid by plywood or, in some circumstances can have adhesives bonded directly. Previous finished decorative timber floors can be problematic due to treatments and lacquers etc. Also older design parquet and block flooring was fitted with moisture tolerant adhesive and was relatively dimensional stable but care must be taken to ensure the conditions of equilibrium are not being changed. Tongue and groove flooring is very rarely suitable to lay anything other than carpet due to the excessive amount of joints and its general unevenness so we would advise securing it and overlaying with plywood

Q: What floorcovering do you intend to install on this particular project?

A: Carpets and other breathable products pose much less risk than resilient ones, as the potential for build-up of moisture and therefore movement of the floor is less. However, even textiles with backings and also underlay systems can cause the same concern. A tile (i.e. ceramics and stone products) requires a subfloor with a much greater degree of rigidity and may need significant bracing with potential for further noggins being fixed as well as a substantially thicker grade of plywood being fixed.

Q: Is this a newly-built property?

R: The question here is twofold, firstly has any timber been seasoned and acclimatised or is there still potential for movement? Secondly the design is probably known so check with the main contractor as to what specification has been put forward by the architect rather than second guessing and taking on the design responsibility. Floors with underfloor heating, for instance, may cause issues if the timber is covered over. The timber may have a limited load bearing so using heavy plywood overlay along with thick stone tiles may be too much.

Q: What floor level are you referring to?

A: Floors at ground level and below can generally be considered the most problematic. This is due to the likelihood of moisture being drawn into the timber from the ground into the warmer occupied room above. Putting materials on the timber which could prevent this moisture movement can result in decay at the worst and movement at the least – both possible issues. Older buildings without a base DPM are most at risk although the standard of timber used in the past is probably far superior to that used in modern building.

Q: How is timber fixed to the substrates?

A: Boarded floors that are screwed at regular centres, typically less than 300mm, are the best case scenario. Nailed down timber should be re-secured using screws. Decorative timber that is bonded in adhesives may need uplifting or overboarding. Floating timber floors may include materials designed as structural such as plywood or chipboard OR may be decorative materials such as engineered and laminate flooring. The latter should be uplifted. The former will need to be assessed further as there is a potential for further bonded layers causing tenting or dishing of the boarded floors.

Q: Is there or was there any other floorcoverings on the timber prior to you being requested to lay new coverings?

A: In general, replacing the old flooring with a similar new flooring is OK, provided the reason for replacement is not due to failures.

In summary: Timber floors can mean many different things to a lot of different people so when the question is raised it pays to delve deeper. Both new and old buildings may have timber floors to which new decorative and functional flooring is to be fitted. The main obstacles to overcome are preventing moisture entrapment in timber and ensuring it is strong and stable enough to take the new flooring system.

There is usually an answer and a manner in which flooring can be successfully laid but unfortunately it is not usually a quick and easy fix. The industry as a whole is always looking at problem solving and products with different qualities and performance characteristics are becoming a real option but often with associated higher costs and further work. Unfortunately designers and architects do not appear to be considering how final decorative and functional flooring will be fitted and design a building with priority on thermal, acoustic, environmental and ease/speed of build among other things so we will always have these issues to overcome.

Martin Cummins, Ultra Floor technical sales manager

T: 01827 871871

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