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Central Heating Can Leave You Sweating

Jim Coulson on why old wooden floors are uneven

I BEGAN this monthly series of articles in CFJ way back in 2012 after receiving an invitation from the Contract Flooring Association to assist them in developing a specification for a better quality of plywood to be used for overlaying old floors.
Of course, ‘Diamond Marked’ plywood partly came out of that work, as some of you may know. But although a plywood overlay is the means of masking the very uneven character of old wooden floors – as has been the case for a very long time – I began to wonder whether anyone involved in such a procedure actually knew why those old timber floors so often needed such ‘cosmetic’ overlay work in the first place.
I’m pretty sure that the vast majority of that sort of upgrading work takes place on old ‘solid wood’ boarded floors, rather than on chipboard floors: and also that the majority of those ‘old floor’ situations are due to uneven softwood floorboards, which are laid on top of softwood joists.
The biggest cause of unevenness in those types of floor is movement of the timber, or to be more precise, shrinkage of both the softwood joists and the floorboards.
That shrinkage will generally have happened in the relatively recent history of the floor, long after it was originally built, back in Victorian, Edwardian or pre-war times. Because those were before it was the norm to have central heating.
(In fact central heating had been invented by then – even the Romans had it; and it was called a ‘hypocaust’, if you’re interested).
But central heating was not by any means universal, and most ‘ordinary’ buildings – houses and flats – didn’t usually run to it: so a lot of the central heating now warming up older buildings was actually retro-fitted, in the 1980’s and onwards. And that is where the trouble usually started!
All wood shrinks as it dries, of course. But what is not always understood is that wood also reaches ‘equilibrium’ (balance) with its surroundings. Timber used in unheated buildings settles down to a level of moisture that is quite a bit higher than it would do in a centrally-heated environment.
In other words, a typical moisture content for floor joists and floorboards in a house or flat without central heating would be around 15 to 16%; whereas the ‘equilibrium moisture content’ (or EMC as it is more usually known) of softwood in a house or flat with central heating would be about 10%.
Moreover, timber which has apparently ‘settled’ in one set of conditions will still change if those conditions should change: so that when central heating is installed and set going, the wood which was previously happy at 16% will very soon reach 10% and the result is a not insignificant loss of dimension.
So floor joists shrink by a couple of millimetres (from 150mm to about 148mm or less) but unfortunately, not uniformly: so some joists may end up at 149mm, others at 147mm and others maybe even as narrow as 145mm.
The result is a very uneven floor level, where previously it was flat! To add insult to injury, as it were, the floorboards will also shrink sideways and thus there will be gaps between adjacent boards of 2 or 3mm – sometimes more, if the heating is turned up high.
So the net result of adding heating into older buildings, without having any consideration for the consequences, is often floors that are no longer completely level, with floorboards that have unsightly gaps between them.
But it’s not the fault of the wood. It’s really the fault of the building designers or owners, who didn’t appreciate the fact that wood is a natural material and will react to changes in its environment.
So if someone then wants to lay down a ‘sensitive’ material (such as an expensive vinyl flooring) onto such a shrunken and gapped floor, then the only practical solution is to overlay that old floor with new plywood…then at least wood is sorting out the job once more!
Jim Coulson FIWSc FFB is director of TFT Wodexperts, based in Ripon, North Yorkshire
T: 01765 601010

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