David Gunton's Hardwood Floors.
Grange Lane, Winsford,
Cheshire, CW7 2PS
Tel: +44 (0)1606 861 442
Fax: +44 (0)1606 861 445

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Thick or Thin?

By David Gunton.

Awarded 1999

The Worshipful Company of Carpenters Special Award in Recognition of Outstanding Achievement in the Restoration, Re-Creation, New Design and Quality of Craftsmanship at Windsor Castle.


This article discusses the Choice of Timber, Thicknesses and Widths. The comments apply to both normally heated rooms and to underfloor heating installations. You should also read the article about Installation over Under Floor Heating.

Choice of Timber.

Almost any durable timber is suitable for use over under floor heating if it has been properly prepared for that purpose and the requirement for environmental stability will be honoured by the building owner / operator in the future. However, there are some rules that help in making the decision about what will be most suitable and practicable.

i. Availability. It is difficult to prepare the chosen timber if it is not easily available in the correct sizes. For instance, a timber that is only available in thick boards will be difficult to correctly prepare. Usually, there will be reasons why it is available only in thick boards. It is likely it does not perform well as thin timber.

ii. Drying properties. Very dense timbers, such as satinwood, are very difficult to dry quickly without surface checking. If such a timber must be used, use it as a veneer or thin parquet.

iii. Timber suitability. Obviously balsa wood is not a suitable timber. Equally there are other timbers such as ebony which are equally unsuitable not only because it is a rare endangered species but is very difficult to dry. If you are thinking of using something unusual, ask us for advice on the suitability of the timber for flooring.


iv. A misconception persists, flying in the face of all logic, reason and evidence, that 'thicker is better' for flooring. This may hold some truth for barns, stables and industrial floors where a goodly thickness is required for wear and strength, but appearance is not critical. For domestic, commercial or office use, which is to say, decorative hardwood floors, it is false. At Windsor Castle, the decorative marquetry floor in the Crimson Drawing Room, which the writer recreated after the fire, was only 2 to 3mm thick of veneer laid over a stable substrate. This veneer floor had lasted for some 140 years and had been in good condition prior to the fire. The marquetry borders to the floors in the State Rooms at Buckingham Palace are similarly constructed and have withstood considerable use and not a little abuse since the building was erected.

v. Use thin timber. Veneer on furniture remains stable in widely variable conditions because it is bonded rigidly to the substrate. It is forced to compress rather than expand and to stretch rather than shrink. In hardwood flooring this property of flexibility is borrowed from the 'plywood bonded to screed' construction and transferred by the rigid adhesive to the parquet fitted above. It has been found that 10mm thickness works adequately. This thickness satisfies the customer that he is getting value for money and a floor that will last. It is similar, if not a little thicker, than that thickness of wood above the tongue and groove of a 20mm thick board. That is the 'wear thickness'. Any floor that is worn or sanded down to the tongue and groove is effectively worn out. All the thickness below the tongue and groove is effectively wasted. It is there only to provide stiffness. Thinner than 10mm flooring would perform more reliably over under-floor heating under wider variations in atmospheric conditions. The wider the section, the thinner it should be to provide the greatest likelihood of dimensional stability. Note that thin veneers of several feet wide remain perfectly stable on furniture and plywood.

vi. Stable species. Species of timber vary in their stability. They are graded into bands of low, medium and high. Jarrah (eucalyptus marginata) is of low stability. For a given increase in moisture content it will expand more than a timber in the medium range such as oak (quercus robur) which, in turn, will expand more than Teak (tectonis grandis), which is in the high stability category. However, the range of timbers listed as of small movement suitable for areas of wide variation in temperature and humidity - Banga Wanga, Loliondo, Missanda, Muhuhu, Panga Panga, Zimbabwe Teak, - are mostly pretty well unobtainable as commercial timbers. Of those listed as suitable for fitting over underfloor heating - Abura, African Mahogany, Afrormosia, Afzelia, Agba, Ayan, East African Camphorwood, Gedu Nohor, Guarea, Iroko, Makore, Muninga, Opepe, Teak - some are unavailable. Most are rich red, dark brown or otherwise strongly coloured or patterned timbers. However, successful installations have been carried out with a wide variety of other timbers including oak, beech and walnut that are classified as medium for movement in service.

vii. The cut of the timber. Quarter sawn timber is more stable than tangentially sawn timber by approximately a factor of four. However, obtaining quarter-sawn timber is difficult. It must be planned well in advance of the requirement for fitting. The delivery period will depend upon the species and the time of the year. For temperate zone hardwoods the end of winter and early spring are the best time to plan quarter cutting because the felling season has just finished and there is usually a reasonable choice of logs from which to choose the best. Autumn is the worst time to attempt to buy the timber. By then the best of the seasons logs have been sold or committed to an inappropriate cutting size. Choice is limited. Obtaining quarter-sawn oak is usually easier than in all other species because the timber is so popular and widely available across Europe.

Wide Plank Flooring.

All wooden floorings expand and contract with changes in moisture content. This propensity is most easily controlled and disguised in thin, narrow, quarter sawn timber. Conventional parquet floors can be used over under-floor heating provided the necessary precautions are taken. However, with board floors, the wider the board, the greater the potential for gaps between boards to appear with changes in the moisture content of the board. The changes can be brought about by seasonal temperature and humidity variations. As a general rule, it is not recommended to use boards over 100 mm wide over under-floor heating. It is recommended that the timber be quarter sawn to provide the best stability. However, there have been so many successful installations that fly in the face of this rule that it is becoming discredited.

The Exceptions to the Rule.

There are always exceptions to the rule. This company has successfully carried out many exceptions to the rules including an installation of boards of Brobdignagian proportions, being up to 685mm wide (27 inches) by 7.3 metres (24 feet) long in 'country' grade, through and through sawn English oak at 32mm thickness over a piped water under-floor heating system. The installation is in a Grade II listed building set at the seashore. The house and the floor within is so low lying that if it were not for the bunds thrown up in the garden outside to keep out the sea, it would flood at spring tides. Yet, the boards have not changed dimension at all since installation. The stability has been achieved by ruthless adherence to all the principles set out in this essay - except those of choice of grade of timber, cut of timber, width and thickness. However, the client was warned that the boards will move if they do not maintain the evenness of the environmental conditions and that, even so, in practice, they must expect some movement.

Storage of the timber.

The timber awaiting fixing must be stored so that its moisture content remains stable. This may be achieved by shrink-wrapping the timber or by it remaining with the supplier until the site environmental conditions are as they will be when the timber is in service. DO NOT store the timber, exposed and unwrapped, in damp conditions. It will pick up moisture from the atmosphere. Later, in service, it will shrink.


This is a complex issue. If the supplier has correctly prepared the timber to the required specification, acclimatisation should not be necessary. In some circumstances acclimatisation causes other problems. If the atmosphere is not as it will be once the flooring is in service, acclimatisation will serve to condition the flooring to incorrect moisture content. If the timber is not exactly to specification it can distort during acclimatisation, thus preventing close jointing during fitting. This is particularly true of wide boards containing knots. The coefficient of expansion may vary considerably along the length of a wide knotty board. Acclimatisation of a 10" wide knotty oak board can result in its width varying. It will be wider across the sections interrupted by a knot than across the clear sections. This will prevent close fixing of the board and may present some open joints.

Some dense tropical timbers are extremely difficult to dry to a consistent level. Acclimatisation of these types of timbers may be helpful, but it is carried out at the risk of some severe distortion of the machined timber. It is the opinion of the writer that it is better to warn the customer that in service there may be some localised shrinkage or surface cracking of the timber that will later be filled and polished.


Hardwood flooring is susceptible to moisture, whether it be by loss, causing shrinkage, or by gain, causing expansion. This cannot be over-emphasised. Significant variations of heat and humidity create strains in the timber, which it or its fixings cannot always withstand. Therefore these variations should be minimised. Correct installation procedures, with suitable adhesives and lacquers, all carried out in accordance with sensible precautions, and ensuring a consistency of temperature and humidity in service, will combine to provide satisfactory, lasting and beautiful flooring. There is a partnership in the maintenance of the stability of the hardwood flooring between supplier, flooring contractor, builder, heating engineer and, most importantly, the customer. Providing the flooring supplier and contractor have prepared, supplied and fitted the flooring with close attendance to the specification, subsequent movement in the timber must not be laid at their door. They are not magicians, capable of transforming nature's changeable organic produce into the immutable inorganic.

For further information, please contact us.


1. Oak and Iron.:

3. Fixing methods.:

3. Fixing hardwood floors over under floor heating.: