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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Installation of Hardwood Floors over Underfloor Heating

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.


Many hardwood-flooring specialists are reluctant to install hardwood floors over under-floor heating. Their reluctance is based upon fear of failure, ignorance and a lack of willingness to train their fitting staff and their timber suppliers to meet the modern needs set by under-floor heating. Within the construction industry, there is also a significant lack of proper understanding of under-floor heating systems and their effect upon hardwood floors.

Timber performance.

Timber is hydroscopic. It takes up free water when wetted. It gives off and takes up water vapour to and from the atmosphere. Timber species vary considerably in their hydroscopicity and their propensity to associated expansion and contraction. Not only do species vary, but, the 'cut' off the log affects the degree of expansion and contraction. Quarter sawn timber expands and contracts significantly less than tangentially sawn timber. A 1" thick tangentially sawn board will have a different co-efficient of expansion on one face to the other. The density of the timber affects the speed with which it will dry. Some very dense tropical timbers can be difficult to dry, even when machined very thin. Veneer will remain stable if securely bonded to a stable substrate and only deterioration of the adhesive will allow it to expand or contract. The Romans, the Greeks, the Egyptians, the Persians, the Incas and Aztecs, all the stone building civilisations throughout history have driven dried wooden wedges into holes in the rock of their quarries. These wedges were then wetted. The expansion of the wood provides the enormous force necessary to split vast slabs of stone from the quarry face.

Warm humid weather causes more expansion in timber than cold rainy days. Warm dry desert atmospheres desiccate timber to the point of cellular collapse, but, equally, cold dry atmospheres can also create excessive dryness. Sailors of timber-decked ships swab the decks, not to provide work for idle hands, but to prevent the timber from shrinking and, later, letting in water when the weather cuts up rough.

The point of the above list of apparently unrelated facts is to demonstrate that timber and moisture have a close relationship, that there can be slow, but powerful, even irresistible, reactions between them, that heat and cold have relevance, but less so than humidity. Under-floor heating is merely adding a new dimension to an existing technical problem with timber. It must be taken seriously, but is not something to get over anxious about.

Many years ago, floors fitted with under-floor heating were overheated to compensate for poor building insulation. High temperatures caused contraction and distortion in hardwood flooring. Today, high levels of insulation, improved ventilation systems, better adhesives and installation techniques enable hardwood floors over under-floor heating to remain stable.

Maximum Water Temperature

The maximum water or element temperature should be no greater than 50 deg. C. The maximum temperature at the underside of the timber should be no greater than 40 deg. C.

Fitting Procedures

Where the under-floor heating element or pipe is buried in a sand and cement screed the following system works. Therefore, it is recommended.

i. Seal friable screed with a suitable surface binding agent; e.g. polyurethane, PVA, epoxy resin or other rigid 'dry hard' surface adhesive. Flexible bituminous compounds are not suitable. It is essential that the screed is level to within + or - 3mm over 3 metres in any direction, dry, flat and smooth without snots.

ii. To that surface is bonded 12mm wbp far eastern plywood of good quality. It is not sensible to use poor quality ply that may delaminate easily, since the strength of the lamination bonds in the plywood will be relied upon in service.

iii. The adhesive used to bond the plywood to the screed is to be a 'dry hard' adhesive. Flexible adhesive such as bitumen emulsions, hot melt bitumen, contact adhesives etc. are not suitable. Laycol, Stauf and Silovil all make suitable adhesives.

iv. It is sometimes necessary to weight the plywood to ensure that good contact with the base is maintained until the adhesive cures.

v. It is essential that the adhesive be allowed to cure and harden before any boards or parquet is fitted.

vi. Over the plywood is fitted boards, geometric parquet or marquetry at 10mm thickness or less.

vii. The hardwood floor is fitted using a dry hard, rigid adhesive such as PVA, Laycol 5500, Silovil two pack polyurethane adhesive or similar.

viii. The parquet is maintained in position until the adhesive cures by pinning with air gun fired 20mm long pins.


Timber is an excellent insulation material, and therefore should be as thin as is practicable. The purpose of the plywood is purely to provide something into which fixings may be made. The parquet may be applied direct to the screed. 12mm WPB plywood is a good and economical underlay. Thicker plywood can be employed, but at the expense of the efficiency of the heat transfer.

Plywood is dimensionally very stable. Its surface has the quality of dispersing small quantities of moisture laterally. MDF, Chipboard, Sterling board and other particle boards are not recommended as the best underlay, though in areas where there is no potential for moisture ingress of any sort, they may be employed. They are not as dimensionally stable as plywood. Free moisture is not dispersed and can cause severe local swelling. The financial savings from using cheaper materials are not sufficiently significant to justify the increase in risk to the long-term integrity of the floor.

If the flooring to be fitted can be satisfactorily fitted to the screed, without the benefit of pinning, reliant entirely upon a rigid adhesive, then the plywood may be dispensed with.

Dry Sand System.

There are advantages to laying the underfloor heating elements into loose dry sand. Other services, otherwise forbidden to be set in screed, can also be run in the dry sand. In future years, well after the initial installation, new services can be introduced with minimised disruption. In the event of damage to the heating pipe, the damaged portion of the pipe can be quickly identified and easily exposed for repair. Wet sand can be quickly and easily removed and replaced without damage to the remaining pipework - whereas wetted screed is difficult to remove, the removal being more likely to cause other damage to the pipework. Loose sand provides a 'softer' heat distribution to the underside of the timber. The sound tone of footfall on the floor is closer to a typical suspended board floor.

The system successfully adopted elsewhere is as follows;

1. Over concrete base, tanalised softwood bearers 100mm x 50mm are rag-bolted at 500mm centres and adhered, continuously along their length, with modern rigid adhesive ( Gripfix, Gripfill, No Nails etc. ) to the base.
2. The bearers are laid 100mm face down at 300 to 500 centres. The spacing depends upon the thickness of the flooring to be laid over, the anticipated loadings and the requirements of the heating engineer. In deciding the optimum bearer centres, if the information is available, it is worth considering the lengths at which the timber will be supplied if solid boards are to be employed. There is nearly 15% wastage of the boards if centres of the bearers are set at 500mm but the boards are mainly in 3.4 metre lengths.
3. Kiln dried sand is poured between the bearers to level with the tops. Plywood stops and polythene sheet may be used to stop sand at open ends. Insulation board is used to separate the sand from external brickwork walls.
4. Over all is laid visquene sheeting to prevent the sand escaping.
5. The underfloor heating pipework is laid over the visquene which prevents it sinking into the sand.
6. Over this may be fitted 18mm plywood and up to 10mm hardwood as described above, Or
7. Over this may be fitted 20mm to 30mm thick solid boards, nailed or screwed and pelleted to the bearers.

Heating Element Location.

The spacing of the tubing is the responsibility of the heating engineer. It should be even. Crowding of tubes, e.g leading to or from manifolds, should be avoided under the timber floor. Such crowding creates areas in which much higher calorific output is concentrated and can lead to localised shrinkage.

Fixings over under-floor heating systems.

As a rule it is best to so organise the specification of the under-floor heating system and the hardwood floor in combination so that no fixings need to be put through the flooring that risk penetrating the pipes. Where the pipe work or electrical heating element is in screed, unless the heating engineer and floor fitter work together to establish with a certainty exactly where they lie throughout the entire floor, it is extremely risky to put any sort of fixing into the screed. Fixings must not penetrate beyond the plywood substrate.

If softwood bearer battens have been set into the screed, fixings may be used to secure the timber floor to these. The flooring craftsmen must be punctilious in fixing only to the battens and must take careful note of the positions of any heating element crossing through notches in the battens.

Other systems are designed that make use of trays carrying the heating element suspended between joists to enable flooring to be fixed to the joists.

Environmental Control.

The problem that under-floor heating presents for the hardwood flooring is compounded by the (often unrealistic) demands and expectations of the customers. For reasons too complex and laborious to go into here, today, customers demand that natural materials give up their peculiar behaviours and remain inert and unchanging under all circumstances - that real wood behave like Amtico or Formica wood effect plastic - that the real behave like the hypereal.

To get close to achieving this state in a hardwood flooring over under-floor heating, the environmental controls must provide a year round consistent environment. The principal that really matters is humidity control. Ideally, the heating, ventilation or air-conditioning systems should have humidity monitoring and automatic controls, so that the eradication of excessive humidity - which will be the most likely cause of problems - is not reliant upon human interaction with the controls.

Really Important. At a suitable point, a heat transfer control monitor should be fixed to the floor or to a monitoring sample of the floor in the boiler room, as close to the output as is reasonable. This will monitor the heat at the timber surface. Excessive heat will cause the system to shut off or to divert the hot water to other areas.

Exterior temperature monitoring warns the system of sudden temperature drops to the exterior of the building. These sudden changes in exterior conditions can cause problems with condensation within buildings. This is particularly true for buildings which have a high occupancy for long periods followed by periods of no occupants during which windows and doors are shut and heating is shut down or significantly reduced, e.g. office buildings, public halls, school assembly halls etc.

Moisture in the subfloor and building.

Prior to fitting hardwood flooring, the under-floor heating system should be fully pressure tested and run, usually for a fortnight, but certainly until all moisture in the screed or plywood subfloor has been driven off. Plaster should be dry and all wet trades finished in the rooms to which the flooring is to be fitted. Relative atmospheric humidity should be no higher than 75% and no lower than 60%.

Laying polythene sheeting over an area of the floor will carry out a simple, but effective, test for moisture. It should be marked as a test and weighted down to prevent accidental removal. The minimum area covered should be 1m x 1m, but it is preferable to cover an area as large as is practicable. Turn the heating system on and leave for 3 days - less if the test proves positive earlier. After 3 days if there are no beads of moisture on or under the polythene, then the floor is dry and fitting may proceed. If there are any beads of moisture or fine misting under the polythene, the floor is not dry and the new flooring must not be fitted until it is.

Heating during and after fitting.

The under-floor heating should be turned off during fitting. Any space heating required during cold weather should be dry electrical heat not blown air heated by propane burning which puts a lot of moisture into the air. After fitting, the flooring should be allowed to rest for a few days to ensure complete cure and hardening of the adhesives used. Once the adhesives have hardened the heating may be turned on. The heating should be raised gently by 1 degree C per day from the prevailing ambient temperature. It must not be turned full on right away.

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3. Thick or Thin?:

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3. Fixing methods.:

3. Thickness of surface.: