Devon Flooring Suppliers offer great advice on choosing underfloor heating underneath oak floors. Many customers are often relieved to finally find a company that knows what they are talking about! Here in Devon, floor suppliers have lots of experience with supplying wooden floors to be used over underfloor heating and can now share the benefit of their experience.

EXPANSION & CONTRACTION:

The most important point to realise, is that a wooden floor is subject to expansion and contraction, according to the amount of moisture in the local environment.  Hence your solid hardwood boards may contract slightly during winter months (as central heating produces a much drier atmosphere) and expand slightly during the summer when windows are open, heating is turned off, and the atmosphere within the house has more moisture. 

This expansion and contraction is magnified when dealing with underfloor heating as the wood is subject to a higher than usual temperature and degree of moisture loss.  There are two main structural options to consider before choosing which type of oak flooring to lay over your underfloor heating.

Traditional engineered boards have a softwood core and are resistant to movement under normal conditions.  However, for the more extreme conditions of under-floor heating we would recommend engineered wide plank flooring.  Engineered  flooring has a unique structure; layers of water-resistant birch ply are glued together in opposing directions to form a 14.5mm base and finished with a 6mm hardwood layer. 

This gives not only the appearance and the durability of a solid wide oak board (180mm face width), but also ensures even better stability than a traditional engineered board. 

In addition the boards are unfinished, leaving you the opportunity to apply a lacquer or oil of your own choice, to produce a more natural looking floor.  Many suppliers can also supply these boards with an oiled finish if required.  Engineered flooring has been successfully used in many situations where different types of under-floor heating have been used and is extremely resistant to movement. 

It is advisable to use the ‘loose laying’ technique as described below to ensure the engineered boards remain as stable as possible. Engineered boards can be glued directly to screed floors (using glues) or secret nailed to battens.

Generally the wider the board the greater the expansion and contraction. Thus flooring suppliers would recommend that the width of the boards is kept to a maximum of 130mm, if using solid oak boards, to minimise gaps.

Opt for solid pre-finished RUSSIAN OAK if you want solid oak boards, as it comes in a range of three widths: 90mm,110mm & 130mm, has a surface of 5 coats of tough lacquer and also has one coat of lacquer on the reverse side to reduce moisture movement. 

Solid hardwood boards are normally air-dried and then kiln dried to between 8 and 11% moisture content.It is not usually recommend further kilning as it can dry the boards too much and damage their structure.   If you are using solid oak boards (remember: the narrower the better) then we would recommend ‘loose laying’ the boards. 

This means a fitter will lay the floor, cut it to size and simply push the tongue and groove boards together rather than fixing or nailing.  The floor can be lacquered or oiled like this and then used as normal (although it may squeak as the joints move).  The under-floor heating should then be GRADUALLY increased over a period of a couple of months until you reach the normal operating temperature of your under-floor heating. 

This will gradually acclimatise the boards to the extra heat & moisture loss rather than the severe impact of extra kiln drying.  Once a normal heating temperature has been reached the boards can be taken up a few at a time and fixed either by gluing or nailing (see below).

We would recommend either secret nailing or gluing boards depending on the method of under-floor heating.   If battens have been laid then the flooring can be secret nailed to this but great care must be taken around any areas close to the pipes. 

It may be advisable to use the SIKABOND T2 glue (supplied in cartridges) to glue boards to any battens that are close to pipes to ensure that there is no damage to the pipes from nails. 

Any resultant water leaks from pipes are disastrous!

If a concrete screed has been used then gluing boards using either SIKABOND T54 or T52 is the best method to ensure a rapid heat transfer. (Any layer of air left when using ‘floating’ boards will act as barrier to the heat transfer – this is another reason why traditional engineered boards are not as suitable for under-floor heating, as they are usually laid as a ‘floating floor’.)