Floor Constructions

There are many factors to take into account when considering whether to install an underfloor heating system in your property or development. Even if you have already decided that underfloor heating is right for your project it is important to know what steps to take to ensure that you're system will perform to its full capabilities and give you the best possible efficiency and performance.

One of the most important considerations when planning your system is the floor construction. Depending on whether you are installing in an existing property or in a new build, the type of floor construction is likely to vary. For example, while the majority of new builds are built with a concrete floor some existing properties floors may have been built using timber joists. This means that the method of installing underfloor heating within the floor will vary as outlined below.


A solid screed floor is the simplest type of floor to install with your underfloor heating system and it is also the most cost effective. The screed is poured directly onto the floor insulation on which the underfloor heating pipes are fastened using pipe fixing clips.

Where possible it is preferable to use a screed floor as it is the best performing heat conductor when poured over the underfloor heating pipework as it provides complete coverage of the floor area.

The two common types of screed used with underfloor heating are liquid screed (Anhydrite) and sand & cement. Liquid screed must be poured to a minimum of 50-55mm depth in domestic applications while sand & cement should be no less than 65mm. In commercial applications these numbers may increase.

Liquid screed can be literally poured from a pump onto the pipework making it a fast and effective option. Sand & cement is somewhat slower in both the time it takes mix and lay the screed but also in the curing time.

Once poured the screed should be allowed to cure before the underfloor heating system is started. Applying heat to a screed floor before it is fully cured can damage the floor with serious consequences. Under no circumstances should the underfloor heating be used to speed up the curing process.

Once fully cured and ready for use a screed floor can take anywhere from 1-2 hours to get up to temperature initially.

Alternatives to clipping pipe directly onto insulation boards are available including Floor Insulation Panels.


This type of system can be applied to both new and existing buildings. It is widely agreed that pouring screed onto timber joists is not suitable due to the weight implications this presents. Therefore, the most common system used with a timber joist floor is the aluminium heat spreader/diffusion plate system.

The aluminium plates are laid on top of the joists and fixed into position at regular intervals using nails or small screws. Once in position the underfloor heating pipe is laid into the grooves within the plates and, when heated, the pipes warm the plates which in turn act as the heat source to warm the floor coverings above.

The plates are flexible and easy to install. They are typically made to a size of 390mm wide (standard joists are 400mm apart) and 1000mm long. They can be cut in order to cover as much of the available floor space as possible ensuring maximum coverage.

Before laying the plates, Celotex insulation boards should be fixed between the joists as shown in the diagram above. Depending on the depth of the joists the insulation boards should be anywhere between 50mm - 100mm. The more insulation that can be laid beneath the plates the better to reduce any downward heat loss. It is also possible to use a mineral wool (Rockwool or similar) on top of the insulation boards to fill any air void between the plates and the boards, as shown above. Some installers state that this helps push the plates upward creating a contact with the chipboard covering above.


If you’re looking for a low profile underfloor heating solution then an overlay system is the answer.

An overlay system comprises of a dry screed board (available in 15mm or 18mm) and multilayer pipe (10mm with 15mm board, 12mm with 18mm board) to provide a perfect solution when build up height is at a minimum.

Overlay systems are ideally suited to retrofit applications and can be laid directly onto an existing concrete floor.

It is a quick and easy system to install and requires no drying times like traditional liquid screeds.