Combymat Overlay Boards

Overlay boards for use with 150W Combymat System

Category: Combymat Overlay Boards
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Combymat Overlay Boards. Allow use of combymat heating mats beneath carpet and vinyl floors

Combymat Overlay Boards are pre-coated with adhesive and allow the Combymat heating system to be installed beneath carpet, vinyl, linoleum and bonded board floors, without needing to cover the heating system with a levelling compound. The Overlay Boards provide a strong, stable and smooth floating sub-floor.

By enabling a dry installation without any wet trade the system saves installation time and costs and also minimises build height; the total build height of the Combymat System with underlay, and the Overlay Boards, is only 13mm. Carpets can be fitted the same day as the Overlay Board installation is completed, any other floor surfaces must be fitted between 24-48 hours after laying the boards.

Each pack of boards covers 2.88m² and for rooms below 10m² we recommend supplying boards to cover at least 15% more than the room area to allow for off-cuts.

For rooms above 10m² we recommend ordering boards to cover 10% more than the room area, but for particularly complicated shaped rooms we recommend 15% more than the room area.

Overlay Board Features

  • Enables carpet, vinyl, linoleum and bonded board floors to be laid onto underfloor heating without any wet trade
  • Dry and dust free installation
  • Creates a very stable, floating sub-floor
  • Very simple to install - boards are pre-coated with a contact adhesive for speedy installation (saves significant time on installation compared to using mats and a levelling compound)
  • Heating system and overlay boards can be installed in a single day
  • Supplied in 2.88m² packs
  • Boards increase build height by only 7mm, roughly half the thickness of a levelling compound
  • Boards are not suitable for use in wet areas such as bathrooms or wetrooms
  • Boards must be acclimatised in the room where they will be installed for at least 48 hours prior to installation


Heat Mat's Overlay Boards are only suitable for use with the Combymat System and must not be used with any other Heat Mat underfloor heating mats or cables.

Carpet and other floor coverings

If Overlay Boards are installed above the Combymat system carpet, vinyl, linoleum and bonded board floors can be laid directly on top, as long as the combined Tog rating is no more than 1.8 Tog. When used with carpeted floors the Overlay Boards provide a resilient protective covering on top of the heating system and also help to spread the heat, lowering the risk of thermal blocks above the heating system.


Overlay boards are very simple to install, the only tools required are a knife, sandpaper and a tape measure.

Before installing the boards they should have been acclimatised in the room where they are to be fitted for at least 48 hours, at a temperature of at least 18°C. When cutting the boards we recommend the use of a retractable concave bladed knife or similar to avoid creating sawdust, which could prevent the adhesive from correctly bonding. The boards are laid in a staggered pattern with the 3mm boards being laid first, and the 4mm boards on top and an expansion gap of 8-10mm should be left around the perimeter of the room. Any irregularities in the final surface can be easily sanded with 120-grain fine sand paper. Full fitting instructions for the Combymat (including Overlay Boards) can be downloaded here.

If the resilient floor covering is going to be glued onto the Overlay Boards the boards must be fitted between 24 and 48 hours before the floor covering is laid. This ensures the boards have settled, but should prevent them absorbing too much moisture from the air. When installing the resilient floor covering it should not project beyond the edges of the boards.

Combining multiple units

The boards utilise an overlap joint configuration to produce a seamless final finish and prevent lines from ghosting through shiny floor finishes. The very low thermal resistance of the Overlay Boards enables valuable heat energy to pass quickly through to the floor surface for a fast warm up time and improved energy efficiency.

Floor sensor positioning

Special attention should be made to the location of the floor sensor and it should be installed as close to the floor surface as possible and be located in an area directly above a heating element in order to achieve accurate control. A wide groove should be made in the top layer of the Overlay Board to take the tip of the sensor and allow the sensor cable to run to the edge of the room without causing a raised area for the final floor covering. The groove and sensor should be entirely covered with flexible filler to ensure an accurate temperature reading and a smooth and level finish to the Overlay Boards.


The packs must be kept dry and should be stored in a moisture free environment with low humidity; the packs must never be stored outside. The boards are supplied in simple to transport packs (1.2m x 0.6m x 2.8cm) and the weight of the packs is roughly 17kg. Pallet deliveries are available for larger orders.

Please note, the Combymat System and Overlay Boards are not suitable for use in wet areas such as bathrooms. The Overlay Boards are sensitive to moisture, so if there is a risk of moisture rising through the sub-floor a damp proof membrane must be installed beneath the Combymat insulation.

Product Code Product Description m² per pack TSI GTIN
CBM-OVE-0280 7mm overlay boards 2.88 403083938 5060424144273