How to install oak flooring over underfloor heating

A helpful guide

We recommend that the installation of engineered oak floors is carried out by professionally trained wood floor fitters. Each fitter will have their own installation techniques, and each property and installation will require a different approach to the next, however the following guidance notes should be helpful.

Engineered oak flooring can withstand moderate changes in temperature and humidity, so can be installed over certain types of underfloor heating (UFH) as long as the heat and humidity are controlled, and are never excessive as this will cause distortion.  

UFH systems are developing at a rapid pace with new technologies being introduced. It is essential that you check with the UFH manufacturer that your chosen UFH system is suitable for engineered wood flooring.

An excerpt from TRADA – A Professionals Guide To Installation. “The top surface temperature of the wood floorcovering should not exceed 27°C for engineered boards, otherwise excessive gapping between boards may occur. This temperature equates to an average power output of approximately 100W/m2.”

Most modern underfloor heating systems are water based.

The following information relates to engineered oak floorboards which are installed over a water based UFH system, with the water pipes laid beneath a concrete screed sub-floor.

Site Conditions

The building must be weather-tight, the heating system working, and all ‘wet’ trades (plastering, painting etc.) must be completed and fully dry. All guttering and rainwater goods must be properly fitted and fully functional. All windows and doors must be glazed, sealed and fully functional.

The underfloor heating system should be run for 3 weeks prior to installation of the flooring to ensure that the entire floor is dry. The room should be ventilated briefly every day during this period. Any cracks which appear in the concrete screed must be repaired before installing the floor.

At the time of delivery and installation, room temperature would ideally be between 18oC to 20oC, and relative humidity would be no higher than 60%. A dehumidifier can be used to assist room preparation prior to installation.

Sub-floor (i.e. the screed over which the flooring is being fitted).

Ensure that the sub-floor is hard, free from dust, and level; according to BS8204-1 2003 the height deviation must be no more than 3mm measured over a 2m distance using a straight edge. Use a suitable levelling compound if necessary. De-nib the sub-floor to remove any sharp irregularities that could cause the engineered oak floorboards to pivot; even a 3mm nib could cause rocking.

The sub-floor must be allowed to dry thoroughly before engineered oak flooring is installed. The water pipes must evenly cover the whole of the area where engineered oak flooring is to be fitted as hot or cold spots can cause distortion. Install a vapour membrane and underlay directly over the dry sub-floor for floating installation, or use a matched liquid DPM and permanently elastic adhesive for glued installations. The sub-floor should have a relative humidity no higher than 40%. This can be measured with a hygrometer.

It is good practice to fit a damp proof membrane (DPM) over concrete sub-floors (i.e. polythene DPM, or a liquid DPM). Any joints in polythene DPM must be overlapped by 200 mm and be taped with polythene adhesive tape. The DPM should be taken up the walls by 25mm or more (this will be hidden behind skirting).

Acclimatising engineered oak floorboards prior to installation.

Your new floor should be in harmony with its surroundings. Engineered oak flooring is kiln dried to a moisture content (MC) of 9% to 11% prior to despatch. It is recommended that the MC should be reduced to below 8% to 9% prior to installation on UFH systems. During acclimatisation, room temperature should be 18oC to 20oC and relative humidity should be 40% to 60%.

Stack the packs of flooring horizontally (flat – no on edge) in the centre of the room where the installation is to take place. Use wooden spacers (approximately 20mm thick and long enough to span the full width of a pack) to separate the packs and allow air circulation between them – use at least three spacers per running metre so that the weight is evenly distributed. Ensure the spacers are identical thickness, and that packs are not in direct contact with the sub-floor. Do not fully open the packs; you may cut the ends of the packs open. Leave to acclimatise in the room for 3-4 days prior to installation.

Turn off the UFH whilst the engineered wood floor is being installed. After completion of the installation turn the UFH on gradually over several days e.g. +2-3oC per day; sudden temperature changes can cause irreversible distortion of the flooring.

Expansion provision

Hardwood flooring is kiln dried to a moisture content of 9% to 11% prior to despatch. As local conditions vary in each property, the moisture content of wooden flooring will fluctuate depending on the heat and humidity. Floorboards, therefore, contract and expand during their life (however we are generally unware of this happening).

It is essential that an expansion gap of at least 15mm is left around the perimeter of the room, and around any obstructions such as columns, radiator pipes, door casings, hearths etc. and in doorways between rooms. In rooms larger than 10m x 8m, and in rooms which are subject to regular fluctuations, it may be necessary to provide larger expansion gaps. Your fitter will be able to advise further. Expansion gaps can be covered with a variety of solid skirting or trims. It is essential that the natural movement of the boards isn’t impeded.

Installation of pre-packed flooring

Take the boards out of the packs immediately before installation; draw from at least 3 packs to ensure a good colour/grade mix which is to your liking and repeat this process as the floor is laid. Decide on which direction to run the planks (usually with the light-fall from windows or along the length of the longest wall.) Use wedges to maintain expansion gaps during installation and remove them after completing the floor installation. Try to stagger joints in adjoining rows. The first and last rows may require cutting down.

Fitting methods

Engineered oak flooring can be fixed to the concrete sub-floor with glue, or it can be floated over underlay.

Stick down

Use a permanently elastic Polymer glue system. The glue should be spread out using a notched trowel to the glue manufacturer’s specification. Only spread as much glue as you can cover within the curing time of the glue, and always follow the glue manufacturer’s instructions.Use wedges to maintain the expansion gaps during installation. Don’t glue the boards together at the joints.

Floating Floor

Engineered oak flooring can be floated over a low tog underlay. In this type of installation the new oak floor is not fixed to the sub-floor, it is held in position by its own weight i.e. it is said to be floating. Tongue & Grooved engineered boards require fixing with PVA wood glue in the header and longitudinal joints when being installed. Do not walk on the floor until the glue has cured, usually 24 hours. Squeeze engineered boards together so they fit tightly. Use installation clamps to ensure a tight fit of the first three rows, and leave clamped together for one hour before proceeding with the remainder of the floor. Use wedges to maintain the expansion gaps during installation. Immediately wipe away any adhesive that seeps from the joint with a damp cloth. The last row should be installed with an installation bar.

A few more thoughts

Oak flooring is a natural product. Each board is unique, and, depending on grade, may have knots, sapwood and colour variation which are not preferred by customers. Unwanted natural defects can be cut out during installation and the resultant cut boards can be used at the end of rows.

We would recommend that an additional 10% of the floor area is purchased to allow for boards which are rejected due to their appearance, and wastage resulting from the removal of unwanted natural defects.