Liquid Screed Under Floor Heating Information

A Guide to Liquid Screed Drying Times

Our clients tell us that there are numerous advantages to working with liquid screed. These include its self-levelling qualities, blemish eradication, and the efficiency of installation using a small specialist team. Top of the list though, is the rapid drying time of flow screed. Traditional screed can take days to be dry enough to walk on, liquid screed takes just 24-48 hours. The process of drying is quicker with liquid screed because it can be laid in a thinner layer than traditional screed. This is due to the enhanced strength of the flow screed, making it more durable than traditional screed even when more thinly spread. Whilst the speedy drying times can normally be relied upon, no two installation environments are the same; temperature and humidity have an important part to play, as does moisture ingress. There are three stages to the drying process and it’s important to pay careful attention to each one to achieve the optimum drying results.

Liquid Screed – first 24-48 hours

This is the most important period for the drying process. Once the pour is complete the liquid screed needs to be left untouched for at least 24 hours. During this period, all doors and windows should be kept firmly closed. At the end of this period the floor screed is normally dry enough to walk on, although you should carefully check all areas before opening the space to ambulatory traffic. Heavier traffic should be avoided until testing shows that the screed can take it. We would recommend that storage of site materials on the screed flooring be avoided until the dry is complete.

Liquid Screed – post 48 hours

Whilst the early stages of drying are dependent on a draught-free environment, after 48 hours the opposite is true and cross ventilation is required to harden the screed. All doors and windows should be kept open to accelerate the drying process. Liquid screed dries at the rate of 1mm per day for the first 40mm and 0.5mm per day for any additional depth. This is contingent on an environmental temperature of 20℃ and 65% relative humidity. As a rule of thumb, the higher the temperature and the lower the humidity the faster the drying time.

Liquid Screed – force drying

Underfloor Heating – If liquid screed is being used as part of an underfloor heating installation the water pipes can be used to speed up dry times after 7 days. This should begin with a water temperature of 25℃ for 3 days, rising to 55℃ for a further 4 days. Humidity should be kept to a minimum throughout the force drying period, so good ventilation is essential.

Dehumidifier – If your liquid screed is being used as foundation to a flooring finish a dehumidifier can help reduce drying times if used in conjunction with the heating of the area. It is important to estimate the number of humidifiers needed to work effectively for the space in which the screed has been laid. Overall, the relative strength of liquid screed necessitating thinner layers than those required for traditional screed mixtures, and the ability to force dry the flooring to complete the dry, makes liquid screed the most cost effective and efficient floor screed currently available.

Preparation

Anhydrite screed often dry with laitance on the surface, the laitance is a weak layer that is often shiny in appearance and looks like a hard crust. The laitance must be removed before tiling commences by mechanically sanding and/or abrading the surface of the screed using a floor sander with a diamond cup. One the laitance has been removed, the screed should be vacuumed to remove all the loose dust, dirt and contaminants

Moisture testing & drying of the screed

Anhydrite screeds must be confirmed dry via consistent moisture readings across the whole floor. When using Anhyfix adhesive. The residual moisture content of the screed must be less than 1% or alternatively the relative humidity must be 85% or below. When using cement-based adhesive the residual moisture content must be less than 0.5% or the relative humidity below 75%. As an approximate guide for drying times, allow 1 day per mm up to an overall depth of 40mm and2 days per mm for anything above 40mm.

The drying of anhydrite screed can be assisted and accelerated once the screed is 7 days old by commissioning the underfloor heating system. Commission the system by turning on the heating at a low temperature and then gradually increase the temperature by no more than 5℃ per day until the maximum temperature of 25℃ on the thermostat is achieved. Maintain this temperature for a minimum of 3 days and then switch of the heating off 48 hours prior to tiling to allow the substrate to cool sufficiently. Alternatively, in cold conditions, reduce the temperature of the screed to below 15℃ prior to tiling

The drying of anhydrite screeds can also be accelerated with the use of dehumidifiers.

Priming

All screeds require priming with a suitable acrylic primer. This is done to stop the adhesives drying too quickly, so maximum strength is obtained. If cement-based adhesives are to be used the floor will need priming until screed is no longer porous. Anhydrite based adhesives generally only require 1 prime.

Laitance removal, vacuuming, moisture testing in preparation for fully bonded flooring applications are all services currently offers by liquid screed floors ltd please ask sales for further information.