Jim Percival, technical director at Palace Chemicals, considers how to deal with unwanted moisture within a tiling installation
TWO different types of moisture may be encountered in a tiling project. The first is construction moisture, which is in the materials that you’re working with, such as the water that is in the concrete, screed and levellers, whereas building moisture enters the building materials during construction or once the building is in use.
A tiler should typically aim to ensure the substrate’s relative humidity is below 75%, whether working on a plaster or cement-based surface.
This should be measured before starting the job, either with a digital moisture meter to provide an instant measurement of moisture content at the surface or with a hygrometer and digital reader which gives you a moisture reading at the screed within 24 hours.
The drying time for concrete or brickwork walls would normally be six weeks before you can tile on to them. Rendering is generally a minimum of two weeks. Plasterboard or tile backer boards can be tiled straight away, while if you’ve got a bonding coat, then it’s four weeks and skimmed plaster on to plasterboard will be about seven days. A sand:cement screed is generally three weeks.
With an anhydrite screed it’s a different situation – a day per millimetre should be allowed for the first 40mm and then one day for every 2mm thereafter. The key requirement is to ensure the moisture content is below 75% and the surface should be cleaned and sealed over with a suitably applied barrier sealer before applying either a cement based adhesive or levelling compound, if levelling is required.
You then have to consider the drying time of the leveller, according to the manufacturer’s instructions, which could be around eight hours or longer in cooler environments. If substrates need force drying, you can bring in heaters and dehumidifiers.
There are also fast track systems, such as Palace Pro Gyp-Base for tiling on to anhydrite screeds which have a moisture content up to 95%. This basically introduces a 4-layer isolation membrane applied over the screed that allows the substrate beneath to dry at a slower rate and allows the tiles to be fixed on to it without any risk of interaction with the anhydrite base.
If the project involves underfloor heating, the substrate must reach the appropriate moisture level before the system is switched on, because force drying it from the inside can ‘shock’ the concrete and cause cracking. The system should then be left for a specified period before full commissioning.
If it’s necessary to force dry or accelerate the speed at which the screed dries, and the floor contains heating pipes, then the temperature in the UFH system should be increased incrementally over a seven-day period, until the planned input temperature is reached. It should then remain at a constant temperature for a minimum of seven days before it’s turned down incrementally, allowing cooling to room temperature before tiling. Then, once tiling is completed, you should leave it for 28 days before turning it back on for in-service use.
Jim Percival
Technical director, Palace Chemicals | www.palacechemicals.co.uk
