Tiling on Gavylon/Anhydrous Screeds

Today’s blog concerns a common dilemma for many contractors. Tiling on gavylon screeds when it is safe to tile on. Firstly you need to identify the type of screed; if it looks smooth and has no visible expansion joints over a large area then it is likely to be a gavylon screed. We recommend checking the specifications with the screed contractor to make absolutely certain.

The most important aspect is the dryness of the screed. Contractors recommend a timeframe which calculates when the screed will be dry; normally 1mm per day for the first 40mm of screed thickness then 1mm every two days for the next 30mm. However, in our experience these timeframes are rarely accurate and cannot to be relied upon.

As the tile flooring contractor it is your responsibility to check that the screed is dry and suitable to be tiled upon. In this instance, dry means a relative humidity reading of 75% or under on as recorded on a moisture content reader (hygrometer). However, screeds are notorious for drying at variable or inconsistent rates and we would recommend sampling a few readings from different parts of the floor before proceeding. The hygrometer needs 24 hours to reach the optimum performance level before a reading can be relied upon and must be sealed to the floor with silicone.

Often the main building contractor is under pressure to complete the site works prior to handover and in turn will be pushing you to lay the floor. However, you cannot do so if the hygrometer reading is above 75%. However, if the reading is between 75 -85% you can proceed with tiling if a damp-proof membrane (DPM) is installed first. Applying the DPM is a messy job and we recommend that you consult both your adhesive and DPM supplier before proceeding.

Previously we have also sanded the screed floor slightly to open up the pores and then ensuring it’s dust-free before applying a primer and then the membrane itself. Fitting the DPM is quite a messy job but we always recommend applying an anti-crack membrane first before proceeding to tile the floor.

As we have said earlier, you must resist the pressure from contractors and/or clients if you consider the moisture levels are too high, or else obtain a ‘cast-iron’ disclaimer from them. As the tiling contractor the ultimate responsibility for the installation of the floor is yours. If it fails due to excess moisture and subsequent movement in the screed floor below, even if the contractor/client authorised you to proceed, then, without a disclaimer, you will be held entirely responsible.

We strongly advise that you obtain a couple of hygrometers as they are likely to be one of the best investments you will ever make.

By sharing our experiences we trust that you will avoid some of these pitfalls. If you require further information or installation advice on tiling floors please don’t hesitate to contact us. Of course if you need a professional installation team you need look no further than Granite House, please call us today to arrange a costing for your tiling project.

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