Tiling On Cracked And Green Screeds | Schlüter-Systems

How To Tile Early, In Safety

Not dealing correctly with problematic substrates is one of the major causes behind the failure of many tiling installations, where stresses from the substrate find their way into the finished covering of stone or ceramic tiles. For example, Schlüter had been called in to advise a tiler who had been asked to lay ceramic tiles in the kitchen of an old house which was being renovated and extended. The existing screed had several cracks in it, and the tiler was worried that in time, the tiles will debond and crack, which is exactly what would happen if the tiles are laid directly on to the screed. Because of different thermal expansion and contraction the screed will move at a different rate from the tile, and without the right solution will lead to disaster. Even if the tile adhesive has a flexible polymer in it, thermal movement in the screed -- particularly at the joint of the cracks -- will be too great for the adhesive to hold the tiles down. However, an uncoupling system -- also referred to as a separation system - will allow the tiles to move independently from the substrate. 

Schlüter®-DITRA 25 has been developed specifically for uncoupling the building structure from the tiles. It's a pressure-stable polyethylene membrane with a grid structure of square cavities and an anchoring fleece laminated to its underside. The fleece backing becomes anchored in the tile adhesive on the substrate. On top of the membrane the adhesive is anchored into the square, cut-back cavities, and the tiles are installed on top. The membrane neutralises the cracks in the screed, and can absorb up to around 2-3 mm of horizontal movement. We were also able to advise the tiler on fixing onto the floor of a new conservatory at the house. BS recommendations are that you shouldn't tile on a new, green, screed, for at least 28 days, but the owners wanted him to do it before that. He was worried that if he did, the installation was likely to fail in the future. 

The recommendation of 28 days is to allow for movement and shrinkage to take place in the green screed. But using Schlüter®-DITRA 25 actually means you can get on to the screed much earlier. Depending on the weather and time of year it could be as little as a couple of days. Basically, you're looking at being able to apply the membrane as soon as the screed can support weight. If you can walk on the screed and your shoe doesn't leave an impression in it, you can install the membrane and tile immediately afterwards. The membrane actually slows down the drying process of the screed, minimising shrinkage and eliminating most of the cracking which could occur. Also -- especially over floating type screeds where you've got insulation -- it'll minimise the curling effect of the screed, so all in all, using Schlüter®-DITRA 25 on a green screed not only protects the tiles, but gives you a better screed as well. And if you can get off site earlier, because you don't need to wait 28 days before tiling, you can get paid earlier too!

Contact Schlüter-Systems Ltd

Units 3-5 Bardon 22,
Beveridge Lane, Coalville,
Leicestershire, LE67 1TE
Phone: +44 (0) 1530 813396

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