Anhydrite screed and Laitance

If you are building a new extension or renovating an existing building the builder might choose to use an anhydrite (calcium sulphate) screed. For the builder it has a couple of benefits, it is pumpable so it can be quicker to install with less manpower, and it is self-levelling.

Once the screed is down and the builder is finished you will probably be very keen to get on with the finishings. Unfortunately the drying and preparation of an anhydrite screed for tiling can be a lengthy and labour intensive process. It can take three or four months for the screed to dry and then meet the humidity requirements for it to be an acceptable substrate for tiling. As it dries a layer of fine particles called laitance is deposited on the surface of the screed.

This laitance is too weak to tile directly on to and should be removed by abrasion 4-6 days after the screed is laid, using a sanding or grinding machine which can be noisy and dirty process, then the loosened dust is vacuumed up.

None of this is ideal and if not done properly it can lead to failure of the tile over time. Fortunately there are now products on the market which help the tiler and the customer avoid many of these issues. Decoupling mats sit between the screed and the tile adhesive and provide a stable background on which to tile without the need to wait for months or to remove laitance, simply vacuum the floor, The mat can be laid in minutes, saving substantial labour costs and tiles can be laid directly on top of it.

If you have an anhydrite screed you want to tile on, be sure to get proper technical advice from your tiler before proceeding to ensure the installation lasts the test of time. At Wirral Fine Tiling we can help advise you on the most suitable matting for your surface.