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Tiling onto green screeds or concrete
Newly-laid floor screeds – also known as green screeds – are usually 1:3 or 1:4 cement to sand at a usual thickness of 50mm. These screeds can be applied both bonded directly onto a concrete base or unbonded (applied on to a polythene sheet damp proof membrane). Both screed and concrete are referred to as being "green" during the period from application through to the time it gains strength and reaches dimensional stability. The question is often: how long does screed take to dry before tiling?
Common issues and queries you can face
New screeds and concrete shrink as they dry
When laying tiles on concrete that has been laid recently, it's worth noting that the screen or concrete contains relatively large amounts of water, which need to be retained while the cement binder is gaining strength. Full strength may not be achieved for a period of 28 days but screeds should be kept moist for at least 7 days to allow sufficient strength gain before drying is allowed. After this period, retained water needs to evaporate away until a normal moisture content is reached. During this time shrinkage occurs to compensate for the volume loss of water. Tiling on a new screed floor too soon whilst the substrate is still stabilising can result in tiles blowing away from their base (heaving) as the screed shrinks below. Concrete substrates take far longer to reach stable equilibrium than sand and cement screeds due to their greater thickness.
New sand and cement screed drying times can be long
Cement-based materials need to retain moisture until the hydration process is complete. To this end a polythene sheet is often laid over newly applied screed for seven days to control evaporation of water and ensure that full strength is attained. In warm and well-ventilated drying conditions, screed up to 40mm in thickness can take 1 day per mm to dry. Thicker substrates will require 2 days per mm. BS 5385-3:2007 Code of practice for the design and installation of ceramic floor tiles and mosaics, specifies that a screed shall be left for at least 3 weeks to dry prior to tiling. However, other documents specify longer periods and BS 8204 recommends 1 day per mm, for thicknesses up to 50 mm. Normally 3 weeks is the absolute minimum.
Use fixing materials that will tolerate movement
Flexible tile adhesives and grouts tolerate some movement in the substrate which can compensate for the shrinkage of the screed as it continues to cure, whilst allowing excess moisture to evaporate through the grout joints.
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Assess and prepare floor
Screeds should be firm enough to walk on, usually at least 3 days after application, before tiling can be started.
Concrete should be at least 7 days old.
All substrates to be tiled must be clean and surface dry.
Particular care should be taken that movement joints are adequately specified.
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Lay tiles
Fix the tiles into a solid bed of weberset pro lite - rapid at 5 mm thick.
Leave joints at least 3mm wide for grouting and make provision for movement.
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Apply grout and silicone
Grout the joints with weberjoint wide flex and use weberjoint premium silicone to fill perimeter movement joints.
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