Tiling to Calcium Sulphate Screeds12th March 2021
What is a Calcium sulphate screed?
Instead of a cement-based binder, these types of screed contain anhydrous (dry) Calcium Sulphate and aggregate(s).
When water is added, the anhydrous (dry) Calcium Sulphate forms either an alpha hemi-hydrate (a stronger and harder crystalline form) or a di-hydrated Calcium Sulphate (more commonly known as Gypsum). This reaction stops when the vast majority of the anhydrous Calcium Sulphate is used up and is generally complete after 3 to 7 days. Therefore, any water remaining within the screed is surplus to requirements and needs to evaporate through the surface of the screed.
BS 8204-7: Screeds, bases and in-situ floorings
The above standard is the code of practice which covers pumpable self-smoothing screeds. The salient points to watch out for are listed below and should be used in conjunction with the other codes of practice that are relevant, specifically BS 5385 Parts 1 to 5, taking into consideration any recommendations for specific conditions, i.e. Part 4 2015.
- Any area to be screeded should be enclosed within a weatherproof structure and any opening should be covered.
- Temperature range of applying a screed is 5°C to 30°C; storage of materials prior to laying is 5°C to 30°C, ideally 20°C.
- Areas to be laid should be free of draughts and be protected for at least 24 hours.
- The atmospheric Relative Humidity (RH) should not fall below 50% RH value – therefore reducing the risk of rapid early drying.
- Levels must be accurate to meet the requirements of the specification for the floor tiles being installed. See Table 4 of British Standard BS 8204-7. If a Surface Regularity of SR1 i.e. 3mm when checked with a 2m straightedge is not achieved, use of a suitable floor levelling compound (following priming the screed) e.g. UltraTileFix ProLevel Two should be considered before the tiling installation can be undertaken.
Typical values at which it would be permissible to lay moisture sensitive floor finishes are <0.5% w/w moisture content, for impermeable floors and <1% w/w for more permeable floor such as carpet tiles etc.
Methods of Testing Moisture
- Using a hair hygrometer or digital 1 hygrometer. Moisture content must be <75% RH (Relative Humidity).
- CM tester (Carbide Bomb 2) may be used. Moisture content must be <0.5% water by weight.
- Laboratory testing of weight loss via oven drying a sample of the screed at 40°C.
NB these results can sometimes be misleading.
TIP 1: This is the best method for testing moisture. However please seek expert advice with regards to the use of a carbide bomb tester.
The graph below represents the approximate time taken to reach 0.5% water by weight in ideal drying conditions i.e. air temperature of 20°C with a relative humidity not greater than 65%.
Approximate Drying Times for Calcium Sulphate Based Screeds
No of Days Drying
1) All laitance*, dust, dirt or any other loose surface material incompatible with the tile adhesive must be removed. This is normally carried out using a sanding machine i.e. a rotary floor scarifier and a 60 grit paper. Remove excess dust completely by the use of suitable vacuum equipment. Seek further advice on the correct method to achieve this from the screed manufacturer.
*Laitance is a week intermediary layer of fine particles deposited on the surface of the screed, generated by water migrating through to the screed surface as drying occurs.
TIP 2: Good preparation for calcium sulphate screeds includes abrading the surface of the screed (as with cement based screeds). This helps to remove surface contamination and things like laitance, and the like, which will be present on most screeds. By exposing the screed’s surface this will also provide a key to the primer and adhesive and this aids adhesion.
TIP 3: Abrading the surface is best carried out 4-6 days after application of the screed and can assist in the drying out process.
TIP 4: It may still be advisable to lightly sand the surface even for low laitance calcium sulphate/anhydrite screeds (screeds modified with a laitance suppressing additive) as this will provide an additional mechanical key to aid further adhesion.
2) The installation should be protected against ingress of water or rain. Accelerated drying of calcium sulphate based pumpable self-smoothing screeds may be used once the screed is at least 7 days old. As a general rule of thumb, the screed drying time is approximately 1mm per day up to 40mm (see graph to left) and will significantly increase for thicker screeds or those in poor drying conditions.
TIP 5: The speed of drying out of the screed can be improved by a) increasing the ventilation in the room, b) application of external heat e.g. space heaters combined with the use of industrial dehumidifiers in order to remove water vapour as the higher temperature encourages natural evaporation of excess moisture, c) where applicable, commission underfloor heating after 7 days.
3) All traffic should be kept off the screed until the screed has hardened sufficiently in accordance with manufacturers recommendations and should be protected from contamination and damage from following trades. The screed should also be protected until the flooring is applied.
TIP 6: If not protected as the project progresses, trafficking by other trades may result in wearing or grinding down of the screeds surface. It may then be necessary to carry out further surface treatment using a suitable sanding machine / vacuum equipment as recommended by the screed manufacturer.
In reality, the risk of an adverse chemical reaction (ettringite attack) with calcium sulphate is reduced when the tiling is carried out using an UltraTileFix rapid setting adhesive due to the specialist cement chemistry. However, we would always recommend that after following the correct preparation of a calcium sulphate screed surface, a primer is applied to the prepared screed surface before application of the tile adhesive.
TIP 7: Priming creates both an effective barrier and assists in consolidating the prepared surface making it more stable. This helps to ensure the tile adhesive is fully bonded to the screed. Use a primer such as UltraTileFix ProPrimer, or a suitable SBR or epoxy based primer.
TIP 8: When selecting the tile adhesive, consideration should be given to factors such as the type and method of cleaning and maintenance for the floor whilst in service i.e. how wet the finished floor may become and what are the risks associated with limited water penetration beneath the tiling layer? Ceramic tile adhesive must meet the requirements for BS EN 12004 “Adhesives for tiles – Requirements, evaluation of conformity, classification and designation.”
UNDERFLOOR HEATED CALCIUM SULPHATE SCREEDS
If underfloor heating is incorporated into the calcium sulphate based screed, allow the screed to dry for a minimum of 7 days before commissioning of the underfloor heating.
Gradually increase the temperature by approximately 5°C per day from the minimum recommended operating temperature until the maximum required working temperature is reached. Maintain this temperature for a minimum of one week. Following this, reduce the temperature gradually to 15°C or turn off the system completely to achieve this prior to tiling. If force drying of the screed is required, please consult the heating system manufacturer.
The moisture content of the screed must be checked by the floor layer prior to tiling.
As discussed, when the screed is ready for tiling we would recommend the following procedure:
- Mechanically prepare the screed to expose a clean base as described above.
- Apply UltraTileFix ProPrimer in two coats. The first coat should be diluted 1:3 by volume with water and allowed to touch dry before applying a second coat diluted 1:1 by volume with water at 90° to the first. This is in order to ensure the entire surface of the screed is covered.
- When the above primer coats have dried, and if necessary, the floor may require levelling using UltraTileFix Level IT Two e.g. over electric heating cables.
- The tiles can be fixed in a 3-6mm solid bed (i.e. ensuring as far as possible no voids are left beneath the tiles) with UltraTileFix ProFlex SP in grey or white. Alternatively, if a slow setting adhesive is required for ceramic or porcelain tiles, use UltraTileFix ProFlex SPES in grey or white.
- When the adhesive has fully cured, the joints between the tiles should be filled with a suitable grout, e.g. UltraTileFix ProGrout Flexible. (NB. A minimum joint width of 3mm for floor tiling is recommended).
Movement joints will be required in these installations. Any movement joints, or joints likely to be subject to movement, in the calcium sulphate screed should coincide with movement joints in the tile bed. Perimeter joints should be a minimum of 6mm in width and extend through the full depth of tile and adhesive bed.
For ceramic/porcelain tiling, these may be filled using UltraTileFix ProSealer (colour to match UltraTileFix ProGrout Flexible.
Movement joints should be incorporated as outlined in British Standard BS 5385-3: 2007 Clauses 6, 8 and 7.1.6. Briefly, this document requires that joints be located:
- Over existing and/or structural movement joints or at day joints (with unbonded/floating screeds) where movement is likely.
- Around the perimeter of the floor and where tiling abuts columns, curbs, steps and plant fixed to the base and at doorway thresholds.
- In large floor areas, tiles should be divided into bays of size not greater than 10m x 10m intervals.
- For floors subjected to significant thermal changes i.e. underfloor heated screeds, tiles should be divided into bays of size not greater than 40m2 with an edge length not greater than 8m. The areas bound by movement joints should be square to rectangular with a width to length ratio not greater than 5 to 8m.
- On suspended floors the bay size should be reduced and additional joints provided over supporting walls or beams.
Further advice should be obtained from The Tile Association Technical Document “Tiling to Calcium Sulphate Based Screeds” and BS 5385-3-2014.