Flotherm liquid screed is a specially formulated calcium sulphate self-levelling (flowing) floor screed. The product is a high quality factory produced screed to EN 13813:2002.




The building must be weather tight prior to the placing of any screed material: the roof; external doors and windows must be in place and closed or covered and taped to prevent draughts. All substrates must be suitable to receive the screed as per current good working practices.

All substrates for bonded constructions should be clean and thoroughly sound; they should be free from standing water, oils, grease, dust, loose particles or any other contaminants which may interfere with adhesion. For floating (or unbonded) and underfloor heating constructions, it is imperative that the highest quality preparation is carried out. All insulation boards should be installed in such a way as to provide a clean even base. A suitable (500μm) polythene sheet should be applied over the insulation. All joints should be lapped to 100mm and fully taped. A suitable 5-10mm PE foam expansion strip should be fitted to the full depth of screed around the entire perimeter of the floor. Corners and joints should be constructed neatly and taped to form a watertight seal. All openings should be suitably sealed to prevent screed material escaping. Heating pipes should be laid as required and mechanically fixed every 300mm. It is recommend to also fill pipes with water to prevent lifting. Isolation strips should be placed in door thresholds where there is likely to be a difference in operating temperature in underfloor heating circuits.

It is recommended to leave designated areas without underfloor heating pipes and clearly mark their location to allow the easy sampling for CM moisture readings without risk of damage to the heating system.

The screed can be mixed at site from a silo; with a suitable mixer pump; or at a ready mix plant and delivered by mixer truck. Generally a 6m3 truck can carry 5m3 of screed without the risk of spillage. The screed should be pumped into place using a suitable pump. The pump hoses should be first primed/lubricated with a slurry of binder and water.

The level should be set with tripod markers and a suitable laser level or water level before pumping commences. After placing, the screed should be lightly tamped with Tbars or similar to ensure full compaction. The screed should be protected from draughts during installation and for the first 3 days. The screed should be dry (less than 0.5% moisture content measured by CM) before covering. This can be accelerated by the use of dehumidifiers or by commissioning of the heating system after 7 days.


Calcium Sulphate based screeds are not suitable for use externally. Joints should be formed in accordance with BS 8204, maximum unbonded areas of 50m², aspect ratios no more than 2.1, form joints at stress points i.e. door openings etc.

Calcium Sulphate based screeds must be suitably primed prior to the application of subsequent cementitious materials, e.g. tile adhesive, (latex) levelling compounds etc. (Please refer to after-care datasheet for further details).

Application Thickness


Once the screed has been installed, it should not be trafficked for a minimum of 24 hours and the building should remain sealed for 2-3 days. The screed should be protected from draughts, extremes of temperature and strong direct sunlight during installation and for a minimum of 3 days. Larsen Flotherm screed should not be covered with polythene or ‘cured’ in the conventional manner. After 3 days adequate ventilation should be created to allow the screed to dry out. Drying can be accelerated by the use of dehumidifiers or by commissioning of the heating system after 7 days. The screed should be protected from extremes of temperature and exposure to moisture. Where possible the screed should be kept clear (e.g. from the storage of materials) to aid the drying process. After 7 days the screed may be forced dried with the use of dehumidifiers or the under floor heating system. Under floor heating systems must always be trialed prior to the installation of floor finishes.

Underfloor Heating

Under floor heating systems should be commissioned with a gradual build up in temperature in line with BS EN 1264 recommendations.

A typical routine would be:

1. Raise flow water temp to 25°C
2. Maintain this temperature for 3 days
3. Raise thermostat temperature by 5°C per day
4. Raise to maximum flow water temperature (<50°C)
5. After testing continue to run heating system for 2 weeks
6. Switch off system 48 hours before checking moisture or installing floor coverings

Always follow the guidance supplied by the heating system manufacturer.

Moisture Content

The screed should be dry before covering. This is more important with gypsum-based screeds than traditional cement based screeds as trapped moisture can have a detrimental effect on the strength and durability of a gypsum screed. We recommend the screed be checked for moisture using a Carbide Bomb type moisture meter. Electrical resistance meters (two pin type) are not suitable.

We do not generally recommend the use of a surface applied DPM with Larsen Flotherm Screeds. If there are issues regarding drying times, consult our Technical Department regarding the use of suitable tanking and decoupling membranes.


As with alpha-hemihydrate screeds, Flotherm screeds generally have low levels of laitance but as per TTA guidelines, all calcium sulphate screeds should be mechanically prepared before tiling.

Floor Covering

Due to the incompatibility between gypsum based materials and cement based, it is imperative that the screed is sealed with a suitable primer prior to the installation of cement based adhesives or leveling compounds. We recommend that this be carried out by priming the surface with Larsen Acrylic Primer in two coats after the surface is brushed/vacuumed free of dust. First coat is applied diluted 1:1 with clean water and the second coat is applied neat. For best results apply with a short pile paint roller and do not apply in thick coats or allow to pond. Alternatively a suitable water based epoxy primer may be used.

Always isolate gypsum-based materials from cement based materials when mixing. A relatively small amount of gypsum contamination in cement-based mixes can cause deleterious expansive reactions. Gypsum based screeds are not suitable for use externally or in areas of permanent dampness (e.g. wet rooms, swimming pools, commercial areas where power washing is used routinely). Gypsum based screeds must be suitably primed prior to the application of subsequent cementitious materials, e.g. tile adhesives. Contact our technical department for further advice. As with all calcium sulphate (gypsum) based screeds, very warm conditions can lead to extended setting times and over-rapid drying, with a risk of cracking. In very cold conditions there is a risk of frost damage and extended drying times.

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