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Pre-installation Preparation Guidance
BEFORE INSTALLATION
The area must be ready to receive the screed before our team arrives. All preparatory works should be complete, including insulation, underfloor heating, and edge insulation.
The substrate should be clean, sound, and free from dust, debris, standing water, and contaminants that could affect the screed installation.
Ensure the building is weather-tight, with windows and external openings secured to prevent water ingress and draughts during and immediately after installation.
ON THE DAY
Provide clear and unobstructed access to the pour area, with suitable access for the screed delivery vehicle and installation equipment.
The working area should be free from other trades, materials, and equipment. Any underfloor heating system must be installed, secured, and pressure-tested before screeding.
Ensure adequate lighting and access to suitable power and water supplies where required. Protect walls, door frames, and other finished surfaces from splashes during installation. Any protrusions, service ducts, and shuttering should be properly sealed before screed installation.
The area must be closed up for the first 48 hours to allow the screed surface to dry without being affected by external influences.
Floor Preparation
Before screed installation, the building should be weather-tight. Ideally, the roof should be complete, with all external doors and windows installed. If this is not possible, all openings must be sealed and made weatherproof using polythene or another suitable protective material to prevent water ingress and drafts.
The floor must be clean and properly prepared before screeding. Remove all dust, debris, and loose material to ensure the floor area is suitable for the installation of liquid screed.
Edge Expansion Strip
Perimeter isolation upstands must be installed to all abutments, including walls and structural elements, as part of correct liquid screed preparation.
We recommend either:
- 20 mm extruded polystyrene insulation boards to reduce cold bridging, or
- A proprietary 8 mm expansion strip, installed to the full depth of the screed
Correct perimeter isolation is essential to allow for movement and to ensure the long-term performance of the screed.
Installing the Polythene Membrane
All liquid screeds require a 500 gauge (125 MU) polythene separating membrane as part of correct liquid screed preparation. Temporary Protective Sheeting (TPS) membranes are not suitable.
Important: Screedflo liquid screed reacts with aluminium. Aluminium-backed products must not be used.
The polythene membrane must be laid:
- Free from punctures, tears, or creases
- With joints lapped a minimum of 100 mm
- Extended up all abutments to finish slightly above the final screed depth
All joints and abutments must be fully sealed using waterproof gaffa tape (not aluminium-backed tape) to create a 100% watertight area.
Care must be taken to remove:
- Air pockets
- Voids beneath the membrane
- Weak points at corners and abutments
All exposed edges, including door openings and level changes, must be provided with temporary formwork to prevent leakage during screed installation.
Underfloor Heating
Where underfloor heating is installed, the system must be laid on top of the separating membrane. Heating pipes or cables must be securely fixed using proprietary fixings, at a maximum of 300 mm centres, to prevent movement during screed installation.
Water-based underfloor heating systems must be filled prior to screed installation to reduce the risk of pipes floating when the liquid screed is poured.
Frequently Asked Questions
Liquid screed can dry out too fast if the building is not weathertight, as it is affected by direct sunlight, drafts, or rain on the surface during the initial 48 hours of setting. After the initial 48-hour setting time, water ingress can slow the drying of the screed significantly.
Animals are attracted to the heat and smell given off by newly laid screed, and if the floor is not closed off and they get in, there can be a lot of mess!
The building should be weathertight prior to commencement. The roof should be covered, and all external doors and windows should be in place. Alternatively, all openings are to be made weatherproof with polythene or another suitable material.
If the floor is not clean, the debris on the floor can float to the surface of the screed and create a lot of imperfections.
If the insulation is rocking and unstable, it will be difficult to set up the levels for the screed installation.
Ensure that all high/low points are removed from the sub-base ( use blinding sand to level off where required) and that insulation boards are laid flat.
Where there are service pipes running across the floor, the insulation should be fitted around the pipes, so that the boards are still sitting flat. Ensure any protrusions that come up through the insulation are wrapped with edge strip and membrane to prevent any leaks.
Insulation boards should be tightly butt-jointed with staggered joints.
The liquid screed needs something soft to expand onto as it dries. If upstands or expansion foam are not installed, there is a risk that cracks will appear if the screed is not allowed to expand onto something soft and put under stress.
Perimeter edge strip should be installed on all walls and around doorways and protrusions in the floor before the membrane is laid. We recommend the use of either 20mm extruded polystyrene board insulation for cold bridging or a proprietary 8mm expansion strip to the full depth of the screed.
Liquid screed reacts with the aluminium backing on insulation boards. If a membrane is not laid, the two products react and produce a gas, which affects the surface of the screed and can sometimes prevent correct setting.
As it is a liquid, the floor should be tanked with a membrane so there is no chance the screed can leak between the insulation boards and lift them.
And the membrane should be cut and taped with waterproof gaffa tape around protrusions, so there can be no leaks that could affect the finished surface.
Heated screeds should include joints across doorways and between areas with separate heating control zones (take advice from the underfloor heating manufacturer). This is because they are put under more stress where the screed goes from a large area through a narrow area and into a hallway or another room. Installing expansion strips in these areas induces a controlled break in the screed where expansion can occur correctly.
Expansion joints can be made using pre-formed 10mm closed cell Polyethylene foam with a self-adhesive T-bar base known as a vertical control joint.
Concrete and anhydrite screed are not compatible. If they come into contact, they can have a reaction, which leads to the production of ettringite, a crystalline material that can cause floor failures. So the concrete and liquid screed must always be separated by a polythene membrane.
Aluminium reacts with anhydrite liquid screed, so always make sure waterproof cloth tape or gaffa tape is used.
A Level Survey detects the variance over the floor slab, and a laser is required to do this.
If critical points (eg bottom of stairs or door thresholds) are not checked, the minimum screed depth may not be achievable, and the insulation may have to be removed and adjusted in height, or door thresholds may need to be raised.
In an area of 100 m2, for example, the builder would need to take about 20 readings across the screed to identify the different depths from the datum point (the finished floor level). This also identifies in advance if the insulation level needs to be raised to reduce the depth of screed to the recommended depths and reduce drying times.
The builder needs to take an average across the 20 readings to calculate the quantity of screed required. E.g., average of 40mm = 4m3, 50mm = 5m3, 75mm =7.5m3 etc.
We recommend clips every 300mm on the UFH, as screed is heavier than UFH pipes filled with water. So the pipes would try to float when the screed was installed if they were not secured into the floor.
Water systems should be filled prior to the application of Screedflo to reduce the risk of pipes floating when the screed is poured.
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