Screed FAQs

What is a flowing screed?

Flowing screed, also known as liquid screed or self-levelling screed, is a screed material for floors that is formulated to be fluid enough to be almost self-levelling.

Why should I use a flowing screed rather than a sand/cement traditional one?

There are many significant and obvious benefits to using flowing screeds. The following is a list of some of them:

1. 1. Flowing screeds are very much faster to install. Over 1000m² per day is possible – more than ten times as fast.
   2. Flowing screeds are thinner for most applications. For example, typically 40mm over insulation rather than 75mm. This saves weight and allows for more insulation to be used in the same overall thickness.
   3. Flowing screeds work very much better with underfloor heating, allowing efficiency gains and much faster response times (from switch on to operating temperature).
   4. Liquid screeds are much more environmentally friendly with far lower embodied carbon.
   5. As a liquid, flowing screeds are always at the correct material density in the finished floor. There is no risk of lack of strength through lack of compaction and density as is common with traditional screeds.
   6. Flowing screeds can be foot trafficked after just 24 to 48 hours.
   7. Health and safety of the installation process is significantly improved.
   8. Pumping flowing screeds to tall buildings is much easier than conveying traditional materials to height.

Can I use a flowing screed with underfloor heating?

Flowing screeds are ideal for use with underfloor heating. They conduct heat much better and they are usually thinner so there is less screed to be heated and cooled and less thickness for the heat to pass through. Even more importantly the liquid screed will, as it is installed, completely surround and encapsulate the pipe so that 100% of the surface of the pipe is in contact with the “heat conductor”. This simply cannot be achieved with traditional screeds and methods.

Are flowing screeds expensive?

Flowing screeds were once a lot more expensive than traditional screeds and the perception remains that this is still the case. However, this is not true; the relative costings have for years been steadily moving in favour of flowing screeds.

Larger projects certainly favour flowing screeds. For example, projects as small as a single house may mean that a small premium may be paid but the last few years of huge growth in the use of flowing screeds to houses suggests that this is more than offset by the benefits.

Can I buy the material and install my own flowing screed?

No. All manufacturers of flowing screeds insist that the material should be installed by only approved screeding companies and this is a position that the FSA fully supports.

Can I become an approved flowing screed installer?

If you are an existing flowing screed contractor then you can apply to any or all of the screed manufacturers in our directory. If you are currently not engaged in the installation of flowing screeds, then please contact FSA or any of the screed manufacturers to discuss the possibility of becoming one.

How flat can flowing screeds be installed?

Very flat and suitable for all floor finishes.

There are two elements to the “flatness” of a screed. These are the deviation from datum (level) and the “Surface Regularity”. These are defined in the appropriate British Standards. Flowing screeds can be installed to the highest standard for both elements.

I have underfloor heating installed with a flowing screed. When can I turn it on?

Underfloor heating can usually be switched on after 7 days but this must initially be done at a low temperature which must be increased gradually in accordance with the manufacturers’ (screeding and UFH) guidelines. This process constitutes the commissioning of the system and should be carried out and signed off by a qualified person. Usually, the system must then be cooled and turned off before installing floor finishes.

How long after installation should I wait before walking on a flowing screed?

Usually this should be between 24 to 48 hours.

What is the minimum thickness for a flowing screed?

This depends on the exact type of screed and the way that the screed is installed. The following would apply to the most common types:

• Bonded screeds – 20 to 25mm, when the screed is bonded to the substrate concrete
• Unbonded screed – 30mm, usually on just polythene over concrete
• Floating Screed (Domestic) – 35mm, on insulation
• Floating Screed (Commercial) – 40mm
• With Underfloor heating – 25mm cover to pipes, with under floor heating pipes (usually 16mm)

Can flowing screeds be used outdoors?

Flowing screeds are not designed to be used outside. Though some could provide the necessary properties in situ there would be problems providing the conditions required during the curing and drying period. There are better materials for external application.

How can I tell if a flowing screed is dry enough for my floor finish to be installed?

The moisture level in the screed must be measured to ensure that the screed is dry enough receive floor finishes. The required maximum moisture level will be as required by the specific floor finish material but typically 75% (as per the British Standards method). Please refer to the FSA guidance sheet “Measurement of moisture in screeds”.

Can I apply a flowing screed over my existing flooring?

Sometimes this is possible. The existing flooring must be sufficiently strong, stable and clean and there will be a minimum thickness requirement. Please call FSA for further advice or, if you have a specific screed manufacturer in mind you may prefer to call their technical desk.

Are flowing screeds environmentally friendly?

In general, flowing screeds are far more environmentally friendly than traditional sand/cement screeds. They contain far or less (or no) cement, and some are easily recyclable come demolition time.

Is there a maximum floor size that can be installed in one go?

There is a maximum unjointed “bay” size but for all liquid screeds it is much larger than for traditional sand/cement screeds. Depending on the exact type of flowing screed this can be from about 5 times as large to more than 20 times.

Does my flowing screed need sanding?

Usually yes, at least once.

There are two occasions on which liquid screeds may require to be sanded.

The first is, when required depending on the type of screed, to remove loose friable laitance from the surface (normally 3-10 days after installation).

The second occasion is to create a mechanical key and suitable surface for the floor finish if it is to be bonded to the screed.

For more information on this please refer to the FSA guidance sheet “Sanding”.

Does my flowing screed need priming?

Usually yes, wherever a floor finish is to be bonded to the screed.

Primers are used primarily to seal the screed surface to prevent the absorption by it of the water from the adhesive or overlayment. Primers provide a consistent surface suitable for adhesives or floor coverings. A smoothing compound, although usually not a finished surface, is a floor covering.

With some combinations of screed and overlayment, a primer is also necessary to prevent any detrimental reaction between the two materials.

The manufacturer’s recommendations must always be followed when installing any coverings above the screed.

How and why do I need to test the flow of flowing screeds?

It is important that flowing screeds are tested at the batching plant and at site. Depending on the flowing screed product the flow range is normally between 230mm-310mm. To flow test the material a jug, tape measure, flow cone and flow plate are needed. The equipment should be clean and dry. Place the cone in the middle of the plate and fill with flowing screed. Lift the cone straight up and allow the screed to level and form a pancake. Measure the pancake in two directions and take an average measurement of the result – record the measurement after 30 to 60 seconds (depending on the exact product). The flow must be accurate for the product to be installed, as per limits set out in specific product information.

What is the purpose of the FSA?

It’s an association that has been created to support screed installers and to promote and develop the profile of flowing screeds throughout the UK and Ireland.  The aim is to raise standards and industry awareness of flowing screeds.

Will the FSA offer any technical advice for its members?

The FSA has technical advice available for its members. If you wish to take advantage of the service, please send your query to membership@flowingscreedsassociation.com. Please provide us with all information regarding your query, including photographs where appropriate, and your full contact details.