The term screed can be a confusing one. Some people refer to the concrete surface as a screed, others may mean a levelling compound, others may refer to the final floor finish. We delve into the topic with comment from leading industry manufacturers.
What is a screed
Screeds are typically a cement:sand layer most commonly used to level out a concrete subfloor, particularly when significant variations in levels are found after pouring the concrete. The minimum thickness of each type of screed varies, based on the duty requirements of the application. i.e. heavy duty applications such as high traffic areas require a thicker screeds.
Floor preparation screeds
Floor preparation systems could include primers, adhesives, repair mortars and self-levelling screeds which are applied to either new or existing concrete subfloors. These compounds are designed and formulated to achieve the smooth flat finish required for the installation of a l ‘soft’ or resilient floor covering, such as vinyl sheeting, luxury vinyl tiles (LVTs), or resin-based floor coatings to prevent aesthetic imperfections and potential wearing damage. They are also installed prior to the installation of large format tiles which require a flat and level surface to minimise the risk of ‘lipping’ of the tiles.
- Self-levelling/ smoothing
- Rapid-setting for quick turn-around times
- Non-shrink
- Dustless
- Chloride free
- Pumpable
- Effective in thinly laid layers
Final finish screeds
A screeded concrete floor, which is left as the final finish requires a totally different formulation from a floor preparation compound, or product system as it is directly exposed to traffic. It will offer different properties or needs to be protected with a resin-based coating or similar, to perform well depending on the project criteria and application area.
Industrial building and other high mechanical load exposure areas often require high-strength, abrasion- and chemical-resistant screeds with higher compressive and flexural strength, and a finish compatible with the requirements of the service conditions.
Floor hardening compounds can also be broadcast and floated into newly installed cement:screed to provide an abrasion resistant and hard-wearing surface. These are available in a variety of colours and are ideal for heavy traffic areas such as loading bays, car parks, warehouse floors, workshops and production areas.
Floor repair
Used when reprofiling a concrete substrate, a concrete repair mortar is a screed or flowable micro-concrete that can be used to patch or repair damaged sections of an existing floor. They are robust enough to be left exposed in high traffic industrial applications, or below a final floor covering. Self-levelling variants are also available.
Self-levelling vs trowel applied floor preparation screeds
These are available with different flow rates, and application methods.
- Self-levelling
- A self-levelling screed is formulated with high-flow properties to produce a level surface which is suited for installation of ‘soft’ or resilient flooring coverings. These products can smooth out imperfections with as little as a 2.5mm thickness to a maximum of 10-16mm, depending on the product used. In large application areas, this type of compound can be pumped for faster installation.
- When variations in the floor are greater than 20mm, or over large spans of floor, a more robust compound is required to correct variations in the floor surface. These can be a micro-concrete, or rapid-setting cement and aggregate screed mix. Installation thickness can be affected by the aggregate size used. This type of product could also be used for patching repairs on concrete floors.
- Trowel-applied
- A trowel-applied screed is ideal for smaller installations or can be used to “screed to falls”, that is to direct the flow of water over a surface towards a drainage point. This is required for showers, flat roofs and balconies to prevent water from sitting on the surface and potentially damaging it, and to prevent ingress into the structure. When used in ‘wet’ areas, a suitable latex additive in the screed mix is essential.
- A trowel-applied screed is ideal when installing underfloor heating, avoiding damage that may be caused by the rolling and spiking associated with self-levelling screeds. The addition of a latex additive in the screed mix provides flexibility for the anticipated thermal expansion and contraction.
Screed vs tile adhesive
Some tiling contractors will forego the use of a screed, choosing to rather use the tile adhesive to build up areas where the floor is not level. In applications where small, isolated areas of the floor surface is uneven, and by no more than 20mm, this may be acceptable. However, in spaces where the bulk of the surface is uneven, the use of a levelling screed is more cost-effective versus the increased tile adhesive use. Exceeding the maximum application thickness of the adhesive may result in a compromised installation and tile adhesives do not have the same compressive strength as a screeding compound. Tiling onto a level surface also allows for a quicker tiling installation overall, as it is labour intensive to work with thick beds of wet adhesive and achieve a level tiled finish.
Selecting the right type of screed, or levelling compound that is fit for purpose requires an understanding of the strengths and properties of each type of compound.
Acknowledgment and thanks go to a.b.e. Saint-Gobain and TAL, for some of the information in this article