Specifying Self Levelling Compounds

What is a self levelling compound?

A 'self levelling compound' is a generic term given to a group of products which are used to create a smooth floor. They are also known as smoothing compounds, smoothing underlayments, levelling compounds, levellers, liquid screeds or flow screeds, but self levelling compound is the most frequently used. In fact a 'self levelling compound' does not naturally 'self level' but instead 'self smooths'. This is because they naturally flow across a substrate but do not necessarily make the floor level. Over a large area for example the variance from one corner to another could be different but may not be apparent to the naked eye. Smoothing compounds can be used to create 'level' floors through the use of a laser and level pins but it is not a wide spread practice unless specified. Due to these reasons at UZIN we refer to 'self levelling compounds' as smoothing compounds.

How important are smoothing compounds?

Whether in a new build or refurbishment, choosing the right smoothing compound is critical for ensuring the success of any final floor finish. The appearance and performance of the floor coverings are determined to a significant extent by the quality of the prepared base on which they are installed.  There are many factors which can affect what smoothing compound should be specified, such as the substrate type, site conditions, occupational use and type of floor covering installed.

Type of substrate and preparation

There are some basic requirements for any substrate, these are defined in the various British Standards and to paraphrase any substrate should be “CLEAN” (free from any materials deleterious to the possible success of any surface preparation) “DRY” (below 75% relative humidity) and “SOUND” (stable, free from signs of movement, be that past or present). Before any work is carried out it is essential to evaluate the substrate in order to provide a specification.

Moisture

Above all it is excess moisture which can cause the most problems to flooring installations. When installing wood floors the moisture reading must be less than 65% RH (Relative Humidity).  For most other types of flooring the reading must be below 75% RH.  If the moisture in substrate reads above these levels there are various methods to suppress moisture, such as a surface applied damp proof membrane (DPM) or underlay isolation systems. There are also innovative and more rapid methods such as the UZIN L3 Gold Moisture Control System, a unique combination of primer and smoothing compound with the moisture suppressant ‘built’ into the smoothing compound.

Type of substrate

The type of substrate can affect which smoothing compound is specified. Cementitious smoothing compounds are the most common form of smoothing compound and can be used over most surfaces if the substrate has been correctly prepared such as cement or concrete. Other surfaces such as wooden substrates need to be handled differently as they will require a fibre reinforced smoothing compound such as UZIN NC 175.  Mastic asphalt substrates are very sensitive to temperature changes and influences from external stresses and almost inevitably contain hairline cracks, where possible calcium sulphate based smoothing compounds such as UZIN NC 105, UZIN NC 110 or UZIN NC 111 BiColor should be used. 

Special care must be taken over anhydrite screeds (also known as gypsum or calcium sulphate screeds), their requirements, limits and performance can be very different to cement based subfloors. UZIN is one manufacture which specialises in systems for these screeds and provides several gypsum based smoothing compounds such as such as UZIN NC 105, UZIN NC 110, UZIN NC 111 BiColor as well as UZIN NC 118 gypsum rapid repair mortar.

Refurbishment projects

When working on refurbishment projects there are a number of issues which need to be considered. The old floor covering may need to be removed, the floor might just need to be cleaned, or ground to remove old residues of cement or adhesive. There may be surface contaminants, laitance or residues which need to be removed. It is recommended that all these contaminates are removed, although some ‘bag and bottle’ smoothing compounds can be used over old waterproof adhesive residues which are well bonded to the substrate such as UZIN L3 Gold.

Type of flooring

Another important factor to consider is the type of floor covering being installed. Some floor coverings place higher demands on the surface strength of a smoothing compound. It naturally follows that with these materials, high-strength cement-based compounds should be used. For example, because of changes due to moisture, wood flooring will build up high shear forces between itself and the substrate. In these cases a higher flexural strength cement-based product should be used, something with a minimum F 7 N/mm² flexural rating such as UZIN NC 170.

Strength

Floors in a commercial setting, which will receive heavy and regular footfall or wheeled traffic, will require a much higher strength smoothing compound than those in say a domestic location. It is very difficult to be precise as to what level of “strength” a substrate may require, but in general up to 25 N/mm2 would be domestic / light commercial, up to 30 N/mm2 would be medium commercial, up to 35N/mm2 would be heavy commercial / medium industrial and everything above, heavy industrial. It is also important to understand that whilst the compressive strength is indicative of a preparation materials suitability, the cohesive strength of the product is as, if not even more, important. UZIN provide a range of different strength smoothing compounds which are suitable for every requirement.

Cheap, low strength smoothing compounds are still prevalent in the industry and with many resilient floor covering manufacturers requiring at least a 25 N compressive strength smoothing compound, there continues to be a flaw in many installations.  Lower strength smoothing compounds are susceptible to indentations that will mirror in the floorcovering, especially if there is heavy traffic and trolley transportation. In mainland Europe scratch testers known as the (RiRi) surface test are used to assess the strength of the floor. Inspection of the diamond formation it produces and clarity of the grid lines help the tester judge the suitability and hardness of the surface of the smoothing compound or subfloor.   

Levels and surface regularities

Ideally a floor should remain level and smooth with no readily visible undulations in depth or surface abnormalities. The desired result can be achieved through good working practice, trowel skills and ‘rubbing down’. Making sure the surface is level can be aided through the use of level pins or pin levelling rakes can be set to ensure that uneven depths are clearly identified. Specifying a good quality smoothing compound can also help the flooring contractor. It is well understood that water mix smoothing compounds traditionally produce better fluidity and a more even finish, although there is constant improvements being made in “bag and bottle” mixes such as UZIN L3 Gold. It is important to confirm the degree of smoothness and or levels required when specifying. Also remember that level isn’t smooth nor is smooth level. The British Standards relevant to our industry specify a “surface regularity” (SR1,2,3) this is a measurement of smoothness, not level.

Speed

Most smoothing compounds will dry and allow moderate foot traffic after approximately 2 to 3 hours, but will not be ready to accept the floor covering for 20 to 24 hours. With project deadlines squeezed ever more tightly, choosing the correct smoothing compound can help expedite projects. Smoothing compounds such as UZIN L3 Gold or UZIN NC 172 BiTurbo dry extremely quickly and allow floor covering to be installed after just 1 to 2 hours.

Sustainability

Due the increasing number of legislations such as REACH and COSHH as well as LEED and BREEAM certification, clients are demanding more safe and sustainable approaches to construction. Smoothing compounds awarded EC1 plus classification are very low emission (TVOC limit after 28 days is less than 60 μg/m³) and provide the most ecological approach for flooring installations. Most UZIN smoothing compounds have been awarded EC1 Plus certification.

Some smoothing compounds such as UZIN NC 170 have Environmental Product Declarations (EPDs). An EPD describes the ecological and technical characteristics of a product over its full life cycle. In doing so, it relies on the life cycle assessment method for a product, makes statements about its energy efficiency and thus enables planners and architects to evaluate the smoothing compounds sustainability. 

Best practice

Due to the increased use of water mix smoothing compounds over the years, priming has become more common. A primer prevents the water in the smoothing compounds being drawn out too quickly preventing pin holing and gives a more homogenous appearance (this also can be aided by spike rolling) and can increases the bond strength between the substrate and smoothing compound. Traditional “bag & bottle products” can be used to avoid priming, but priming provides a better guarantee that no failure will occur.

As with all smoothing compounds, correct mixing and application methods are paramount, overwatering, wrong temperatures, poor airflow etc. will all lead to potential problems. Specifying a higher quality smoothing compound may allow greater water stability, be more tolerant of poor conditions and are less likely to ‘separate’. 

Conclusion

Specifying the right smoothing compound can be diverse and complex. Ultimately, the individual project, site conditions and the user preferences must determine the steps taken.  Smoothing compounds can help expedite projects and aid with environmental building criteria and certification. The strength of smoothing compounds also plays a vital and fundamental part in the appearance and durability of the floor, a factor which is often over looked when specifying.  The UZIN Technical Team are pleased to provide help, support and advice if required.