Successful screeding projects are grounded on correct screed mixing, adequate drying times, full compliance with specifications, including flatness and levelness, and appropriate screed care after the installation. The next five points explain some of the mistakes that can be made when installing screeds.
1. Incorrect screed proportions and mixing
Site-mixed screeds can bring a series of positives in terms of cost, flexibility, quality control, and waste management. But if the screeder adds too much cement, aggregates, additives, or water, or does not mix screeds properly, the resulting construction may fail to develop the strength required to tolerate the traffic and loads expected. On the other side, using recommended amounts of materials and opting for forced-action mixing can help achieve a high-quality substrate. Once the screed has hardened, a BRE Drop Hammer test can be conducted to confirm the ability of the sub-floor to withstand the type of exposure intended.
2. Insufficient curing and drying
Buckling floors, blisters on vinyl, musty smells, and mould growth are just a few consequences of installing final floors before the screed has dried completely. Curing traditional screeds, laid at a depth of 75mm, for 7 days and then allowing them to dry for 110 days can prevent all these issues and deliver a solid, durable construction that can take on any type of floor finish. For projects with short time frames, screeders can choose anhydrite screeds, which require approximately 80 days for complete drying, or fast-drying screed formulations, which can dry in as little as 3 days. These screeds can reach maximum strength after 28 days under the correct conditions.
3. High surface regularity index
Bulging or sagging sections in the screed may lead to serious serviceability issues. Although the screed does not have to be perfectly flat, maintaining flatness within previously established limits can help avoid localised wear marks, impact at high spots, and difficulty installing equipment directly on the floor. A 2m long straight edge in conjunction with a feeler gauge can be used to measure deviations, which should not exceed 10mm in utility standard floors, 5mm in normal standard floors, and 3mm in high standard floors, as defined in the British Standards.
4. Excessive departure from datum
Permissible departure from the datum level depends on the size of the screeded area. Whilst a variation of ±15mm is considered satisfactory for large areas, finer tolerances may be required in small rooms. To keep deviations from datum within reasonable limits, in line with project specifications, and avoid a series of problems, such as excessive wear and tear at high spots and the inability to install specialised equipment directly on floors, professional screeders use self-levelling compounds that help them achieve perfectly level sub-floors. To confirm compliance with project specifications, specialists use laser levels, which are high-precision instruments that measure departure from datum accurately.
5. Lack of temporary screed protection
Temporary screed protection is very important to maintain the integrity of the screed until the installation of the final finish. To select the right type of protection (e.g. reinforced paper, cardboard, corrugated plastic, or polypropylene mesh sheeting), expected traffic to newly screeded areas must be considered. Since builders may decide to proceed with other tasks before the screed develops the strength required to withstand traffic and loads, providing temporary screed protection is the responsibility of the builder.
To find out more about screeds and screeding, we invite you to contact our experts at BuilderScreed or download our guide, “Six Steps to Getting a Perfect Screed and Under-Floor Heating System”.