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ROCEDURE ieving a long-lasting, professional flooring finish when working over calcium sulphate screeds.


of steps that should be undertaken before any other work takes place, including ensuring that the subfloor is suitably sound, smooth and dry. The removal of any laitance (the crust of fine particles formed on the surface of the screed as it dries) should take place seven to 10 days after the screed is installed, prior to the laitance hardening, which will prolong the drying time of the subfloor. If not removed, laitance can also cause subsequently applied subfloor preparation products to debond from the substrate, causing floor failure.


A moisture measurement test should then be conducted to ascertain if the subfloor is dry enough to proceed directly to installing floorcoverings. If a subfloor has a relative humidity (RH) reading above 75% (or 65% if wood floorcoverings will be installed), contractors will need to allow further time for the screed to dry, otherwise there is the risk that excess subfloor moisture will attack floorcoverings and adhesives, potentially resulting in floor failure.


DRYING TIME As a rule of thumb, calcium sulphate screeds dry at a similar rate as sand/cement screeds i.e. 1mm per day. Therefore, contractors should be patient and allow enough time for the subfloor to dry sufficiently. This process can be accelerated by ensuring adequate ventilation, by opening windows or using a dehumidifier and/or turning on underfloor heating, if present, at a low temperature.


FAIL TO PRIME, PREPARE TO FAIL Once the relative humidity of the subfloor is sufficiently low, the next step is to prime. Priming a calcium sulphate screed is essential to the overall appearance and performance of the floor as it prevents ‘pinholing’ in subsequently applied


smoothing underlayments. Bubbles, in the form of air slowly escaping absorbent surfaces when drying, burst upon reaching the surface, resulting in small pinholes which compromise the smoothing underlayment’s integrity and can affect the appearance of the finished installation. Priming also helps to promote the adhesion of a subsequently applied smoothing underlayment and prevents it drying too rapidly, which can cause it to become weak.


“Priming a calcium sulphate screed is essential to the overall


appearance and performance of the floor as it prevents ‘pinholing’ in subsequently applied smoothing underlayment.”


F. Ball’s Stopgap P121 acrylic primer is specially formulated for application over calcium sulphate screeds, which are denser than concrete or sand/cement screeds. The primer contains finer particles than other products in the F. Ball range, helping it to bond with calcium sulphate screeds. Stopgap P121 should be applied in two coats; the first diluted with one part water to one part primer, followed by a second coat applied neat.


COMPATIBLE SMOOTHING


UNDERLAYMENTS Screed manufacturers advise that a calcium sulphate-based self-levelling compound should be used to create a base for floorcoverings when dealing with this type of subfloor. The application of cement-based products onto damp calcium sulphate screeds can result in a chemical reaction that causes the formation of ettringite, a crystalline material that can cause floor failure where the two materials meet.


Unlike most cement-based products, F. Ball’s Stopgap 1100 Gypsum calcium sulphate-based levelling compound is tension free and does not shrink during the drying process, which limits the amount of stress at the bond interface. On all surfaces, applied at a nominal thickness of 3mm, the compound will be ready for contractors to install textile floorcoverings after 24 hours, and resilient floorcoverings can be installed after 48 hours.


Following these steps will result in a hassle-free installation, though F. Ball’s technical service department is still on hand during working hours to answer queries, should any arise.


www.f-ball.com/en twitter.com/TContractFloors SCREEDS, RESINS & COATINGS | 23


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