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Preparing a concrete surface for a new floor

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It is essential to prepare the floor surface before laying the new floor. A spirit level is essential for determining the slope of the floor.

Preparing concrete floors

Dry, smooth and flat concrete floors are suitable for any kind of tiling. If the floor is very uneven - differences in depth greater than 12mm, use a sand and cement screed of at least 12mm thick. Use a coat of proprietary sealer on the uneven surface before laying the screed, especially if the original surface is dusty. Leave the floor for approximately four weeks before commencing tiling. Tiles can then be laid directly onto the screed.

A water-based levelling compound is suitable for levelling a concrete floor, if the depth is less than 12mm. It comes in powder form and is mixed to a paste and then a thin layer - no more than 4mm (3/16 inches), spread over the floor.

Before beginning, repair any cracks deeper than about 3mm (1/8 inches) by cleaning the cracks and removing any loose debris - with a hammer and chisel if necessary. Apply PVA adhesive and then fill the cracks with a sand and cement mortar. Deeper cracks (10mm) should be cleaned, and using a bonding agent filled with sand/cement using a 3/1 mix. Allow the area to dry and make sure the floor is clean before applying the levelling compound. Use a wire brush to clear out any cracks or corners and remove any oil or grease by washing the entire surface with detergent.

Mix up the compound, following the manufacturer’s instructions - it should be free of lumps and have a creamy consistency. Start in a corner of the room furthest from the door and trowel, using a steel float outwards to an even thickness of approximately 3mm (1/8 inches). Keep the level horizontally even, but it is unnecessary trying to smooth over any marks from the surface, as the compound will seek its own level. Continue across the room until the entire floor is covered.

When the floor is hard enough to walk over, smooth out any small lumps with a little water and the steel float. Although it is safe to walk on the surface after an hour or so, the covering should wait for a few days to allow the compound to harden.

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