TECHNICAL BACKGROUND ARTICLE FROM ARDEX UK
Date: 18 February 2005 •
Sub-floor preparation for ceramic floor tiling
Cyril Potter, Chief Chemist at Ardex UK looks in detail at sub-floor preparation for ceramic floor tiling.
Any preparation work carried out on sub-floors is intended to provide a surface that is clean, dimensionally stable, rigid and sufficiently flat so that the ceramic floor tile can be well adhered and provide a hard wearing and durable floor finish.
SUSPENDED TIMBER SUB- FLOORS.
Where wood based sub-floors are to be tiled the tile fixer has to ensure that the sub-floor is rigid and at the correct moisture content.
The first requirement can give problems if the sub-floor has to be strengthened to achieve a rigid base for the ceramic floor tiles.
Fixing new plywood to stiffen the existing sub-floor will raise the floor level and this can cause problems where the new tiled floor abuts existing floors.
The recommendations given in the BS Code of Practice are to ensure that the plywood sheets and boards are conditioned to the correct moisture content and then the backs and edges should be sealed with a suitable sealer to prevent the ingress of atmospheric moisture.
Note that an adhesive specifically recommended for use on wood based sub-floors should always be used and any priming recommended for use with the adhesive should be applied to the upper unsealed plywood surface.
The main cause of problems on wood based sub-floors is the difficulty in conditioning any new wood based sheets or boards to the moisture content they will have once the floor is in use.
The problem applies to new build construction and refurbishment work where the new sheets and boards may be stored under conditions of high humidity so that the boards take up moisture vapour and expand.
If the tiles are fixed whilst the boards have too high a moisture content cracking of the tiles can occur.
Guidance on the suggested moisture contents of wooden sub-floors and boards to receive ceramic tiling under different heating conditions are given in The Tile Association guide 'Tiling to Timber Sheets and Board, Timber Substrates and Alternative Products'.
The lowest moisture content of the boards is necessary where under tile heating is used on a wooden sub-floor.
Wood based boards with piped integral heating should always be brought up to operating temperature to ensure that they are of appropriate dryness when the ceramic floor tiles are subsequently fixed.
When fixing plywood sheets and boards over existing floorboards it is also good practice to check that the old floorboards are well screwed down and rigid.
It is always advisable to pre-smooth the existing floorboards with a suitable latex smoothing compound to ensure that the new plywood sheets fit flush so that no voids are present where localised movement under loads can occur.
When fixing plywood sheets try to use one large sheet if possible or take steps to minimise the number of board joints.
If 2 boards are necessary then try to set out the tiling so that the tile joints coincide with the joint between the boards.
If movement occurs at the plywood board joint, from subsequent drying of the plywood, replacing the grout along one joint is easier than replacing a cracked tile.
The plywood boards should be laid broken bonded and running in the opposite direction to the joints between the existing floorboards.
Where large areas of tiling are required on existing wooden sub-floors it is better to utilise some of the proprietary intermediate substrate materials specifically designed for use on wood based sub-floors.
