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Plywood outlawed as wall tiling substrate

As part of the five year standards review, the new version of BS 5385: Part 1: 2018 has recently been published.  This document considers important changes which have occurred within the tiling industry since the previous 2009 version of this standard was published, such as the increasing prevalence of thin panels and large format wall tiles, and the use of natural stone tiles.

For example, previously it stated in BS 5385 Part 1: 2009 that: “the use of sheets or boards that are subject to movement from changes in moisture content should be avoided if at all possible. If such boards (plywood board, chipboard, some fibre building boards) have to be used, they should be restricted to small areas and installed in such a way that they provide a dimensionally stable and rigid background.”

However, since the last review of BS 5385 Part 1, the quality of plywood available from a tiling perspective has decreased significantly, with cheaper imports flooding the market.  While higher quality exterior grades of plywood are available, they are significantly more expensive.

It is important to consider that wood is a hygroscopic material, which means that its moisture content will change dependent upon any changes in the environmental conditions on site.  Therefore, the dimensionally stability of wood-based boards cannot be assured, and this may ultimately lead to failure of the tiling installation.  Since BS 5385: Part 1 was last revised, there has been a huge increase in the use of tile backing boards of varying types including foam-core, fibre cement and other materials such as Magnesium Oxide.

Tile backing boards are ideal for wall tiling as they offer a dimensionally stable wall background to moisture and thermal movement.  Plus, they offer an additional benefit over plywood including increased weight limits for tiling. The recommended maximum weight of tiling for plywood is 30kg/m².  This is less than plasterboard (32kg/m²) and significantly less than tile backing boards which can hold anything from 60kg/m² to 100 kg/m², dependent upon the advice given by the board manufacturers.

A competitive tile backing board market means that prices have also been driven down, no longer being cost prohibitive for fixers and contractors. As wood is a hygroscopic material it will be affected by is changes in humidity and temperature on-site and as such there is always a risk when tiling to plywood or other wood-based sheets. With this in mind and following discussions with industry experts it was agreed to omit the use of plywood from BS 5385: Part 1 as a suitable background for the direct fixing of ceramic or natural stone tiles for internal wall tiling.

 However, while plywood is not recommended as a background for direct wall tiling, it can still be used as a structural board when overlaid with a suitable tile backing board, particularly where installation of mechanical fixings is required e.g. for mesh backed natural stone where it is not possible to remove 75%  of the mesh backing.

While plywood is not recommended as a wall substrate for the direct fixing of tiles, the advice for floor tiling remains the same as per BS 5385 Part 3: 2014.

It was agreed that for existing timber floors, plywood overlay is still acceptable to tile to in small internal dry areas, if it is a minimum 15mm thick and of an appropriate quality exterior grade.

This is because many houses still have timber floors, particularly at first floor level, which may require over boarding with plywood to ensure the floor is suitably rigid and strong enough to receive a tiled finish and any addition loads.

However, despite the inclusion of plywood, fixers and contractors should still be aware of the limitations of wood as a substrate for floor tiling – particularly in certain environments – such as wet, frequently damp areas or those subjected to high humidity.

Increasingly, preparatory tile backing boards are being specified as an alternative to plywood when installed over timber floor and BS 5385: Part 3: 2014 considers these boards in clause for new timber bases.  

Other changes
Previously in internal dry wall areas it was recommended that tile adhesive should cover a minimum of 50% coverage spread evenly over the back of the tile. However, driven by necessity, with the increase in the size and types of tiles i.e. larger formats and thin ceramic panels now available on the market, BS 5385 Part 1: 2018 advises; “Tiles with a surface area of less than 0.1 m², but which weigh more per square metre than 70% of the background’s capacity to carry the weight, should be solidly bedded e.g. the maximum weight of tile that can be supported by Gypsum plaster = 20 kg; whereas 9 mm thick porcelain tiles, which weigh approximately 18 kg/m², weigh more than 70% of 20 kg (14 kg) therefore, they should be solidly bedded regardless of their size.”

Included within the scope of BS 5385-1: 2018 are large format ceramic tiles, ceramic panels i.e. tiles with a surface area >1m² (any edge length >1200 mm) and thin tiles i.e. ceramic tiles and panels with a panel thickness of ≤ 5.5 mm. To reflect this, additional changes have also been made in the minimum recommended grout joint width, dependent on the tile/panel size, e.g. the minimum grout widths vary by tile facial area – an example as follows:

1) For tiles with a facial area of less than 0.1m² with no side >600mm long, a minimum joint width of 2mm is required.

2) Tiles with a facial area 0.1m² to 1m² with no side >1200mm long, a minimum joint width of 3 mm is required.

3) Joints between ceramic panels should be increased pro-rata to panel size (e.g. for a 3m long ceramic panel the minimum required joint width between panels is 5mm).
Not included in the scope of BS 5385 Part1: 2018 are:

  • Natural Stone Slabs i.e. stone which is more than 12mm thick,
  • Agglomerate stone
  • Metal, plastic resin, mirror or glass tiles of a similar construction
  • Ceramic panels and thin tiles which are less than or equal to 5.5mm thick.

Always refer to the manufacturer of these products for further advice.
For more information please consult BSI Document BS 5385 Part 1:2018 or consult BAL Technical Advisory Service on 03330 030160.
W: www.bal-adhesives.com

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