Plywood is often used in the construction of drywall to provide a pattress for fixing items to a wall. Also, the introduction of plywood or other wooden sheets in addition to or as a replacement for plasterboard may affect the fire or sound performance of the wall and could render any certification void, leaving the specifiers open to huge cost implications to correct matters. Joe Cilia, FIS technical manager, considers the implications.

Plywood (ply) and Oriented Strand Board (OSB) are often the default solution when it comes to constructing drywall so that it can take a load; be it to ensure that a flat screen TV can be installed, ceramic or stone tiles fixed to the face of a partition or lining or to comply with the Lifetime Homes Standard so that handrails can be installed retrospectively.  But is ply the right choice? What are the implications? Is there a right way? And are there any alternatives?

When it comes to meeting the pull-out pull-down and eccentric downward load tests in BS 5234-2:1992, the options are:

  • install a pattress
  • install a fixing channel
  • use a higher density board designed for the purpose.

The choices of pattress material are either OSB or ply, both of which are available in a range of thicknesses, grades and from a number of countries including Canada, Brazil and China. Ply is also available in moisture-resistant and Water and Boil Proof (WBP) variances. The stud and system manufacturers all offer a range of fixing channels with calculated loadings to allow specific items such as handrails to be installed. While the choice of Gypsum and cement-based boards with and without fibrous infills and backings is growing.

The FIS advisory service has visited a number of sites where we were asked to comment on subsequent bowing of a wall or where tiles had cracked after occupation. In a high proportion the cause was traced back to the inclusion of ply either as a pattress or as a facing material for tiling. However, the cause of the distortion was often different, which means that there are a number of things that should  be considered during the  design and installation to ensure a satisfactory completion and handover.

The first thing is to understand what load is going to be applied to the wall or lining and whether there is  evidence to support the solution you have proposed? Has the pull-out strength of the board been tested? Secondly, how is the pattress going to be included in the construction: is it going in front of the studs, between them or as a facing?

If the pattress is going in front of the studs, is the ply available in the same widths as plasterboards, or does it need cutting? They are unlikely to be the same thickness, so there may be implications to consider where double board thicknesses are used. If they are to be installed between the studs, they might require cutting if you are not using a proprietary product designed for use as a backing substrate. If this is the case, what are the knock-on implications caused by increased noise and dust and the risks in using saws?

Check with your supplier about any implications to performance certification or warranties before deciding on a solution, as the consequences of checking after the event could prove very costly.

Lastly, what do the suppliers of the pattress material say about site conditions, installation and suitability for use?

Ian Rochester of the Wood Panel Industry Federation (WPIF) said: “Prevention – the best way to prevent any issues with wood apart from correct fixing and expansion gaps is to condition it to as close to in-service conditions as possible before fixing to reduce the amount of movement that will occur once installed.” This is probably the most simple and effective advice to give when working with wood as most problems with wood are generally related to moisture (usually too much).

The Panel Guide V4 from WPIF states: “The moisture content of panels at the time of installation should be as close as possible to the in-service moisture content. Panels are normally manufactured at low-moisture contents, between two per cent and 13 per cent, and may still be very dry at the time of delivery. Where panels are to be used in warm, dry areas, it is important that the moisture content of the panels is kept as low as possible. This requires storage in an internal, dry environment that is preferably heated. Any protective wrapping should be left in place until shortly before installation. If the storage conditions are close to the final in-service conditions, then the panels can be unwrapped and conditioned by loose-laying (on floors) or horizontal stacking with spacers between each. A minimum conditioning period of 48 hours is recommended but longer periods may be required, depending upon the conditions required and the initial moisture content of the panels.”

Having considered all of this in the design and planning stages to ensure you have the right solution and that the materials have been conditioned before installation, what about the intricacies of installation, and the impact on the finish?

Where board is installed to the face of the studs, either as a first layer or finishing layer, the boards should be pre-drilled, ensuring that the recommended centres and distances from the edge of the board are closely adhered to. This is to allow a small degree of flexibility as the board takes on and releases ambient moisture during operation. If the boards are to be tiled ensure that the guidance from The Tile Association (TTA), WPIF and supplier have been read and understood regarding sealing and balancing, as well as conditioning.

If the boards are going to be installed between the studs use of a proprietary support plate or zed channel is recommended, again ensuring that the side as well as a face fixing is used. Check with the system manufacturer about maximum depths that the brackets can accommodate too, and understand that there will be additional build-up of material at these points which may impact the final flatness of the wall.

If the pattress is going to be used in a kitchen, bathroom or where a TV is going to be installed, consider how the services can be accessed; should cut-outs for wires, plumbing or conduit be pre-cut and, if so, will this impact on sound and fire performance?

TTA says 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 (ply, 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. The backs and edges of such boards should be treated against the ingress of atmospheric moisture that would result in movement and warping. Tiles should not bridge joints between boards.

Sheets or boards should  be adequately braced to provide a rigid surface, be free from any springiness and surface undulations, and undergo no subsequent distortion during and after completion of the tiling. Wherever possible, the boards should be screwed, not nailed, to the supporting framework.

In general, where the sheet or board has a smooth and a rough side, the latter should be used for tiling. The surface to receive the tiles should be clean and free from dust and other forms of contamination.

Care should be taken to ensure such wood-based sheets are not installed in a condition where their moisture content is higher than the ambient equilibrium moisture content once the tiled installation is in use. Failure to observe this can lead to subsequent warping and distortion of the sheets with consequent cracking and delamination of the tiling.

One possible solution to all of this is to consider using a dense gypsum or cement board designed to take an applied load in the form of a shelf or handrail and accommodate a tile finish.