There is some confusion among specifiers concerning the properties of “exterior grade” plywood. This article by Dr Vic Kearley of TRADA seeks to clarify the situation and outline improvement actions that are underway.
Most confusion arises from a misinterpretation of BS EN 636:2003 Plywood – Specifications, which classifies plywood for use in dry conditions (EN 636-1), humid conditions (EN 636-2) and exterior conditions (EN 636-3). Here, “exterior conditions” corresponds to external weathering which can be expected to include periods of persistent wetting. It appears that some specifiers wrongly assume that plywood specified or labelled as “EN636-3” is suitable for external use without any further protection. However, the standard refers mainly to the quality of the glue bonds. It indicates these are likely to perform satisfactorily if exposed to long-term wetting out of doors. The method used to assess glue bond quality is similar to the older “WBP” test that