Materials Wood-based panels
Smartply Site Protect is a coated site hoarding panel with an exterior polymer coating. Photo Medite Smartply
Here are two illustrations of how this might translate in the decision-making process for biological durability: 1. Exterior cladding in residential low rise Brief stage • Has to be painted and have a minimum 10-year life • Must be able to take paint and edge sealing • Will require regular maintenance • Possibility to be replaced if necessary after 10 years Specification stage In this case an ‘exterior’ plywood with a durability claim of UC 3.1 or above, exterior MDF, or a marine plywood could be used. Reasoning As the cladding should have a painted appearance, is accessible and regular re-painting is anticipated, the biological durability of the cladding is not an issue due to the use of paint, so a regular exterior grade plywood could be used. But a marine plywood or an exterior MDF (painted or modified*) could equally be used, as they have inherent biological durability and may maintain the cladding for more than 10 years. 2. Cladding on a block of flats where access is difficult Brief stage Has to have a good appearance but without paint, no maintenance and last for 25 years when it will be replaced. www.trada.co.uk
Specification stage Modified MDF or a treated ‘exterior’ grade plywood with a durability claim of UC 3.1 or above, and a preservative treatment suitable for the desired service life. Reasoning Access is not possible except for very infrequent maintenance by contractors for which painting is not anticipated. So any product used must be able to withstand the weather for 25 years without any maintenance. Using these illustrations, a picture can be built up of the decisionmaking process and the considerations needed to specify appropriately. While the explanation of the system of classification for durability may appear complex, in practice, as can be seen in the examples, the process is relatively straightforward. n *MDF made with wood fibres chemically modified such that they become resistant to moisture ingress and as a consequence enhanced durability is imparted. The commercial production of these fibres is registered under the brand name Tricoya.
Further Information Specification and use of wood based panels in exterior situations [WIS 2/3-11] is available at www.trada.co.uk
About the author Ian Rochester is Technical Affairs Manager at the Wood Panel Industries Federation. Timber 2018 Industry Yearbook
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