Panel Products - Design & Workmanship

How design & workmanship on site will affect the performance of wood-based panels

Wood-based sheet materials when correctly specified and utilised are robust materials. Much depends on the actual conditions, but it is possible to state some general rules applicable to all but very temporary exterior use of wood-based panels.

Edges of boards must be sealed to minimise absorption of water. Such sealing may be with:-

  • special sealing compounds, such as 2-part epoxy

  • non-setting mastics if the panel is set in frames

  • polyurethane, acrylic or rubber-based paints

  • wooden beading bonded with exterior adhesives

  • metal or plastic capping or channels fixed with non-setting mastic

  • a minimum of two or three coats of paint applied to all edges.

‘Special’ edge sealing at the mill is also available for some products on request

Good design and site workmanship are essential in all cases. In addition to edge sealing, the top and bottom edges of boards should be chamfered, bevelled or rounded to promote shedding of water and to maintain good paint adhesion.

Designs which incorporate a measure of protection to wood-based boards should also be considered, as this can prolong the service life of panels when subject to external conditions. For example, the inclusion of large eaves overhang to prevent driving rain from hitting panels, slopes or gradients to decks in order to shed water, and designs which incorporate and/or allow for the free flow of air around panels should be considered. Detailing and design must be such that water is shed effectively and does not soak into the backs of panels, especially in areas such as soffits, where cavities must be adequately ventilated to allow dispersal of moisture.

Failure to leave expansion gaps around the edges of panels can lead to buckling (3 mm on each edge is generally considered adequate), while clearance at panel joints should be such that free drainage of water can take place. Mastic joints between boards are unlikely to be satisfactory due to the cumulative movement across the width or length of the board being concentrated in the area of the joint. Exposed or concealed joints may be used but, in each case, should provide an expansion/drainage gap wide enough to facilitate re-treatment of the board edges on redecoration.

Similar considerations apply when detailing the junctions between wood-based panels & masonry. Adequate clearance should be left at the junction to allow for panel expansion, drainage and prevention of capillary absorption of water from the porous masonry, and to allow periodic reinforcement of the edge sealing. The bottom edges of the boards should stand well clear of any flashings or sills for similar reasons.

Non-concealed fixings should be non-ferrous or of a suitable grade of stainless steel in order to prevent unsightly iron-staining. Particular attention to this is necessary when translucent finishes are used, since any staining due to moisture or corrosion products is readily visible against a relatively uniform background.

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