What is WBP plywood?

WBP stands for Water Boil Proof: a designation for plywood whose veneers are bonded with an adhesive that withstands boiling in water. The board must survive a prolonged boil test without the plies separating. WBP plywood is therefore suitable where it may get wet, such as exterior work, formwork and damp rooms. It is often loosely called exterior or marine-type plywood, although marine plywood is a separate, stricter grade.

WBP is about the glue, not the timber

This is the key point: WBP only describes the glue bond. It says nothing about the durability of the timber species. A WBP-bonded board made from non-durable timber can still rot outdoors, even though the plies stay together. Waterproof glue does not mean the board can be left unprotected outside.

Current designations

WBP comes from an older British standard. In Europe, bond quality is now specified under EN 314-2. Bond class 3 is intended for exterior conditions and corresponds to what used to be called WBP. In addition, EN 636 classifies plywood by use; EN 636-3 is plywood for exterior use. Look for these designations when buying, not just the word “exterior”.

Uses

• External cladding, sheds, roof decking and fascias (when finished).
• Concrete formwork, often as film-faced plywood.
• Boat building, for which separate marine grades exist.

Tips for exterior use

1. Choose a durable species or a board made specifically for outdoor use.
2. Seal the edges. End grain and drilled holes absorb the most water.
3. Paint all faces, including the back, so the board does not take up moisture unevenly and warp.
4. Let water drain: leave a small gap at the bottom and ventilate behind cladding.
5. Use stainless or galvanised fixings.

Related terms

• Plywood
• Film-faced plywood
• Three-ply plywood
• Plywood underlayment
• Fascia board

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