Plywood
All plywood for exterior use is glued with the same phenolic glue. It carries the mark WBP=water boil proof.
The durability of plywood is entirely dependent on the species of wood used in the veneers. Often the manufacturer saves money by having a good but thin surface veneer, but species with low rot resistance in the core veneers.
The difference between "WBP Exterior" plywood and "Marine plywood" is in the species of timber accepted for the veneers and thickness and number of layers. "Marine plywood" should have a minimum of 5 layers in plywood between 6 and 9 mm thickness. More layers give more equal strength along the sheet and across the sheet. The thickness of the layers should be :
Outer layers : Minimum 1.4 mm Inner layers : Maximum 2.5 mm
Unless the veneers have been treated against rot at the factory, the durability of the plywood is dependent on the type of wood used. The following list of species for marine use has been prepared by Lloyds in England:
Common nome
Agba
Gedu Nohor
Guarea ldigbo
African Mahogany
Makore
Light Red Meranddi Light Red Seraya Sapele Utile
Natural Durability
Durable
Moderately durable
Durable
Durable
Moderately durable Very durable Moderately durable Moderately durable Moderately durable Moderately durable Durable
Douglas fir and Gaboon/Okoume are acceptable provided they are given preservative treatment at the factory. Gaboon/Okoume has very low natural durability.
In plywood construction it is important to seal all edges with epoxy glue. Especially where the deck overlaps the side there are problems of rot if the edge is not well sealed before fastening the sheer batten.
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