We are very supportive of alternative ways to use the vast timber and fiber resources of British Columbia. “Engineered” wood products employ a lot of folks here and have provided for some amazing structural materials to replace traditional wood posts and beams. For exterior wall sheathing there are two “engineered” materials that are made and commonly used in British Columbia. These sheet products, plywood and osb (oriented strand board) replaced a long time ago the traditional “shiplap” dimensional lumber which has serious structural limitations. Both plywood and osb are widely used in the Province as an exterior sheathing material. From our experience though, in the Lower Mainland of B.C., only plywood should be used.
Plywood is created by laminating veneers of softwood (generally spruce or fir) with layers of waterproof adhesive in a laminar form. Spruce is a reasonably durable softwood while fir is extremely durable. The veneers are generally fully coated between each ply with waterproof glue. If moisture reaches plywood it generally does not cause decay in a hurry and, if it does, it is often in the outermost surface but not structurally compromising the sheet.
OSB is created by gluing together an amalgamation of “strands” of various species of softwood. Not generally fir but spruce, pine, and popular. Pine and popular having very low durability if wet. Further, the wood “strands” or chips being oriented in various directions tend to allow decay, if it is occurring, to spread much further into the sheet as there is no solid laminar layer of waterproof glue to stop it. OSB is coated with waxes to try to make the exterior more water repellent but it seems to do little once the material is cut or penetrated with fasteners and if the material gets wet.
We have taken moisture damaged walls apart where both OSB and plywood have been used in the same area for sheathing and found intact but water stained plywood right next to completely decayed OSB. We believe that, when it comes to exterior sheathing, plywood is the only way to go in the Lower Mainland.