Today builders can choose from several types of outdoor plywood. The correct choice mainly depends on the use, but ultimately, it all depends on the budget. The grade that may be used for most outdoor projects is CDX plywood, which is available in almost any store.
Since outdoor plywood does not imply using it for furniture finish and inside the house in general, the appearance of CDX plywood will always be inferior to that of the more expensive grades for interior finish. This plywood is not as beautiful as the finish grade plywood. Such plywood often has visible spots, knots, and darkening on it. It is usually made of coniferous wood species, which are somewhat cheaper than birch.
3/4 Baltic Birch Plywood (13-ply, 48 x 96 sheet size)
3/8 Baltic Birch Plywood - 10mm (7-ply, 48 x 96 sheet size)
1/2 Baltic Birch Plywood - 12mm (9-ply, 48 x 96 sheet size)
7/8 Baltic Birch Plywood (15-ply, 48 x 96 sheet size)
Types of outdoor plywood
CDX is the most common plywood type for exterior finish. It comes of any thickness; the most popular is 3/4, 5/8, and 1/2 inch. The appearance of plywood grades varies from cabinet (A) to construction (C and D) grades. Outdoor plywood is marked X, which makes it suitable for outdoor use. In addition to CDX, more expensive grades, such as ABX, ACX, etc. may be used. However, if the budget is limited, and the appearance is not very important, CDX will serve its purpose well for many years.
It is important to use moisture-proof plywood with the X glue type or similar impregnation, otherwise, the plywood will soon start delaminating and deforming, and will eventually swell and fall apart. In an extreme case, such plywood may be painted, but it is much more convenient to use the CDX plywood.