Plywood is used for applications that require highly durable material that can resist cracking, breaking or warping. However, different applications require different strengths of plywood. Further, plywood itself has different strengths. For instance, flexibility may be considered a weakness for a roof but a strength for a curved part. Resistance to heat or water may also play a factor. Detailed standards exist for different types of plywood.
Softwood plywood is used primarily in the industrial sector for such applications as furniture frames, shipping containers and pallets. The American Plywood Association rates softwood according to several durability levels. Exterior grade softwood may be used outdoors. Exposure 1 softwood is suitable for outdoors in moist climates. Exposure 2 is a slightly inferior version of exposure 1. Interior grade softwood may only be used indoors. Softwood is also graded in part by strength. All grades of plywood above D must be free of splits, stitches and knotholes that measure more than 2 1/2 inches across the grain.
Hardwood plywood (e.g. lumbercore) often gets used in construction and furniture. The American National Standards Institute (ANSI) standards for hardwood include several factors that measure strength. For instance, a section of A grade plywood can have no more than two repaired splits. Further, it must not have too many pin knots or small burls. For water applications, all grades of plywood above 4 Back must be free of knotholes although they may be acceptable if they have been repaired.
As it name implies, aircraft plywood is used for planes. The quality requirements for this material are more stringent than hardwood. Aircraft plywood must meet the MIL-P-6070 specification that specifies testing for deviations of thickness, moisture content, tensile strength, gluing strength, bending, torsion and immersion in water.
Tropical- or marine-grade plywood may be used for boats but also appears in construction, especially in Southeast Asia. It must meet the U.S. Products I-74 standard. Marine-grade plywood has fewer knots than industrial as well as a greater overall rigidity. This type of plywood is also particularly resistant to fungi that can weaken waterborne wood over time. Generally, boat projects should use marine-grade plywood with at least a B grade, though knots and holes of inferior ratings can be repaired to save money.