Plywood is an assembly of thin sheets of wood layers alternating in grain direction, glued and compressed together.
There are 2 types of plywood used in construction: Interior (moisture resistant) and exterior (waterproofed). Exterior plywood surfaces are grouped into ‘grades’ ranging from A through D, with A being the highest quality and D being the lowest:
A Grade - Very smooth surface with patches permitted
B Grade - Smooth surface with patches and plugs permitted
C Grade - Knot holes permitted up to 2-1/2" wide
Exposure 1 plywood is fully waterproofed but is not rated for permanent exposure to moisture. Exposure 2 is an interior plywood type with intermediate glue which is only water resistant. Structural grade plywood is specially made for engineered applications like box beams.
Types and prices of plywood available will fluctuate based on geographic and economic conditions. Below is a more detailed explanation of surface grades and associated allowable quality:
A – Both Veneers virtually free from defects
A/B – Reverse veneer only a few small knots or discolorations
A/BB – Reverse side allowing jointed veneers, knots, plugs.
B – Both veneers only a few small knots
B/BB – Reverse side allowing jointed veneers, large knots, plugs
BB – Both sides allowings jointed veneers, large knots, plugs
X – Knots, Knotholes, cracks, etc.
Below are certain uses based on different grades:
AA-AD – Interior – Cupboards, shelving, paneling, furniture.
BB Plyform – Concrete form plywood
CDX – Wall and roof sheathing, primarily exterior
AA-AC – Exterior – Fences, signs, siding, soffits, etc.
The wood used for plywood is either softwood or hardwood. Softwood is most common for construction and is made up commonly of Douglas-Fir or Spruce-Pine-Fur (SPF). 4×8 sheets are the most commonly manufactured size with 4×10 sizes also readily available. Roofing plywood is commonly at least 5/8″ thick and subfloor sheathing is commonly 3/4″ minimum thick. Subfloor sheathing is commonly tongue and groove which keeps the boards from shifting up and down at the joints.