plywood sheathing

plywood sheathing

Plywood is an assembly of thin sheets of wood layers alternating in grain direction, glued and compressed together. 

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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

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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.

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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.

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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.