Exterior and structural plywood (fir)

Overview

This entry is for North American plywood built to the APA PS 1 or PS 2 standards, in Exterior or Exposure 1 bond classes: the fir and softwood panels that go on roofs, walls and fences, and into cheaper furniture. APA says PS 1 is prescriptive (it specifies the veneers and layup) and PS 2 is performance-based (it specifies how the panel must perform). Canada has its own CSA O121 standard for Douglas-fir plywood, and APA member mills can stamp plywood made to it. Douglas fir is the species most associated with this panel, though other softwoods are used too.

Pros

• Strong, stiff and inexpensive per square foot; APA lists mezzanine floors, trailer liners, furniture manufacturing and more as end uses.
• Exterior-bond panels are made with a fully waterproof bond and are designed for long-term weather exposure (APA).
• Rated and stamped, so you know span rating, bond class and grade. APA span ratings such as 32/16 give the maximum spacing between supports.
• Many surface grades, from A (smooth) to D (utility).
• Widely stocked in 4x8 and 4x10.

Cons

• Fir faces blotch, show heavy grain and have patches and knots in the lower grades.
• Exposure 1 panels (CDX) are not suitable for permanent outdoor exposure (APA); only Exterior panels are.
• Core gaps and patched voids in C and D plies.
• Heavy: a 3/4 in sheet is awkward to carry alone.
• Raw edges absorb water and swell if left uncoated.

Variants and subspecies

• Grades (APA): veneers run N, A, B, C-plugged, C and D. Sanded A-A, A-B and B-B are two-side-appearance or utility; A-C, Exterior only, has one appearance face and is used for soffits and fences; A-D and B-D are Interior and Exposure 1. The strongest species fall in Group 1 (APA classes over 70 species into five groups).
• Minimum plies by thickness (APA): 3 plies up to 3/8 in, 4 plies at 15/32 and 1/2 in (still 3 layers), 5 plies at 19/32 to 3/4 in.
• CDX: see CDX sheathing plywood. Exposure 1 panels include "CDX".
• Marine: see Marine plywood.
• Plyform: concrete-form plywood, Class 1 B-B, Group 1 faces, mill-oiled; see Formply (concrete form plywood).
• Sanded and furniture grades: see Sanded pine plywood and ACX plywood.
• MDO and HDO: overlaid sign and form boards; see MDO and HDO plywood.

Working and finishing

Cut with a sharp carbide blade, face up for a hand-held circular saw. Seal the edges of Exterior and Exposure 1 panels before they see weather, since the plies wick water. Prime and paint, or use a pigmented stain; fir will not take a clear natural look evenly. Screw rather than nail for hold, and pre-drill near edges.

Staining and finishing colour

Fir is a blotchy and strongly grained wood, and plywood fir faces show cathedral and rotary patterns. Heavy-bodied stain or paint hides patches. A clear finish ambers and highlights the contrast between earlywood and latewood.

Sustainability and handling

APA says PS 1 structural plywood is outside the EPA TSCA Title VI composite wood rule (checked 2026-09-30, EPA FAQ), and Columbia Forest Products says PF is the glue in exterior structural softwood plywood. Sheets are heavy. Store flat, off the ground, covered and ventilated.

Popular projects

Garden and storage sheds, soffits and fences (A-C), subfloors, workshop furniture, ramps, stage and set work, and boats (marine grade).

Buying notes

Look for the APA stamp showing grade, span rating and bond class. Toledo lists 3/4 in 4x8 CDX at $37.95 and 3/4 in 4x10 BB/CC exterior-glue plywood at $99.95. Check sheets for flatness and for core voids at the edges before buying.