The key difference between pressure-treated exterior grade plywood and standard exterior grade plywood lies in rot/mold/insect resistance, determined by the treatment process. Both use waterproof "Exterior" glue (phenol-formaldehyde or similar), but only pressure-treated plywood has added chemical preservatives.
| Feature | Pressure-Treated Exterior Plywood | Standard Exterior Plywood |
|---|---|---|
| Core Distinction | Impregnated with chemical preservatives under pressure | Relies solely on waterproof glue bonds |
| Rot/Insect Resistance | High (10-40+ years, depending on preservative retention) | Low (Untreated wood veneers will rot if chronically wet) |
| Moisture Tolerance | Withstands prolonged exposure and ground contact | Requires protection from persistent moisture |
| Visible Identification | Green or brown tint from treatment chemicals | Natural wood appearance (no color tint) |
| Metal Fastener Note | Requires corrosion-resistant (stainless, hot-dip galvanized) | Standard fasteners typically acceptable |
| Cost | 30-50% higher | More economical |
Key Applications:
Pressure-Treated Exterior Plywood:
Ground-Contact Structures: Deck substructures (joists, bearers), retaining walls, fence posts, landscape timbers.
High-Moisture Exposure: Boat docks, ramps, marine bulkheads, foundation perimeter forms (left in place).
Areas Prone to Wetting: Bathroom/kitchen subfloors (over concrete), crawl space linings, exterior stair stringers/treads.
Applications Requiring Long-Term Rot Resistance: Permanent exterior work tables, shed floors, greenhouse bases.
Standard Exterior Plywood:
Protected Exterior Sheathing: Roof sheathing, wall sheathing (behind siding/cladding), soffits/fascia (if properly primed/painted and ventilated).
Temporary Construction: Concrete forms (where reuse is intended), site hoarding.
Interior Wet Areas: Tub/shower surrounds (when properly waterproofed/tiled), utility room subfloor (not over concrete slab).
Exterior Signs/Projects: Painted signs, project boxes (kept dry/off-ground).
Crucial Considerations:
"Exterior" ≠ Rot-Proof: Standard exterior plywood's "exterior" rating only refers to the glue bond. The wood veneers themselves have no inherent rot resistance. If it stays wet, it will rot.
Drying is Essential: Even pressure-treated plywood performs best if allowed to dry thoroughly before sealing/painting and in service.
Fasteners Matter: Always use fasteners rated for pressure-treated lumber (stainless steel or hot-dipped galvanized) with PT plywood to avoid rapid corrosion.
End Sealing: Seal cut edges of both types for maximum longevity, but it's critical for standard exterior plywood.
Grade: Both types come in various face veneer grades (A-D). Pressure-treated often starts with a lower structural grade (like CCA or CDX).
In short: Choose pressure-treated when the plywood will have ground contact, chronic high moisture, or needs decades of rot resistance. Choose standard exterior plywood for applications where it's protected from persistent moisture (like roof/wall sheathing) or temporary uses, where its lower cost is advantageous. Never use standard exterior plywood where it might stay damp.
