Plywood

Plywood in roofing refers to the structural wood panel installed over rafters or trusses that creates the roof deck —the surface your metal roofing system is fastened to. It’s critical for load distribution, fastening strength, and long-term durability.

Types of Plywood

Plywood (traditional layered sheets)

Made from thin wood veneers glued in alternating grain directions

Strong, stable, better resistance to moisture than OSB

Holds screws very well (important for metal roofing fasteners)

Typically more expensive

OSB (Oriented Strand Board)

Compressed wood strands bonded with resin

Most common in modern construction (including Alberta)

Uniform, cost-effective, structurally reliable

Slightly more sensitive to moisture exposure (edges can swell if not protected)

Chipboard / Particle Board (rarely used for roofing)

• Made from small wood particles and adhesive
• Low structural strength and poor moisture resistance
• Not recommended for roof decking, especially under metal roofing

👉 Quick takeaway:
For metal roofing, plywood or OSB only. Chipboard is a no-go.

Read more: 4 Types of Roof Sheathing, Functions and Benefits

 Standard Sizes

Most roof decking panels come in:

• 4 ft × 8 ft (1220 mm × 2440 mm) — industry standard
• Occasionally longer sheets (e.g., 9 ft or 10 ft) for specific framing layouts

Panels are installed with staggered joints to improve strength and reduce weak points.

Plywood Thickness

Common thicknesses:

• 3/8″ (9.5 mm) – too thin for most roofing applications
• 7/16″ (11 mm) – common minimum (especially OSB in tract homes)
• 1/2″ (12.5 mm) – better rigidity, widely used
• 5/8″ (15.5 mm) – recommended for premium metal roofing systems
• 3/4″ (18.5 mm) – used for high-load or specialty builds

👉 In Canada (snow load + temperature swings):
5/8″ is often the sweet spot for strength, screw holding, and long-term performance.

Application in Metal Roofing

The roof deck (plywood/OSB) plays a bigger role than many homeowners realize:

Structural base

• Distributes snow load, wind uplift forces, and foot traffic

Fastener holding

• Metal roofing screws must anchor securely into solid decking
• Thicker panels = better screw retention = less risk of loosening over time

Surface for underlayment

• Ice & water shield and synthetic underlayments are installed directly on top

Flat, even plane

• Critical for standing seam and metal panels to sit straight
• Imperfections in decking = visible waves in the metal roof

Repair stage (before metal install)

• Any rotten, soft, or delaminated plywood must be replaced
• Uneven decking is corrected before installation

Pro Notes (What Contractors Look For)

• No sagging between rafters
• No moisture damage or mould
• Proper fastening to framing (nails/screws at correct spacing)
• Expansion gaps between sheets (important for OSB)
• Clean, flat surface before underlayment

Plywood (or OSB) isn’t just “wood under the roof” — it’s the structural foundation of your entire metal roofing system.

If the decking is weak, thin, or damaged, even the best metal roof won’t perform properly.