How to Install Metal Roofing Over Plywood

Metal roofing offers exceptional durability, energy efficiency, and low maintenance when properly installed over plywood sheathing. This comprehensive guide outlines every phase from substrate preparation to final flashing, ensuring a robust installation that withstands harsh weather.

Why Choose Metal Roofing Over Plywood?

Metal roofs excel in longevity, often lasting 50 years or more, while plywood provides a solid, continuous deck for secure fastening. This combination transfers loads effectively from snow, wind, and rain to the structure below. Unlike open framing, plywood sheathing creates a stable base that prevents sagging and supports heavy panels.

Key benefits include resistance to fire, pests, and rot when paired with proper barriers. Homeowners save on energy bills due to metal’s reflective properties, reducing cooling costs by up to 20% in summer.

Essential Tools and Materials Checklist

Gather these items before starting to streamline the process:

• Measuring tape and chalk line
• Utility knife or roofing knife
• Hammer and 1/2-inch roofing nails
• Drill with 1-1/2-inch galvanized metal screws
• Metal snips or nibbler tool
• Safety gear: gloves, goggles, harness
• Plywood (minimum 15/32-inch thick, APA-rated)
• Underlayment (asphalt-saturated felt, synthetic, or polymer)
• Slip sheet (smooth building paper or #15 felt)
• Metal panels, ridge cap, gable trim, pipe boots
• Butyl tape and sealant

Step 1: Preparing the Roof Deck

Begin by safely removing any existing roofing materials to expose the plywood sheathing. Work in sections if the roof is steep, always using fall protection.

Inspecting and Repairing Plywood

Examine every sheet for damage. Look for soft spots, delamination, or water stains indicating rot. Probe with a screwdriver; replace any questionable areas immediately. Plywood must meet American Plywood Association (APA) standards, featuring an APA stamp with span rating and minimum 15/32-inch thickness for structural integrity.

Match replacement plywood to existing span ratings, typically 24/16 or 32/16 for roofs. Secure new sheets with 8d nails at 6-inch edges and 12-inch field spacing. Clean the entire deck of debris, nails, and protrusions to ensure a smooth surface.

Step 2: Installing Protective Underlayment

Underlayment acts as a secondary waterproof barrier, shielding plywood from moisture infiltration. Choose high-quality synthetic or polymer options for better breathability and tear resistance over traditional asphalt felt.

Application Techniques

Start at the eaves, rolling out underlayment horizontally with the lower edge flush to the drip edge. Overlap subsequent courses by 4-6 inches, and vertically by 6 inches at ends. Secure with 1/2-inch roofing nails spaced 12-16 inches apart, doubling nails in valleys and eaves.

For steep roofs, use fully adhered systems with ice-and-water shield in vulnerable areas like eaves and valleys. This prevents ice dams from forcing water under panels.

Step 3: Adding the Slip Sheet Layer

A slip sheet prevents damage from metal panels’ thermal expansion and contraction, which can tear underlayment over time. The National Roofing Contractors Association (NRCA) recommends #15 asphalt-saturated felt or smooth building paper for this purpose.

Roll out the slip sheet over the underlayment, overlapping seams by 6 inches. Nail sparingly to avoid puncturing the primary barrier. This low-friction layer allows panels to slide slightly without compromising waterproofing.

Step 4: Laying and Fastening Metal Panels

Position the first panel at the eave’s left end, overhanging by 1-2 inches for proper drainage. Align ribs precisely and secure with self-tapping, neoprene-washered screws at 12-16 inch intervals along ribs, avoiding flat areas to prevent leaks.

Overlaps and Sequencing

• Horizontal overlap: 1 inch at panel ends
• Vertical overlap: 6-12 inches between courses
• Fasten only through high ribs

Work upward and rightward, staggering seams. Cut final panels with metal snips, deburring edges. For ridge areas, leave a 1-2 inch gap for closure strips.

Step 5: Installing Trim and Flashings

Trim completes the system, directing water and enhancing appearance. Apply butyl tape under gable and eave trims before screwing every 12 inches.

• Eave Trim: Secure over panel overhangs.
• Gable Trim: Run full length, overlapping panels.
• Ridge Cap: Use foam closures, mastic sealant, and screws.
• Pipe Flashing: Install rubber boots with sealant around vents.

Common Mistakes to Avoid

Steer clear of these pitfalls for a professional result:

• Inadequate plywood thickness leading to deflection.
• Skipping slip sheets, causing underlayment tears.
• Over-tightening screws, stripping holes.
• Poor overlaps allowing water entry.
• Ignoring local building codes for wind uplift ratings.

Maintenance for Longevity

Inspect annually for loose fasteners, debris, or dents. Clean gutters and check flashings. Metal roofs require minimal upkeep, but prompt repairs prevent minor issues from escalating.

Frequently Asked Questions (FAQs)

Can I install metal roofing over old shingles on plywood?

It’s possible if the deck is sound, but removing shingles allows better inspection and ventilation. Consult local codes.

What screw length for metal over plywood?

Use 1-1/2 to 2-inch screws to penetrate plywood at least 1 inch without hitting rafters.

Is OSB acceptable instead of plywood?

OSB works if APA-rated and 7/16-inch minimum, but plywood offers superior moisture resistance.

How many panels overlap vertically?

Typically 6-12 inches; follow manufacturer specs for your panel profile.

Do I need a drip edge with metal roofing?

Yes, install under underlayment at eaves for optimal water shedding.