How To Install Tin Roof Over Shingles: A Complete Technical Guide

Installing a tin roof directly over existing asphalt shingles is a viable, cost-effective roofing method that eliminates costly tear-off labor and landfill disposal fees, provided the underlying roof deck remains structurally sound, supports the combined dead load, and utilizes proper furring strips to prevent condensation and galvanic corrosion.

Pre-Operation & Equipment Checklist

Before beginning any over-roofing project, you must verify that your structure can handle the added weight. Asphalt shingles typically weigh two to three pounds per square foot, while exposed fastener metal panels add one to one-and-a-half pounds per square foot. Most residential roof rafters easily support this aggregate load of roughly four pounds per square foot, but multi-layered shingle tear-offs may still be legally or structurally required if you exceed two existing layers.

• Essential Gear, Tools, and Materials:

  
  
  
  • Corrugated or standing seam metal panels (galvalume or painted steel)
  • 1x4 or 2x4 wood purlins (furring strips) treated for exterior use
  • #10 x 2-inch or 3-inch wood screws with EPDM neoprene washers for fastening purlins
  • Metal roofing screws with EPDM washers (color-matched to panels)
  • Synthetic high-temperature underlayment or ice and water shield (if local code requires over purlins)
  • Circular saw with a metal-cutting carbide blade or electric metal shears
  • Cordless impact driver, chalk line, utility knife, caulking gun, and heavy-duty snips
  • OSHA-compliant personal fall arrest system (harness, lanyard, anchor points), safety glasses, and cut-resistant gloves

• Mandatory Prerequisite Knowledge and Standards:

  
  
  
  • Verify local building codes regarding maximum allowed roof layers. Many jurisdictions prohibit installing metal over more than one layer of shingles.
  • Inspect the existing roof deck for soft spots, sagging rafter lines, water stains in the attic, or missing shingle tabs. Replace compromised decking before proceeding.
  • Calculate thermal expansion tolerances; metal panels expand and contract continuously with ambient temperature shifts, requiring proper fastener torque to prevent washer distortion.

• Estimated Budget and Duration Benchmarks:

  
  
  
  • Average material cost ranges from $3.50 to $7.00 per square foot depending on panel style and gauge.
  • A standard 2,000-square-foot roof installation typically requires 2 to 3 days for a skilled two-person crew.

Step-by-Step Metal Over Shingle Installation Workflow

Step 1: Roof Inspection and Surface Preparation

Begin by thoroughly walking the roof to identify any high spots, protruding shingle nails, curled edges, or loose granules. Sweep the entire surface clean of leaves, twigs, and loose debris. Hammer down any raised shingle nails or staples flush with the roof deck. If your existing shingles have severe cupping or heavy curling, use a flat shovel or shingle scraper to knock down high ridges, ensuring the surface is as flat and uniform as possible so that the overlying furring strips rest evenly without bridging gaps.

Warning: Never install metal roofing directly against old asphalt shingles without a separation layer or purlins. Trapped heat and moisture between the flat shingle surface and the metal will accelerate the deterioration of both materials.

Step 2: Installation of Furring Strips (Purlins)

Layout 1x4 or 2x4 wood purlins horizontally across the roof slope, starting flush with the eave line and spacing them vertically according to your panel manufacturer's specification (typically 12 to 24 inches on center). Secure the purlins directly through the shingles and roof decking into the underlying rafters using #10 wood screws long enough to achieve at least 1-inch of solid penetration into the structural wood.

Pro-Tip: Snap chalk lines across the shingles for every purlin row to maintain absolute straightness and parallel alignment, preventing skewed metal panels later.

Step 3: Fastening Underlayment and Eave Trim

Roll out high-temperature synthetic underlayment or self-adhered ice and water shield directly over the newly installed wood purlins if specified by your local building code or metal manufacturer. Next, install the metal drip edge and gable/rake trim along the perimeter edges using compatible roofing fasteners. Ensure the eave trim overlaps the gutter line properly to direct all runoff away from the fascia board.

Step 4: Placing and Securing Metal Panels

Square the first metal panel in the lower corner of the roof, aligning the edge precisely with the gable trim while allowing a 1-to-2-inch overhang at the eave. Drive metal roofing screws equipped with EPDM neoprene washers through the flat valleys of the panels (for exposed fastener systems) directly into the wood purlins below. Do not over-tighten the screws, as crushing the rubber washer will compromise the watertight seal; the washer should expand just slightly past the metal screw head edge.

Step 5: Installing Ridge Caps and Flashing Details

Continue laying adjacent panels across the roof, overlapping side ribs or interlocking standing seams securely. Once the field panels reach the peak, install foam closure strips along the ridge to seal out insects and wind-driven rain. Fasten the ridge cap flashing over the peak, overlapping sections by at least 6 inches and sealing joints with approved exterior polyurethane sealant. Finally, install any necessary pipe boots, chimney flashings, and valley trims, sealing all exposed fastener heads around penetration flashings.

Technical Specifications and Material Comparison

Common Site Failures and Field Fixes

• Root Cause: Water leaking around roof penetrations due to rigid sealant failure.
  
  
  • Actionable Fix: Remove old degraded silicone or butyl tape, clean the metal surface with denatured alcohol, and apply a fresh bead of high-grade polyurethane or polyether roof sealant designed specifically for metal-to-metal bonding.
• Root Cause: Fastener backing out and leaking due to thermal expansion cycles.
  
  
  • Actionable Fix: Locate backed-out screws using a visual check, remove them, and replace them with a slightly wider oversized repair screw (#14 stitch screw with a fresh EPDM washer) to re-engage stripped wood purlins.
• Root Cause: Oil canning or buckling visible on flat panel surfaces.
  
  
  • Actionable Fix: Check if underlying purlins are uneven or if panel fasteners were driven in at an angle causing lateral stress. Loosen adjacent fasteners slightly to relieve tension and ensure thermal expansion paths are clear.

Frequently Asked Questions

Do I need to remove old shingles before installing a tin roof?

In most cases, no. Installing metal roofing directly over a single layer of asphalt shingles is fully acceptable, provided the roof deck is structurally sound and local building codes permit a maximum of two total roof layers.

Will installing metal over shingles trap moisture and cause rot?

When installed correctly using wood purlins (furring strips), an essential air gap is created between the old shingles and the new metal panels. This thermal break and ventilation space prevents condensation accumulation and premature rot.

Can I install standing seam metal over shingles?

Yes, though standing seam systems typically require a solid decking substrate or specialized clips mounted to wood purlins. Always consult the specific panel manufacturer's installation manual to ensure compatibility with over-shingle application methods.

How much weight does a metal roof add to existing shingles?

Exposed fastener or standing seam metal panels typically weigh between 1.0 and 1.5 pounds per square foot. This lightweight addition is well within the load-bearing capacity of standard residential roof framing.

Do metal roofs attract lightning?

No. Metal roofing does not attract lightning strikes any more than other building materials, and because metal is non-combustible, it actually offers superior safety and fire resistance during a lightning strike compared to wood or asphalt.

Protect your investment and upgrade your property's durability by scheduling a professional structural assessment for your metal-over-shingle roofing project today.