How To Install A Tin Roof Over Existing Shingles: A Professional Guide
Installing metal roofing over existing asphalt shingles is a viable, cost-effective method that eliminates the need for expensive tear-offs while providing a superior weather-resistant barrier. This process relies on the strategic installation of a sub-purlin system or a synthetic underlayment to create a thermal break, ensuring structural integrity and preventing moisture accumulation between the metal and the old shingle substrate.
Pre-Installation Planning and Structural Assessment
Before beginning the installation, you must confirm that the existing roof structure can support the additional weight of the metal panels, which typically add approximately 1 to 2 pounds per square foot. Verify that the roof deck—whether plywood, OSB, or spaced planking—is free of rot, sagging, or delamination. Building codes in many jurisdictions permit one layer of reroofing over an existing layer, provided the structural load-bearing capacity is sufficient. If you discover more than one layer of shingles, you are legally and structurally required to perform a full tear-off to the decking.
- Essential Equipment: Variable speed drill with magnetic hex sockets, circular saw with carbide-tipped metal cutting blades, tin snips, aviation snips, chalk line, tape measure, safety harness, and fall protection gear.
- Core Materials: Metal roofing panels (26-gauge or 29-gauge galvalume or steel), 1x4 or 2x4 pressure-treated wood furring strips, synthetic roofing underlayment (high-temp rated), color-matched metal screws with EPDM washers, and ridge cap flashing.
- Standards & Compliance: Confirm your local building department’s stance on "recover" roofing, as some municipal codes mandate specific underlayment requirements (such as ice and water shield in northern climates) when covering existing shingles.
- Time Benchmark: A standard residential roof of 1,500 square feet typically requires a 3-person crew to complete the installation in 2 to 3 days, assuming favorable weather conditions.
Procedural Installation Workflow
Step 1: Surface Preparation and Inspection
Begin by clearing all debris from the roof surface. Inspect the entire perimeter, paying close attention to the eaves, rake edges, and valleys. Flatten any curled or missing shingles to ensure a relatively uniform plane. If the existing shingles are significantly uneven, the metal panels may buckle or appear wavy. You must also remove or trim back any protruding nails or fasteners that could puncture the new underlayment.
Step 2: Underlayment Application
Roll out a high-temperature synthetic underlayment directly over the old shingles. This acts as a secondary moisture barrier and prevents the bitumen in the shingles from reacting with the underside of the metal panels. Use a high-temp rated synthetic to ensure the material does not melt or become brittle under the intense heat trapped between the metal and the shingles. Ensure a 6-inch overlap on all seams and secure the material with cap nails if necessary.
Step 3: Installing the Purlin Grid
To create the necessary air gap and provide a solid substrate for the metal screws, install a grid of 1x4 pressure-treated furring strips (lath). Space these strips horizontally across the roof at intervals dictated by your specific panel manufacturer, usually 24 inches on center. Secure the furring strips through the shingles and into the roof rafters using 2.5-inch or 3-inch exterior-grade wood screws.
Warning: Do not drive screws solely into the roof sheathing; you must hit the rafters to ensure the metal roof can withstand wind uplift pressures. Use a chalk line to keep the rows perfectly straight, as crooked lath will result in misaligned panels.
Step 4: Installing Eave and Rake Trim
Before installing the panels, install the eave drip edge and rake flashing. These pieces are crucial for directing water away from the fascia and into the gutters. Position the eave trim so that it overlaps the gutter system by at least 1 inch. Secure these trims to the furring strips, ensuring a clean, straight edge for the panels to align against.
Step 5: Panel Placement and Fastening
Starting from one gable end, lay the first panel, ensuring it is square with the rake trim. Use color-matched screws with pre-installed EPDM rubber washers. Fasten the panels through the "flat" or "rib" of the metal, depending on the manufacturer's specification. Tighten the screws until the washer compresses slightly against the metal, creating a watertight seal. Do not over-tighten, as this can crack the washer and lead to leaks.
Pro-Tip: Always install screws in the "flat" area between the ribs to ensure maximum structural support. If you are using a through-fastened system, ensure you are utilizing the specific fastener pattern recommended for your wind zone classification.
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Technical Specifications and Material Comparisons
| Parameter | 1x4 Furring Strip Method | Direct Attachment Method |
|---|---|---|
| Thermal Break | High (Ventilated air gap) | Negligible |
| Wind Resistance | Superior (Anchored to rafters) | Moderate (Anchored to deck) |
| Complexity | High (Two-stage installation) | Low (Single-stage) |
| Condensation Risk | Minimal | High |
| Structural Load | Adds ~1.5 lbs/sq ft | Adds ~0.5 lbs/sq ft |
Common Site Failures and Field Fixes
- Failure Scenario: Oil Canning (Wavy Metal): This is often caused by over-tightening fasteners or installing panels over an uneven surface. Root Cause: Excessive tension on the fastener or a lack of flatness in the underlying lath. Fix: Loosen the affected screws and ensure the furring strips are shimmed to create a perfectly flat plane before re-securing.
- Failure Scenario: Moisture Accumulation: Condensation forms on the underside of the metal when there is no proper ventilation. Root Cause: Failure to install a thermal barrier or lack of airflow. Fix: If the roof is already closed, install vented ridge caps to encourage convective airflow from eave to ridge.
- Failure Scenario: Fastener Back-out: The screws begin to loosen over time due to thermal expansion and contraction. Root Cause: Fasteners were driven at an angle or under-torqued. Fix: Replace loose screws with slightly oversized "stitch" screws or move the fastening point by 1 inch and seal the old hole with high-grade metal sealant.
Frequently Asked Questions
Do I need to remove the existing shingles before installing a metal roof?
In many cases, no. If you have only one layer of shingles and the roof deck is structurally sound, you can install metal panels directly over the top. However, always check local building codes to ensure your specific region permits "recover" roofing.
How do I prevent rust on my new tin roof?
Most modern metal roofing is made from galvanized steel or Galvalume, which features a zinc-aluminum alloy coating designed to resist corrosion. To maintain this, ensure all cut edges are sealed with touch-up paint and avoid using non-compatible fasteners that could cause galvanic corrosion.
Will a metal roof make my home louder during a rainstorm?
Contrary to popular belief, a properly installed metal roof with an underlayment and a furring strip system is not significantly louder than shingles. The air gap between the metal and the deck, combined with the underlying shingles, acts as a sound buffer that dampens vibrations.
How much weight does a metal roof add to my home?
Metal roofing is lightweight, generally weighing between 0.5 and 1.5 pounds per square foot. This is significantly less than asphalt shingles and most other roofing materials, making it a safe choice for most residential structures.
Secure Your Investment with Professional Installation
Installing a metal roof over existing shingles requires precise alignment and strict adherence to fastener patterns to ensure a decades-long lifespan. Consult with your local metal roofing distributor to select the proper panel profile and color for your specific climate requirements.
