How To Put A Tin Roof Over Shingles: A Comprehensive Installation Guide

Installing a metal roof over existing asphalt shingles involves verifying that the roof deck can support the added weight, applying a separator layer like synthetic underlayment or 1x4 furring strips, and securing the metal panels using specific fastener patterns. This "roof-over" technique is permissible under most building codes if there is only one existing layer of shingles, providing enhanced thermal resistance and significantly reducing labor costs associated with a full tear-off.

Structural Assessment and Material Procurement Requirements

Before beginning the installation, you must perform a rigorous evaluation of the existing roof structure. The International Residential Code (IRC) generally allows for a maximum of two roofing layers. If your home already has two layers of shingles, a third layer of metal is prohibited because the combined weight could exceed the load-bearing capacity of the rafters or trusses. You must also inspect the decking from the attic side to ensure there is no pervasive rot, water damage, or sagging. Metal roofing is incredibly durable, but it cannot fix a failing foundation.

Essential Equipment and Gear

• Safety Gear: OSHA-approved fall protection harness, non-slip roofing boots, and cut-resistant gloves for handling sharp metal edges.
• Cutting Tools: Electric metal shears or "nibblers" (avoid using an abrasive circular saw blade as it can melt the protective galvalume coating and lead to premature rust).
• Fastening Tools: Impact driver with a depth-sensing nosepiece to avoid over-driving screws.
• Measurement Tools: 100-foot tape measure, chalk line (non-permanent blue or white), and a framing square.

Mandatory Materials Checklist

• Metal Panels: Typically 29-gauge or 26-gauge "Ag" panels (ribbed) or standing seam panels.
• Furring Strips: 1x4 kiln-dried pressure-treated lumber or 20-gauge steel hat channels.
• Fasteners: #10 or #12 hex-head wood screws with integrated EPDM rubber washers, sized at least 1.5 inches for purlins or 2 inches for direct-to-deck applications.
• Closures and Sealants: Solid or vented foam closure strips and high-grade butyl tape.
• Trim Components: Eave trim (drip edge), gable/rake trim, ridge caps, and transition flashings.

Project Benchmarks

• Estimated Budget: $3.50 to $7.00 per square foot depending on the gauge of the metal and the complexity of the roof's geometry.
• Duration: 2 to 4 days for a standard 2,000-square-foot residential roof with a three-person crew.
• Primary Standard: Compliance with ASTM A792 for Galvalume-coated steel or ASTM A653 for galvanized steel.

Step-by-Step Execution for Over-Shingle Installation

Step 1: Preparing the Existing Surface and Layout

Begin by removing any existing ridge caps, plastic vents, and oversized plumbing boots. The goal is to create as flat a surface as possible. If the shingles are curled or badly buckled, they must be flattened or removed in those localized areas.

Sweep the roof thoroughly to remove debris that could be trapped under the new panels. Use a chalk line to mark the location of the rafters. This is critical because when you install furring strips or panels, you want the primary fasteners to bite into the structural members, not just the 7/16-inch OSB or plywood decking.

Pro-Tip: If you choose to install the metal directly over the shingles without furring strips, you must use a high-quality synthetic underlayment or a "slip sheet" (usually 30lb felt or a dedicated synthetic barrier). Asphalt shingles are abrasive; without a barrier, the vibrations and thermal expansion of the metal will cause the shingles to "sand" the protective coating off the underside of your metal panels, leading to "back-side" corrosion.

Step 2: Installing Furring Strips (Purlins)

The most authoritative method for putting a tin roof over shingles is the use of 1x4 furring strips. This creates an air gap between the old and new roof, providing a thermal break and allowing moisture to evaporate.

1. Install the first horizontal row of 1x4s along the eaves.
2. Install subsequent rows every 24 inches on center up to the ridge.
3. Ensure you double the furring strips at the ridge and around any penetrations like chimneys or skylights to provide extra surface area for flashing.
4. Fasten the 1x4s using two 3-inch deck screws at every rafter intersection.

Warning: Never use common nails for furring strips. The expansion and contraction cycles of the metal roof create "thermal shock" that can pull nails out over time, leading to panel rattling or wind uplift failure.

Step 3: Installing Eave Trim and Drip Edges

Install your metal drip edge or eave trim along the bottom edge of the roof. This trim should be fastened directly to the furring strips. If you are using a profile that includes an integrated "starter" hook, ensure it is perfectly level.

Apply a bead of butyl tape or high-grade polyurethane sealant along the top edge of the drip edge where the metal panels will overlap. This prevents capillary action from drawing water back up under the panels during wind-driven rain.

Step 4: Squaring and Laying the First Panel

The first panel is the most critical component of the entire roof. If it is off by even an eighth of an inch at the top, that error will multiply across the length of the roof, resulting in a "sawtooth" pattern at the eaves.

1. Place the first panel on the roof, allowing for a 1-inch to 1.5-inch overhang at the eave.
2. Measure from the edge of the panel to the gable end at both the top and the bottom.
3. Adjust until the panel is perfectly square with the eave, not necessarily the gable (as gables are rarely perfectly square).
4. Secure the panel with a single screw at the top to allow for minor adjustments as you check the squareness one final time.

Step 5: Fastener Patterns and Torque Specifications

Modern "Ag" panels are fastened through the flat part of the metal, immediately adjacent to the ribs. This is a shift from older methods of fastening through the top of the rib, which often led to compressed ribs and leaks.

1. Space fasteners every 24 inches vertically (into the purlins).
2. Horizontally, place a screw on both sides of every major rib at the eave and the ridge.
3. In the middle of the panel, you can transition to a "staggered" pattern, fastening on one side of every other rib.
4. Torque Control: Tighten the screw until the EPDM washer is compressed flush with the metal. If the washer "mushrooms" out or begins to spin, it is over-tightened. If you can turn it with your fingers, it is under-tightened.

Step 6: Installing Closures and Ridge Caps

Before installing the ridge cap, you must install closure strips. These are foam inserts shaped like the profile of your metal panels. They prevent birds, insects, and wind-blown snow from entering the gap between the metal and the ridge.

1. Apply butyl tape to the underside of the closure strips.
2. Press the closure strips onto the panels about 2 inches down from the peak.
3. Set the ridge cap over the peak and fasten it through the closure strips and into the ribs of the metal panels using longer 2.5-inch "stitch" screws.

Comparative Technical Specifications for Metal Roofing Systems

The following table compares the two primary methods of installing metal over shingles: the Direct-to-Deck method and the Furring Strip (Purlin) method.

Common Site Failures and Field Fixes

Failure Scenario 1: Oil Canning (Wavy Appearance)

Root Cause: This is often caused by over-tightening fasteners or installing panels over an uneven shingle surface without furring strips. The metal is constrained and cannot expand when it heats up, causing it to bulge. Actionable Fix: Loosen the fasteners by a quarter turn to allow the metal to "breathe." If the cause is an uneven deck, the only permanent fix is to remove the panels and install furring strips with shims to level the plane.

Failure Scenario 2: Galvanic Corrosion

Root Cause: Using copper flashing or pressure-treated lumber (with high copper content) in direct contact with Galvalume or galvanized steel. The dissimilar metals create an electrolytic reaction that eats through the tin. Actionable Fix: Always use a barrier (like a rubberized membrane or heavy-duty plastic) between pressure-treated wood and the metal panels. Ensure all flashing is compatible (aluminum or steel, never copper).

Failure Scenario 3: Leakage at the Neoprene Washers

Root Cause: Fasteners were driven at an angle or under-driven, preventing the EPDM washer from creating a 360-degree seal. Actionable Fix: Remove the faulty screw. If the hole is stripped, move to a "replacement" screw (a #14 diameter screw which is slightly wider). Ensure the new screw is driven perpendicular to the metal surface.

Frequently Asked Questions

Is it legal to put a metal roof over shingles?

In most jurisdictions, the International Residential Code allows for two layers of roofing. If your home currently has one layer of asphalt shingles, adding a metal roof as a second layer is typically compliant with local building codes. However, you should always check with your local building department regarding specific wind-load or snow-load requirements.

Does a metal roof over shingles make the house hotter?

When installed with furring strips, a metal roof can actually make the house cooler. The air gap created by the 1x4s acts as a thermal buffer, and the reflective properties of the metal (especially "Cool Roof" rated coatings) bounce solar radiation away rather than absorbing it like dark asphalt shingles.

Will installing metal over shingles void the manufacturer's warranty?

Most reputable metal manufacturers (like McElroy Metal or ABC) provide warranties for "recover" installations provided that a proper separator, such as a synthetic underlayment or furring strips, is used. Always review the specific technical manual for your panel brand to ensure compliance with their "Substrate Requirements."

Is a tin roof over shingles louder during rain?

The "drumming" effect of rain on metal is significantly mitigated when installed over shingles. The existing asphalt layer acts as a sound dampener. If you use the purlin method, the noise increase is negligible inside the living space, especially if you have a standard insulated attic.

Upgrade Your Home Infrastructure

Transforming your residence with a high-durability metal roof provides a 50-year solution that eliminates the waste and expense of traditional shingle disposal. By following these precise engineering standards, you ensure a weather-tight, energy-efficient barrier that enhances both the structural integrity and the resale value of your property.