How is a metal roof installed over shingles?

A metal roof goes over shingles in three stages: an inspection to confirm the old roof can stay, a base that separates or lifts the metal off the shingles, and then the panels, flashing and trim. The base is where the methods differ. Here is what a roofer does at each stage, so you can follow the quote and ask the right questions.

First, can the shingles stay?

Installing over the old roof, without removing it, is called a recover. McElroy Metal, a panel maker, says in many cases a metal roof can go directly over one layer of existing shingles, and that local code and the condition of the existing roof determine eligibility.^[2]

The roofer looks for the conditions that usually force a tear-off: rotted or warped wood or large gaps between deck boards, more than one layer of existing shingles, or a structure that sags across the ridge or truss lines. A sagging roof should be inspected by a structural engineer before anyone adds a new covering. Local code may also limit how many layers a roof can carry, so the building department's answer matters. Expect the roofer to go into the attic as well, to look at the underside of the deck for stains and soft spots.

Method 1: furring strips over the shingles

In this approach, wood purlins or sub-girts are fastened over the old roof as furring strips and attached to the structure below, and the metal panels are fastened to that framing.^[1] It gives the panels a flat, even base and leaves room for air to move under the metal.

There are trade-offs. McElroy notes that the furring adds weight, so someone has to determine that the structure can carry it,^[1] and that with the cost of the furring material and labor, the final price can be similar to a tear-off.^[1] If the furring is pressure-treated, there is a corrosion issue: copper in pressure-treated lumber can aggressively attack metal panels,^[1] so McElroy calls for a barrier of #30 felt or synthetic underlayment between treated furring and the panels.^[1]

Method 2: panels straight through the shingles

The least expensive method is direct attachment: panels are fastened through the shingles into the wood deck below.^[1] It saves the furring, but it has two drawbacks. First, steel can telegraph unevenness in the deck and shingles through the panels,^[1] which is a cosmetic issue, not a leak. Second, direct attachment makes above-sheathing ventilation hard to achieve,^[1] which costs some of the energy benefit described below.

A separation layer is not optional here. A barrier layer between the shingles and the metal is required, to keep the shingles from wearing through the panels as the metal expands and contracts.^[1] Common barriers are #30 felt, synthetic underlayment or thin foil-faced insulation.^[1]

Method 3: a manufacturer's recover system

Some panel makers sell systems built for this job, with clips that hold the panels above the shingles without a full furring grid. The point of the raised design is airflow. Above-sheathing ventilation creates a vented air space between the metal panel and the substrate, so heat that would pass into the house vents back outdoors.^[3] McElroy also notes that leaving the existing shingles in place adds a layer of insulation between the metal and the living space.^[3] If a contractor proposes one of these systems, ask for the maker's installation guide and warranty terms in writing.

Underlayment, slope, flashing and trim

Whatever the base, the rest of the job follows the panel maker's manual. Panels have minimum slopes. Metal Sales, for example, says its Vertical Seam panel has a minimum recommended slope of 3:12 over open framing and 1:12 over solid substrate,^[4] so a low-pitch porch roof may need a different panel than the main roof.

Prince William County's building division points out that the Virginia code calls for an ice barrier along the eaves for metal roof shingles, among other coverings, in areas with a history of ice forming there and backing up water.^[5] Ask your roofer whether that applies to your panel type and your county.

The crew then replaces the flashing at walls, chimneys, valleys and vent pipes, adds closures and trim at the eaves, rakes and ridge, and fastens the panels to the pattern in the manual. Old shingle flashing is rarely reused; metal panels need flashing designed for them. Panel makers are blunt about foot traffic during this work: "Do not use the roof panel as a walking platform."^[4] Crews walk on supports and use fall protection.

Permits and the contract in Virginia

Under Virginia's building code, replacing an unlimited amount of roof covering on a one- or two-family house or townhouse needs no permit, unless the house is in an area where the nominal design wind speed is greater than 100 miles per hour.^[6] Separately, the building official may require a permit application for replacement roofing in a locally designated historic district.^[6] Because a recover adds a layer rather than replacing one, ask your local building department how it treats the job; see do you need a permit to replace a roof for more.

In Virginia, the contract must be signed by you and the contractor before work starts or any payment is accepted.^[8] It has to list the materials and work, and include a statement that the contractor will comply with local requirements for building permits, inspections and zoning.^[8] Have it name the method (furring, direct or recover system), the barrier layer, the panel and coating, and who handles any deck repairs found along the way.

When to call a licensed pro

From the first inspection to the last trim piece, this is a job for a licensed roofer. Falls from roofs accounted for nearly 1,200, or 34%, of construction fall deaths between 2003 and 2013,^[10] and new metal panels are slick underfoot. In Virginia, roofing contracting is the licensed specialty for installing the materials that form a watertight, weather-resistant roof surface.^[7] Check a contractor using DPOR's License Lookup or by calling (804) 367-8511,^[9] or follow how to check a contractor's licence.

Before you decide, read what a metal roof costs in Virginia, whether a metal roof is worth it and what happens during a full roof replacement, or browse more exterior answers. Loclis lists licensed businesses from Virginia's DPOR register: see metal roofing contractors or exterior contractors in Blacksburg.

When you are ready, describe your metal roof project once and licensed local roofers send quotes or offer an estimate visit.