You can install metal roofing directly over existing shingles in most cases, but the shingles must be in good condition and firmly attached

Metal roofing can go over asphalt shingles without removing them first, which saves labour and disposal costs. The shingles act as a base layer, and the metal panels fasten through them into the roof deck beneath. However, this only works if your existing shingles are not curled, cracked, or lifting — if they are, they must come off first. The metal roof's weight and fastening pattern depend on the shingle condition and your local building code, which varies by region and sometimes by municipality.

Before you start, check your local building department's requirements. Some jurisdictions allow one layer of roofing over shingles; others do not. Some require an inspection before installation. A few places have rules about the total weight your roof structure can carry. A quick phone call to your building department costs nothing and prevents having to tear off and redo work.

Key Takeaways

  • Metal roofing can be installed over shingles only if the shingles are flat, firmly nailed, and free of major damage — curled or lifting shingles must be removed first.
  • Your local building department may restrict how many roof layers are allowed or require an inspection before installation, so check their rules before you buy materials.
  • You will need to install furring strips (thin wood strips) over the shingles to create a level surface and proper fastening points for the metal panels.
  • The metal panels fasten through the shingles and furring strips into the roof deck, and the fastening pattern must match your roof pitch and local wind requirements.
  • Flashing around chimneys, vents, and valleys requires careful installation to prevent water from running under the metal panels into the shingles below.

Inspecting your existing shingles before you start

Walk your roof in daylight and look for shingles that curl upward at the edges, have missing granules (bare spots showing the mat underneath), or are cracked or split. Press on the shingles with your hand — they should not move or feel spongy. Lift a few shingles gently at the edges to check that nails are still holding them down; if nails have backed out or are missing, those shingles need to be re-nailed or replaced before the metal roof goes on.

If you find soft spots, water damage, or rot in the roof deck when you lift shingles, the deck must be repaired or replaced before any new roofing goes on. This is not optional — metal roofing will trap moisture against a damaged deck and make the rot worse. If more than 10 to 15 percent of your shingles are damaged, most roofers recommend removing all the shingles and starting fresh, because the cost of repairs often approaches the cost of removal.

Installing furring strips to level the surface

Furring strips are thin wood boards (usually 1 by 2 inches or 1 by 4 inches) that run horizontally across your roof, perpendicular to the slope. They create a level, solid base for the metal panels and lift them slightly above the shingles, allowing air to circulate underneath. This ventilation helps prevent moisture buildup and keeps the metal cooler in summer.

Furring strips fasten to the roof deck through the shingles with corrosion-resistant fasteners — usually ring-shank nails or structural screws — spaced 12 to 16 inches apart along each strip. The spacing between strips depends on the metal panel profile and your local code, but typically ranges from 16 to 24 inches. Strips must be straight and level relative to each other, or the metal panels will not sit flush and water will pool in the valleys between panels.

Fastening metal panels over shingles

Metal roofing panels fasten through the furring strips into the roof deck using fasteners rated for metal roofing — usually self-drilling screws with rubber or neoprene washers. The washers compress when the screw is driven, creating a watertight seal around the fastener hole. Fasteners go through the ribs or flat areas of the panels depending on the panel type; your metal roofing supplier will specify which.

Fastener spacing and pattern vary by panel profile, roof pitch, and wind load requirements in your area. Coastal regions and areas with high wind speeds require more fasteners, placed closer together. A typical pattern might be one fastener every 12 to 16 inches along the panel length, but your local building code or the panel manufacturer's installation guide will specify the exact requirement for your location. Fasteners driven too tight will compress the washer and create leaks; too loose and they will not seal. This is a common source of problems, so take time to get the tension right.

Flashing around penetrations and edges

Flashing is metal trim that directs water away from roof penetrations like chimneys, vents, and skylights, and along the edges where the metal roof meets walls or gutters. When installing metal roofing over shingles, flashing must overlap the metal panels and extend under them slightly, so water runs down the metal and away from the shingles below.

Valleys — the low points where two roof slopes meet — need special attention. Some metal roofing systems use open valleys with a metal valley flashing underneath the panels; others use closed valleys where the panels overlap. Open valleys are easier to flash over shingles because the flashing sits on top of the shingles and the metal panels fasten on either side. Closed valleys require the panels to be cut and fitted precisely, which is harder over an uneven shingle surface.

Roof edges and rake trim (the trim along the sloped edges) must be installed after the metal panels, so they overlap the panel edges and prevent water from running up under the panels. Gutter attachment points may need to be adjusted if the metal roof raises the roof height by more than an inch or two.

Ventilation and moisture management

Metal roofing over shingles creates an air space between the metal and the shingles. This space must be ventilated — air must be able to flow in at the eaves and out at the ridge — or moisture will condense and rot the shingles and roof deck. If your roof does not already have soffit vents (vents in the eaves) and ridge vents, you will need to add them before the metal roof goes on.

In cold climates, an unventilated air space can cause ice dams — water from melting snow refreezes at the eaves where it is colder, backing up under the roofing. Proper ventilation keeps the roof deck cold and prevents this. In hot climates, ventilation reduces heat buildup under the metal panels, which can extend the life of both the metal and the shingles underneath.

When to remove shingles instead of roofing over them

Removing shingles before installing metal roofing costs more upfront but solves several problems. You can inspect the roof deck fully and repair any damage before the new roof goes on. You avoid the weight of two roof layers, which matters if your roof structure is old or if local code limits the total weight. You eliminate the air space that can trap moisture or create ice dams. And you get a cleaner, flatter base for the metal panels, which reduces the chance of fastener or flashing problems later.

If your shingles are old, damaged, or if you are in a high-wind or coastal area where every detail matters, removing them is often the better choice despite the extra cost. A roofer can give you a rough estimate of removal cost versus the cost of repairs needed if you roof over damaged shingles.

Frequently Asked Questions

Do I need a permit to install metal roofing over shingles?

Most jurisdictions require a roofing permit before you start. The permit process includes a building department review of your plans and an inspection after installation. Some areas allow homeowners to pull their own permits; others require a licensed contractor. Call your local building department to find out what is required in your area.

How much does it cost to install metal roofing over shingles versus removing them first?

Roofing over shingles saves the cost of shingle removal and disposal, which typically runs 10 to 20 percent of the total roofing cost. However, if shingles are damaged and need repair, or if your roof deck needs work, those costs add up quickly. Get quotes from roofers for both methods so you can compare the total cost, not just the roofing labour.

Can metal roofing go over multiple layers of old shingles?

Most building codes allow only one layer of roofing over existing shingles. If you have two or more layers of old shingles, they must be removed before the metal roof goes on. Check your local code to be sure.

What happens if water gets under the metal panels into the shingles?

Water trapped between the metal and shingles will rot the shingles and roof deck over time. This happens when flashing is installed incorrectly, fasteners are loose, or the air space is not ventilated. Proper flashing, tight fasteners, and ventilation prevent this. If rot does occur, the metal roof must be removed to repair the deck, which is expensive.

How long does metal roofing last over shingles compared to over bare deck?

The metal panels themselves last 40 to 70 years regardless of what is underneath. However, the shingles and roof deck underneath age faster because they are in a warm, moist environment. If the shingles were already 15 to 20 years old when you roofed over them, they may need replacement before the metal roof does, requiring removal of the metal roof to access them.