Putting a Metal Roof Over Shingles: When It Works and When It Does Not

Yes, you can put a metal roof over shingles in some cases, but only over one flat, dry layer on a sound deck, and tear-off is often the safer call in Ontario. Over shingles you cannot inspect the deck, install ice and water membrane properly or be sure of the warranty terms. Decide by checking the roof, the attic and the paperwork.

Can you put a metal roof over shingles? The short answer

Sometimes. Some manufacturers allow it, and it is done. But an overlay is not a shortcut that suits every roof. It makes sense only when all of these are true:

• There is only one layer of shingles on the roof.
• The shingles lie flat. They are not curled, cupped or missing in patches.
• There are no leaks, stains or damp insulation under the roof, and the deck feels firm and does not sag.
• The panel maker's instructions allow installation over existing roofing.
• Your contractor, and where it applies your building department and insurer, agree.

If any one of these fails, tear off the old roof. The rest of this page explains why, and how to check each point.

How a metal roof is installed over existing shingles

There are two common methods. Details vary by panel type and manufacturer, so follow the maker's instructions.

1. Strapping. Wood or metal strips, often called furring or battens, are fastened through the shingles into the deck or rafters. The panels are then screwed or clipped to the strapping. This leaves an air gap under the metal and gets past an uneven surface.
2. Underlayment over the shingles. A layer of underlayment goes over the old roof, and the panels are fastened through everything into the deck.

In both cases the fasteners must reach solid wood, not just shingle and thin material. The roof also gets thicker, so wall and chimney flashings, vent pipes and edge trims have to be raised or extended to suit the new height. Those details are where leaks start, so ask how each one will be handled.

Why tear-off is often the safer call in Ontario

• Hidden rot. A full tear-off lets the crew inspect the deck, and soft or rotted sheathing is replaced before new roofing goes on. Over shingles, a soft spot stays covered until it sags or leaks.
• Ice and water membrane. This self-adhering membrane goes along the eaves and in the valleys, where freeze-thaw cycles and ice dams do their damage. It is made to stick to the roof deck. Natural Resources Canada recommends an ice-shield membrane when re-roofing.
• Trapped moisture. Old shingles can hold damp, and a layer of metal over them slows the drying. Strapping with an air gap helps, but it does not remove the risk.
• Uneven surface. Curled edges and ridges in the old shingles can show through the panels and make the panels look wavy or add noise.
• Extra load. Natural Resources Canada notes that an extra layer of shingles puts added load on the roof structure and that removing the existing shingles is typically recommended. Metal is lighter than a second shingle layer, but the old layer still stays on the roof.
• The next replacement. When the metal reaches the end of its life, someone has to remove the metal, the strapping and the shingles.

Warranty, insurance and permits for a metal roof over shingles

Warranty. A panel manufacturer may require a specific method over existing roofing, or may exclude it. Ask for written confirmation that the warranty applies to an installation over shingles, and ask what the installer's workmanship warranty covers. The roofing warranty guide shows what to look for.

Insurance. According to the Insurance Bureau of Canada, insurers generally prefer roofs that are updated within 20 years, and they ask about roof type when they rate a home. Tell your broker exactly what is being installed and ask in writing how an overlay is treated.

Permits and inspections. Ask your municipal building department whether the job needs a permit and what they would want to see. The Ontario Building Code requirements for roofing give background. A buyer's inspector may later ask how many layers are under the metal, so keep the invoice, photos and warranty papers.

Check your roof before you decide

Do not climb on the roof yourself. Start with what you can see safely.

• In the attic. Look at the underside of the deck for dark stains, daylight, damp insulation, mould, sagging between rafters and frost on nail tips.
• From the ground. Look for curling or cupped shingles, bald spots where the granules are gone and lifted or missing shingles after wind. Binoculars help.
• The layers. A roofer can count the layers at the edge. More than one means tear-off.
• The history. Past leaks, or ice dams every winter, point to a problem the overlay will cover but not cure.
• The ventilation. An overlay does not fix a blocked intake or weak exhaust. Poor ventilation shortens shingle life and feeds ice dams. See the soffit ventilation guide.

If the roof is already failing, the repair versus replace guide helps you decide how far to go.

Overlay vs tear-off: cost and time

An overlay skips tear-off labour and disposal, and it can be quicker. It also adds strapping or underlayment, longer fasteners and extended flashings, so the saving is smaller than most owners expect. If the deck turns out to need repair, the repair is more awkward once the metal is on.

A tear-off leaves the deck open to the weather while the crew works, so a good crew plans around the forecast. In return you get a clear look at the deck and a clean surface for the membrane and underlayment.

This page publishes no prices. Ask each roofer to price both options in writing, with the same panel, trim, flashing and ventilation work, and compare them line by line. Skipping tear-off to save money on a roof with known problems is a false saving.

When to have a roofer look at it

If you are weighing an overlay, get the roof and attic looked at before you choose. D&D Exterior Finishing installs asphalt shingles and metal roofing, including standing seam and corrugated steel. A full tear-off lets the crew inspect the deck, replace any soft sheathing and install ice and water membrane along the eaves and in the valleys, with synthetic underlayment, drip edge and step and counter flashing. Every estimate is free and written, and it lists the scope, materials, colours and warranty terms.

Ask for both options on your roof. See roofing installation and replacement for what the work includes.

Metal roof over shingles questions homeowners ask

How many layers of shingles can you have before a metal roof goes on?

An overlay is only considered over a single layer. If there are already two layers, tear them off. Panel makers may set stricter limits than that, and your municipal building department can tell you what applies locally. A roofer can count the layers at the roof edge during a quick look.

Will a metal roof over shingles void my warranty?

It might. Some panel warranties require a specific installation method over existing roofing or exclude it. Ask the manufacturer and the installer for written confirmation before you sign. Also ask what the installer's own workmanship warranty covers, because that is separate from the product warranty.

Does a metal roof over shingles cause leaks or condensation?

Not on its own, but it can hide an existing leak, and old shingles can hold damp under the metal. Strapping that leaves an air gap, good underlayment, correct flashing and balanced attic ventilation reduce the risk. A deck inspection and a tear-off remove most of it.

Do inspectors or insurers care if there is metal over shingles?

They may. Insurers ask about roof type and age, and a building department may want to know how the roof is built. A buyer's inspector may ask how many layers sit under the metal. Tell your broker before work starts and keep the invoice, photos and warranty papers.

Devon Moore, Operations Lead Co-Founder & Operations Lead — D&D Exterior Finishing

Devon Moore is the co-founder and Operations Lead at D&D Exterior Finishing, specializing in siding, roofing, windows, and exterior renovations across Waterloo Region.