Can You Put a Metal Roof Over Existing Shingles? (Victoria Guide)

Over-shingle proposals come up most on simple, younger roofs: a 1990s Langford or Gordon Head gable with one layer of laminate shingles lying flat, dry decking and a plumbing stack or two. They come up least sensibly on the older core. Fernwood and Fairfield houses often have plank or skip sheathing under two layers of duroid, with curled tabs and a moss mat on the north slope; covering that with metal locks in every problem the tear-off would have found. Highlands and Metchosin roofs under conifers are usually too mossy to qualify, whatever the age.

Three local checks before you take the idea seriously. Phone the building department: Saanich (250-475-5457), the City of Victoria (250-361-0344) and Langford (250-474-6692) all list contacts, and none publishes a rule on re-cover layers online, so the answer has to come from them. Ask whether a heritage designation applies if the house is pre-1940 in Oak Bay, James Bay or Fairfield. And if the home is a Westhills or Royal Bay townhouse, the strata will have a say on both the material and the method.

Building codes generally limit the number of roofing layers a structure may carry and require that the roof can support the added load; how that is applied to a metal re-cover on a specific house is a building-department decision, and no Greater Victoria municipality publishes the answer online. A Victoria contractor's guide states that a change of material 'will need a permit' while a like-for-like re-roof typically does not. Treat that as a claim to verify, not a rule.

Manufacturers are the other authority. Their installation instructions say whether their panels or shingles may be installed over an existing layer, on strapping or direct to deck, with what underlayment, and over what condition of shingle. Install outside those instructions and the product warranty may not apply. Ask for the relevant page of the instructions with the quote.

Metal roofing is light relative to a layer of asphalt shingles, which is why the Metal Roofing Alliance notes it can sometimes go over an existing layer. The catch is that the old shingles stay on the roof as dead load, so the structure carries shingles plus metal plus any strapping. On a conventionally framed post-1970 house that is usually fine; on an older house with undersized rafters, a sagging ridge or previous alterations, a professional may want to inspect the framing first. Victoria's snow loads are modest, but a wet December on a moss-holding shingle layer under metal is weight too.

The common ways an over-shingle job loses its manufacturer coverage, all of which are avoidable.

• Installing over a condition the instructions exclude: two layers, curled tabs, moss, wet shingles
• Omitting the required underlayment or slip sheet between the shingles and the metal (asphalt and metal in direct contact can be excluded)
• Wrong fastener length for the added thickness, so screws miss or barely bite the deck
• Strapping spaced wider than the manufacturer's pattern for the panel gauge
• No ventilation path, so condensation forms under the panels over a warm shingle layer
• A crew that is not trained or certified for the product where the warranty requires it

Tear-off is an inspection. It reveals rotten sheathing, old leaks around chimneys, rusted valley metal and inadequate ventilation, and it lets the crew install ice-and-water membrane at eaves and valleys and raise new step and counter flashings to the correct height against walls and chimneys. Going over the top keeps the old flashings buried, thickens the eave and rake edges, and leaves the deck condition unknown under a roof you expect to last 40 years or more.

The saving is real but small. Our model prices tear-off labour and disposal at roughly $0.75 to $1.25 per sq ft of roof, so a 1,200 sq ft roof saves about $900 to $1,500 against a metal job that starts around $15,100 for a 2,000 sq ft two-storey home. If the deck turns out to need replacement at $7 to $10 per sq ft, the saving is gone many times over.

If the roof qualifies and the numbers still make sense, insist on the following in writing.

• Building-department confirmation for your municipality, with the name and date of the call or the permit number
• The manufacturer's instructions page allowing installation over shingles, and the warranty terms that apply
• Underlayment or slip sheet specification, strapping pattern or direct-to-deck method, fastener type and length
• Treatment of eaves, rakes, valleys, walls and chimneys, including new flashings where they can be installed
• A ventilation plan and a clause covering what happens if soft decking is found during installation