Re-cover or Replace: What to Do With an Old Flat Roof

Re-cover, tear-off, or something in between

The National Research Council of Canada (NRC) uses three terms for this work:

• Re-cover: a new roof system goes on over the existing one.
• Tear-off: everything comes off down to the deck, and the new roof goes on the bare deck.
• Partial tear-off: the old roof comes off only where it has to — usually where the insulation is wet — and the new roof goes over the whole area. A variation strips the old membrane everywhere but keeps the dry insulation.

A re-cover has real advantages. The building stays closed up during the work, so there is less risk of a leak. You keep the insulation you already paid for. You skip the labour and landfill costs of demolition. The trade-off is that you can trap moisture inside the roof, and you cannot inspect or fix a deck you never uncover.

This applies to every common flat roof: SBS modified bitumen, EPDM, TPO and built-up. The materials have to suit each other, too. Some membranes do not bond to others — SBS modified bitumen and PVC, for example — so the new roof has to be chosen to match the old one or separated from it by a layer of board. Tar-and-gravel roofs have their own quirks on top of that.

Wet insulation is the deal-breaker

The biggest question is whether the insulation under your membrane is dry. The Canadian Roofing Contractors Association (CRCA) is blunt about it: new roofing should never be installed over wet or deteriorated insulation. Where it is found, it comes out and is replaced with new material of the same thickness.

Here is why it matters:

• It stops insulating. A long-used industry rule of thumb treats insulation as wet once it has lost a fifth of its R-value.
• It damages the deck. The CRCA warns that wet roofs can lead to rusting steel decks, rotting wood and spalling concrete.
• It blisters the new roof. NRC research found that blisters in modified bitumen roofs grow from trapped moisture. Water expands about 1,250 times when it turns to vapour, so a tiny amount in a void can push up a blister on a hot day.
• It weakens the boards. Gypsum cover boards and the paper facers on insulation lose strength when wet, and a wet gypsum board can no longer hold an adhered roof down against wind.

A new membrane does not fix any of this. The CRCA notes that re-covering will not solve moisture problems caused by condensation, air leakage or leaky rooftop units. Find the source first, or the new roof inherits the problem.

How to find out what is under the membrane

You cannot see wet insulation from the surface. There are three common survey methods:

• Infrared scan. A thermal camera looks for warm and cool patches, because wet insulation holds and moves heat differently from dry. It scans the whole roof. U.S. cold-regions researchers found in 1977 that night scans work best, once the sun is out of the picture.
• Nuclear gauge. It takes readings on a grid by counting hydrogen, which water contains. So do bitumen and concrete, so the readings are relative, not absolute.
• Capacitance meter. It reads an electrical field, usually on a grid. It does not work on conductive surfaces such as EPDM or foil-faced membranes.

All three detect moisture indirectly. The NRC says every survey should be checked by cutting into the roof, and the published test standards for these methods call for core samples to confirm what the instruments suggest.

The reverse is also true: a few test cuts on their own are not enough. The CRCA warns against relying on a handful of simple cuts to decide whether water is trapped. For a re-cover, the NRC recommends several large cuts — at least 600 mm (2 ft) square — to check the condition of the deck and how well it is fastened, plus a look at the underside of the deck wherever you can get to it.

One caution: if your roof is still under warranty, get the warranty holder’s permission before anyone cuts into it. Otherwise you could void the warranty.

The deck, the weight, and how many layers

A re-cover adds weight, and a Winnipeg roof also has to carry snow. The NRC says a re-cover is not viable if the deck or the structure under it cannot take the extra load, and that it is often wise to check the structure against today’s snow loads — especially for buildings such as schools and hospitals. If there is any doubt, have an engineer look before you choose.

If the deck is deteriorated or poorly fastened, the NRC’s advice is plain: tear the roof off so the deck can be repaired.

Then there is the number of layers. Canada has no clear national rulebook for work on existing buildings; the NRC itself describes the result as a patchwork across the country. The U.S. International Building Code draws a hard line: a roof that already has two or more layers, is water-soaked, or is too deteriorated to be a sound base must be torn off to the deck. Peeling off only the top layer and re-covering what is underneath does not count. One existing roof is a sensible limit here too. Before you commit, ask the City of Winnipeg what it requires for your building.

Flashing heights and drains

Every layer you add raises the roof surface. The walls, curbs, door sills and drains around it stay where they are.

Flashings need height to keep water out. The Roofing Practices Manual of the Roofing Contractors Association of British Columbia calls for membrane flashings to reach at least 200 mm (8 in.) above the finished roof surface in most places. If a re-cover leaves a parapet, curb or door sill too short, it has to be raised. Some things, like windows and door sills, can be costly or impossible to raise.

The CRCA’s reference manual goes further: when reroofing, old flashings should be cut out and the walls and curbs brought up to the standard for new construction. Oakwood makes its own flashings, parapet caps and scuppers in our sheet metal shop, and our crews raise and replace existing roof curbs, so these details can be built for the new roof height rather than patched around it.

Drains matter just as much. The CRCA defines problem ponding as water that stays on a roof 48 to 72 hours when conditions favour drying. A new roof over an old one still has to drain to the same drain bowls, so check that water will actually reach them.

Warranty and insulation

Ask about the warranty before you pick an option. A 1999 CRCA bulletin, citing U.S. industry data, reported that re-covered roofs had problems at about twice the rate of new roofs, and that most manufacturers and contractors were reluctant to warranty a re-cover. Some products have changed that since: Soprema, for example, offers certain re-cover systems with a 10- or 15-year warranty, subject to its eligibility conditions. Terms vary by manufacturer and by roof, so get them in writing.

Then think about insulation. A re-cover keeps what you have. A tear-off exposes everything, and the NRC calls reroofing an opportunity to improve thermal performance, drainage, and resistance to snow and wind. Since January 1, 2024, Manitoba’s energy code for buildings has adopted the 2020 National Energy Code of Canada for Buildings. Ask your designer and the City how it applies to your roof before the design is set.

Re-cover or tear-off: a quick checklist

A re-cover can be a sound choice when all of these are true:

• A moisture survey, confirmed by core cuts, shows the insulation is dry — or the wet areas are small and will be cut out and replaced.
• Large test cuts show the deck is sound and well fastened.
• There is only one roof on the building now.
• The structure can carry the extra weight along with its snow load.
• Flashings, curbs and door sills will still have enough height, and water will still reach the drains.
• The new membrane suits the old one, and the manufacturer will warranty the result.
• The leaks come from a worn-out membrane, not from condensation, air leaks or rooftop units.

Plan on a full tear-off when any of these apply:

• Wet insulation is spread across much of the roof.
• The deck is rusted, rotted, spalled or loose.
• There are already two layers of roofing.
• The existing membrane is badly blistered, buckled or water-soaked.
• The roof failed early and nobody has found out why.