Category: Flat Roofs

The 10 questions

1. What is your RBQ licence number? Check it in the Régie du bâtiment du Québec register.
2. Do you have liability insurance? Ask for current proof.
3. How many layers will you remove?
4. Which membrane and brand? See elastomeric vs TPO vs EPDM.
5. Are you adding insulation? Type, thickness and cost.
6. How will you handle flashing and drains? See flashing.
7. What if the deck is rotten? Written unit price.
8. Who handles the permit and colour requirements? See white roofs.
9. What warranty, on what? Membrane and labour, in writing.
10. What payment schedule? Never pay in full before the work is done.

Red flags

• No RBQ licence or refusal to provide it
• Price far below the others with no explanation
• Pressure to sign the same day
• Large cash deposit
• Verbal or vague quote

Estimations.pro is a matching service and does not perform the work. Always check the contractor’s RBQ licence and insurance.

Why a white roof?

A black roof can get scorching hot in summer, heating the top floor and the neighbourhood. A light roof reflects much of the sun’s energy, keeps the unit below cooler and reduces thermal stress on the membrane.

What the rules usually require

• A white or light covering with a minimum solar reflectance
• Or a green roof
• Or light-coloured gravel ballast
• Possible exceptions depending on slope, use or building type

Which membranes comply?

White TPO is naturally very reflective. Elastomeric membranes come with white or light granules made for these rules. Standard EPDM is black and fits less well. See elastomeric vs TPO vs EPDM.

General information. Municipal bylaws change and vary by borough: check with your borough before work starts.

The 8-point checklist

1. Drains: remove leaves and debris, make sure water flows freely.
2. Debris on the membrane: branches and leaf piles hold moisture.
3. Flashing: look for lifting around chimneys, parapets, vents and skylights. See flat roof flashing.
4. Membrane: blisters, cracks, bare spots, punctures.
5. Ponding: note where water sits after rain.
6. Skylights: seals, frame and glazing.
7. Heating cables: test before the freeze, with an electrician if needed.
8. Top-floor ceilings: report any new stain to the roofer.

Never go up on a wet, icy or unguarded roof. During winter, see should you clear snow from a flat roof? and, if a drip appears, what to do about a leak.

General advice. Working at heights is dangerous: hire an insured professional.

Decision table

Check typical ages in how long a flat roof lasts and amounts in our cost guide. Most leaks are fixed with a targeted repair if you act fast.

The 25% rule is a general benchmark, not an absolute rule. Only an inspection shows your roof’s real condition.

When to clear it

• After a big storm followed by rain or freezing rain
• When drifts pile up against parapets or equipment
• When drains are frozen and water can’t escape
• On older buildings or those with known structural weaknesses

Overload warning signs

• Interior doors suddenly sticking on the top floor
• New cracks in ceilings or walls
• Unusual creaking
• A sagging ceiling or deformed beams

If you see these signs, leave the area and call professionals right away.

Clearing without damaging the membrane

• Plastic shovels only, never metal tools or picks
• Leave a few centimetres of snow on the membrane
• Clear drains and drifts first

Prepare in fall with our checklist. Frozen drains often cause leaks at the thaw.

General advice. If you see signs of structural overload, leave the area and contact a professional immediately.

Where flashing is found

• Along parapets and walls
• Around brick chimneys
• Around plumbing vents and exhausts
• Around drains and strainers
• Around skylights and roof hatches
• Under HVAC equipment and supports

Why it leaks

• Movement from seasonal expansion and contraction
• Too-low upstands that let meltwater in
• Aging sealant and metal collars
• Sloppy work during a reroof or equipment install

What to require from the roofer

• Removal of the old flashing, not just sealant on top
• A membrane upstand of sufficient height
• Well-fastened, sealed counter-flashing
• Before-and-after photos and a written warranty

General information. Have your flashing inspected by an RBQ-licensed roofer.

Most duplexes, triplexes and small buildings in Montreal have flat roofs. Here are realistic benchmarks to budget and spot a quote that doesn’t add up.

Price per square foot by roof type

What drives the bill

• Layers to remove and the condition of the roof deck
• Details: every drain, chimney, vent and skylight needs careful flashing. See flat roof flashing.
• Access: narrow lanes, height, crane or hoist
• Colour requirements in many boroughs. See white roofs in Montreal.

Before replacing everything, check whether a repair would do: repair or replace a flat roof. Then use our 10 questions for a roofer.

Indicative prices before taxes (GST 5% and QST 9.975%), based on Quebec market ranges in 2025 and 2026. Only written quotes give your actual price.

At a glance

How to choose

• Montreal plex: light-coloured elastomeric is often the best mix of toughness and compliance.
• Rooftop deck or foot traffic: elastomeric resists punctures better.
• Tight budget, large area: TPO is often the most affordable.
• Simple shapes, building movement: EPDM is a good option.

Budget it with our flat roof cost guide and check the white roof rules.

Indicative prices before taxes, based on Quebec ranges in 2025 and 2026. Always compare written quotes.

On a flat roof, water can travel far before showing up, so the stain is not always under the leak.

The 5 most common causes

• Flashing: where the membrane changes direction. See flat roof flashing.
• Clogged or frozen drain: leaves, debris or ice block the water.
• Ponding water: poor slope creates puddles that wear the membrane.
• Aging membrane: cracks, blisters, missing granules.
• Punctures: equipment, aggressive snow removal, falling branches.

What to do in the first hours

1. Protect the inside: bucket, tarp, move furniture.
2. Electrical safety: shut off the circuit if water reaches a fixture or outlet.
3. Document: photos and dates for your insurer.
4. Stay off an icy roof: let a pro check the drain and membrane.
5. Call a roofer: a temporary fix keeps insulation and drywall dry.

If water reached the insulation, fast drying limits mould: see our water damage restoration page. Prevent the next leak with our pre-winter checklist.

General advice. For any electrical or structural risk, call a professional right away.

Signs your flat roof is near the end

• Cracks, crazing or blisters in the membrane
• Surface granules disappearing on elastomeric
• Puddles lasting more than 48 hours after rain
• Flashing lifting in several places
• Repeated leaks despite repairs
• Stained ceilings on the top floor

How to make it last

• Yearly inspection, ideally in fall. See our pre-winter checklist.
• Keep drains clear, especially during leaf season and freezes.
• Fix small issues fast: a resealed flashing prevents costly leaks.
• Careful snow removal when needed. See should you clear snow from a flat roof?

When your membrane passes two-thirds of its lifespan, start budgeting with our cost guide.

Indicative lifespans based on Quebec ranges. Installation, slope and maintenance can change them a lot.