What counts as a flat roof
According to the City of Montreal, a roof with a slope under 2:12, or 16.7%, is a flat or low-slope roof. From 2:12 up, it is a sloped roof.
On a sloped roof, water runs off quickly and the shingles overlap. On a flat roof, water lingers. Flat roofing has to be watertight from edge to edge.
Where you find flat roofs in Montreal
Flat roofs are everywhere in the central neighbourhoods. The row duplexes and triplexes of Le Plateau-Mont-Royal, Rosemont, Villeray, Verdun, Le Sud-Ouest and Hochelaga almost all have one. You also find them on apartment buildings of every size, and on commercial and industrial buildings with much larger surfaces.
In plex neighbourhoods, buildings touch and yards are small. Access to the job site is often through the back lane.
Flat roofing systems
Three families of roofing cover almost every flat roof in the city.
- Elastomeric membrane. Two layers of modified bitumen, with a cap sheet covered in granules. It is the most common system on plexes. An elastomeric membrane lasts 25 to 35 years, depending on the manufacturer and the quality of installation.
- Tar and gravel. Several layers of felt and hot asphalt, protected by a layer of gravel. Many older Montreal roofs still have it. A tar and gravel roof lasts 15 to 25 years.
- Single-ply membranes, TPO and EPDM. A single sheet of plastic or synthetic rubber, installed mostly on large surfaces. A TPO or EPDM membrane lasts 20 to 30 years.
A flat roof is a stack of layers, from the wood deck up to the membrane. Our quote says which layers are kept and which are replaced.
Where the problems start
A membrane in good condition rarely leaks in the middle. Flat roof leaks often start where the membrane stops, turns or is pierced.
- the base and top of the parapets, the low walls around the roof
- around the drain
- plumbing vents, ventilation outlets and electrical masts
- skylights and roof hatches
- anchors for a deck or railing added later
Features of the Montreal flat roof
- Parapets. They often separate your roof from your neighbour's. If the parapet masonry is in poor shape, have it looked at before a new membrane goes against it.
- Skylights. The membrane has to run up their frame. An old skylight is easier to replace while the roof is open.
- Rooftop decks. They sit on the membrane and hide it. They must be taken apart to redo the roof, then put back without piercing or crushing the new roofing.
- Mechanical equipment. Heat pumps, air conditioners, fans and ducts sit on bases the membrane must go around. Ask us who disconnects, moves and reconnects them.
How a flat roof replacement goes
- The visit. We go up on the roof, measure, and look at the details and the access.
- Quote and permit. You compare offers, then the permit application is filed if your borough requires one.
- Preparation. Container, access from the street or lane, protection of the facade and yard, notice to tenants and neighbours.
- Tear-off. Old layers come off in sections, so every open area is closed before the end of the day.
- Roof deck. Rotten boards are replaced.
- Insulation and slopes. Vapour barrier, insulation and cover board go down, sloped toward the drain.
- Membrane and flashing. The roofing covers the surface, then runs up the parapets and around every obstacle.
- Finishing. Metal flashing, drain and strainer, yard clean-up, and a final check of the roof.
Permits and borough requirements
The rules vary by borough, type of building and type of work. According to City of Montreal pages consulted on October 3, 2026, replacing flat roofing requires a permit in most boroughs. Minor repairs and maintenance generally do not.
When a flat roof is replaced, most boroughs require a white roof, a reflective material or a green roof to reduce heat islands. The required solar reflectance index (SRI) ranges from 56 to 82 depending on the borough.
Heritage buildings and heritage areas have additional standards. Check with your borough before signing a contract.
What affects the price
- the surface and the number of details (parapets, vents, skylights)
- access, including building height, the lane, and room for a crane or container
- the tear-off and the number of old layers
- the condition of the roof deck, only visible once the roof is open
- added insulation or corrected slopes
- the system chosen and the warranties requested
For buildings with 1 to 5 units, the City offers the RénoPlex program. The conditions are on montreal.ca.
What to ask for in a flat roof quote
A one-line quote cannot be compared. Ask us, as you would any contractor, for a document that states:
- the proposed system and the full name of the cap sheet, with its colour
- what is torn off and what is kept, layer by layer
- how replacement deck boards will be billed
- how the parapets, flashing, drain and skylights are handled
- the manufacturer and workmanship warranties, in writing
- who files the permit application
- our RBQ licence number, which you can check in the public register of the Régie du bâtiment du Québec
Frequently asked questions
How long does a flat roof last?
A flat roof lasts 15 to 35 years depending on the system. Expect 25 to 35 years for an elastomeric membrane, 15 to 25 years for tar and gravel and 20 to 30 years for TPO and EPDM, depending on the manufacturer and the quality of installation. Keeping the drain and flashing in good shape counts toward that lifespan.
Do you need a permit to redo a flat roof in Montreal?
Yes, in most Montreal boroughs replacing flat roofing requires a permit. Minor repairs and maintenance generally do not. The permit counter of your borough gives the answer for your address.
Can a flat roof be repaired instead of replaced?
Yes, a flat roof can be repaired when the defect is local and the rest of the roof is sound. Replacement is needed when leaks multiply, the insulation is wet or the membrane reaches the end of its life.
Can you put a deck on a flat roof?
A rooftop deck needs two checks. An engineer must confirm the structure can carry the load, and the membrane must be protected from the deck supports. A deck also falls under the borough's planning rules. Talk to your borough before you start.
What are the disadvantages of a flat roof?
The main disadvantage of a flat roof is that water lingers instead of running off quickly. The roofing must be watertight from edge to edge and the drain must stay clear. A leak can travel a long way through the insulation before it shows on the ceiling.
Is a flat roof cheaper than a pitched roof?
No. Per square foot, a flat roof generally costs more to redo than a sloped roof with asphalt shingles, based on prices roofers post on their websites. A flat roof membrane often needs more layers and more detail work around parapets and drains.