In short
- An Ontario firm's 2026 list puts a flat roof at $5–$15 per square foot installed, by membrane.
- A Canada-wide guide (April 2026) prints $11–$25 per square foot including tear-off, so the sources differ by a wide margin.
- A garage-size 500 sq ft flat roof runs $2,500–$7,000 on the Ontario list; a 1,000 sq ft section $5,000–$14,000.
- Tear-off, deck repair, drainage and permits are quoted on top on the Ontario list.
- Flat roof repairs run $300–$1,500 on the same list.
Typical prices in Ottawa
| Job | Typical range | Notes |
|---|---|---|
| Modified bitumen, 2-ply torch-on | $5–$8 per sq ft | One Ontario firm’s 2026 list, installed; Canada-wide guide $15–$25 with tear-off |
| EPDM rubber membrane | $6–$10 per sq ft | Same Ontario list; Canada-wide guide $11–$17 |
| TPO membrane | $7–$12 per sq ft | Same Ontario list; Canada-wide guide $12–$18 |
| Flat section of 500 sq ft | $2,500–$7,000 | Ontario list: bitumen $2,500–$4,000, TPO $3,500–$6,000, PVC $4,000–$7,000 |
| Flat section of 1,000 sq ft | $5,000–$14,000 | Same list, by membrane |
| Tear-off and disposal | $1.50–$3.00 per sq ft | Ontario list, extra |
| Tapered insulation for drainage | $1.50–$3.50 per sq ft | Ontario list, extra |
| Flat roof repair | $300–$1,500 | Ontario list; typical repair |
Sources. Custom Contracting, Flat Roof Cost Ontario 2026, updated June 2026, seen 29.10.2026 (one Brantford firm’s Ontario-wide list). RenoQuotes, Flat Roof Replacement Cost per Square Foot in Canada in 2026, updated 24.04.2026, seen 29.10.2026 (Canada-wide). HomeStars, Roofing Cost Guide for Ottawa, updated 21.08.2026, seen 29.10.2026. CAA-Quebec, Ice buildup along the roof edge, updated 02.02.2026, seen 29.10.2026.
Two sources, two answers
We found no Ottawa-only flat roof price list. The numbers below come from an Ontario firm and a Canada-wide guide, and they disagree. The Ontario list prints $5–$15 per square foot installed, from $5–$8 for two-ply modified bitumen up to $7–$12 for TPO and $8–$14 for PVC. The Canada-wide guide, updated in April 2026, prints $11–$17 for EPDM, $12–$18 for TPO and $15–$25 for modified bitumen, and says removal of the old roof is included.
A roof torn off, insulated and drained costs near the high figures; a membrane on a sound deck costs near the low ones. Ask for the scope behind each number. Repairs follow the same logic; see the roof repair cost guide.
What the membranes are
Modified bitumen comes in two plies, often torch-applied, and has a long record in cold weather. EPDM is a rubber sheet, flexible in the cold and simple to repair. TPO is a white or grey single ply with heat-welded seams. PVC suits roofs with grease or chemical exposure. The service page on flat roofs describes each variant and what the company does on site.
Size and the add-ons
The Ontario list shows a 500 sq ft garage at $2,500–$4,000 in modified bitumen and $3,500–$6,000 in TPO, and a 1,000 sq ft section at $5,000–$8,000 and $7,000–$12,000. These are totals with membrane, insulation and standard flashing.
Extras come after inspection: tear-off and disposal of $1.50–$3.00 per sq ft, deck replacement at $3.00–$6.00, tapered insulation to remove standing water at $1.50–$3.50, edge metal at $8–$15 per linear foot, and a curb or flashing for each rooftop unit at $200–$800. The Canada-wide guide adds insulation up to R-60 at roughly $2–$8 per sq ft. A line-item quote shows which apply.
Why water decides the price
A flat roof has a slight slope towards a drain. Where water stands for days, membranes age quickly. The Ontario firm cites a 1:50 minimum slope in the code and tapered insulation as the fix for ponding. Ask the company how it will move water on your roof, and where the drain, scupper or edge is.
In winter the same roof meets ice dams and heavy snow. CAA-Quebec says flat roofs are affected near roof drains. The snow on the roof guide covers what the weight means and who should clear it.
Repair or replace
The Ontario list gives a typical flat roof repair at $300–$1,500 and suggests repair when the membrane is under ten years old with one isolated problem and the repair costs under 30% of replacement. Widespread blistering, several active leaks, or wet insulation point to replacement. For the question in general, see repair or replace.
Permits, contract and timing
Whether a permit applies depends on the work. The roof permits in Ottawa page lists the City’s rules; the contract should say who applies. Membranes go on in dry weather, so spring booking suits summer work. Expect the Ontario firm’s 2–4 days of installation to move with the weather.
Who you are dealing with
We forward the size, layers and pooling notes to one independent Ottawa firm. It inspects the roof, picks the membrane system and writes the price line by line. Use the figures here to check its quote.
Questions to ask the company
- Is the price for tear-off and a new membrane, or for a new membrane over the old one?
- Which membrane do you propose, and why that one for this roof?
- How will the water reach the drain, and is tapered insulation in the price?
- What happens to the price if the deck under the old roof is wet or soft?
- Who handles the flashing at walls, vents and the roof drain, and what does it cost?
Have this ready for the call
- Approximate roof area in square feet, or length and width
- A photo of where water stands after rain
- What you know of the layers: how many roofs are on, and the age of the last one
- Photos of the edges, the drain and the walls the roof meets
- Whether the space below is heated living space, a garage or an addition
Questions people ask
Why do flat roof prices differ so much between sources?
The sources count different things. One Ontario list prices membrane, insulation and labour per square foot and quotes tear-off and deck repair separately. The Canada-wide guide includes removal of the old roof. Compare any figure against a written scope: what comes off, what goes on, what is extra.
Which flat roof material lasts longest?
On the Ontario list, modified bitumen lasts fifteen to twenty years, EPDM 20 to 25, TPO 20 to 30 and PVC 25 to 30. The Canada-wide guide gives longer ranges for the same membranes. Ask the company for the manufacturer’s and the labour warranty terms.
Can a flat roof go over the old membrane?
Sometimes. A second layer saves the tear-off ($1.50–$3.00 per sq ft on the Ontario list) but traps any wet insulation below. If the insulation is spongy or the deck is soft, the old roof comes off. Ask the company what it found in the layers and how.
Do flat roofs get ice dams?
Yes, according to CAA-Quebec, often near roof drains. Heat loss melts snow, the water refreezes at the cold edge or around the drain, and the ice holds the water back. The ice dams page explains the cause, and the fix starts with the heat leaving the ceiling below.
How big is a typical home flat roof?
Home flat roofs are usually a garage, an addition or the low part of a split-level. The Ontario list prices 500 sq ft as a garage or small addition and 1,000 sq ft as a residential flat section. Measure the roof, or send a plan, before you ask for a price.
How long does a flat roof replacement take?
The Ontario firm quotes 2–4 days on average for installation. A tear-off, a wet deck or rain can stretch that. Ask the company for dates in the contract, and for what it does if weather stops the job with the roof open to the sky, and who pays for temporary protection.
