Tear-off, recover or rebuild
Flat roof replacement
Replacing a flat roof is a chance to fix what caused the failure: slope, drainage, insulation and flashings, not only the top layer.
- Houses to large commercial roofs
- Five membrane options
- Slope and drainage checked in the quote
Signs the roof is past repair
Several of these together usually mean replacement: insulation that is wet in more than isolated spots, a surface that soaks up when pressed, widespread blisters or cracking, seams opening in many places, leaks after every storm, or a membrane that has reached the age range for its type. See how long a flat roof lasts.
One bad spot is not enough. A roof with one failed flashing and sound membrane should usually be repaired. The repair or replace guide helps you decide.
Tear-off, recover or rebuild
| Approach | When it fits | What to know |
|---|---|---|
| Full tear-off | Wet insulation, rotted deck, more than one existing layer | Highest cost and most disruption, but you get a clean deck and a chance to correct slope. |
| Recover over the existing roof | Dry, flat, single-layer roof in sound condition | Cheaper, but it buries problems. Rules on the number of roof layers differ by municipality, so check with the local building department. |
| Rebuild the assembly | Ponding, no slope, or an old roof with poor insulation | Add tapered insulation and new drains so water leaves the roof. |
Layer limits and permit rules differ by municipality. See [re-roofing permits in Ontario](/guides/re-roofing-permits-ontario/).
How a replacement is run
Roof assessment
Size, access, existing layers, drains, penetrations and what lies under the roof inside.
Options and quote
Usually two membrane options with a written scope, so you can compare like for like.
Permits and notice
Where the municipality requires a permit, it is arranged before work starts. Occupants and neighbours are told the dates.
Tear-off and deck check
Old layers come off, the deck is inspected and any rotted boards are replaced.
New assembly
Vapour control as needed, insulation, membrane, flashings and drains installed in order, with edge metal last.
Final walk-through
We walk the finished roof with you or your property manager and photograph the completed details.
Replacement starting prices
| Item | Starting price |
|---|---|
| Small flat roof replacement (garage, porch or addition) | from $4,000 per roof |
| House flat roof replacement (about 1,000 sq ft) | from $8,000 per roof |
| EPDM roof replacement | from $8 per sq ft |
| TPO roof replacement | from $9 per sq ft |
| PVC roof replacement | from $11 per sq ft |
| Modified bitumen (torch-on or SBS) replacement | from $8 per sq ft |
| Built-up (tar and gravel) roof replacement | from $9 per sq ft |
| Commercial roof replacement | from $8 per sq ft |
Starting prices in CAD before 13% HST, based on typical Ontario contractor rates (mostly Toronto and the GTA). The final price depends on roof size, access, the layers to remove, the membrane, drains and flashings, and is confirmed in your written quote.
Large roofs cost less per square foot than small ones. Tear-off, wet insulation, new drains and tapered insulation add to the total. Flat roof cost in Ontario goes through each factor.
Choosing the membrane
Each membrane has trade-offs. EPDM is a rubber sheet that handles movement well, with a commonly quoted life of 20 to 30 years. TPO is a white heat-welded sheet, commonly 15 to 25 years. PVC resists grease and chemicals, often used on restaurant roofs. Modified bitumen is layered and tough underfoot, and tar and gravel is the traditional multi-ply system.
Questions about replacing a flat roof
Can I stay in the building during a replacement?
Usually yes. Noise starts early and tear-off dust is possible, but the interior is not opened. Condos and offices often schedule noisy stages around building rules. See the condo roof planning guide.
Does weather stop the job?
Rain, snow and strong wind do. Membranes also have temperature limits for adhesives. Ontario flat roof work concentrates in late spring through fall for that reason.
Should I add insulation while replacing?
It is the cheapest time to do it, and tapered insulation can add slope at the same time. Ask for it as a line in the quote.
Do I need a permit?
It depends on the municipality and on how much of the roof structure changes. Check with your local building department and read our permits guide.
Leak, damage or an old flat roof?
Send a few details and photos. We reply by email with next steps and a written quote.