Flat Roof Replacement for Commercial Buildings
Low-slope tear-off and rebuild on occupied buildings across Toronto, the GTA and southwestern Ontario.
A flat roof needs replacing when moisture has spread through the insulation, the membrane has failed across the field rather than at details, repairs keep returning to the same areas, or the deck is deteriorating. NorthCan Roofing confirms all four with a moisture survey and test cuts before quoting a tear-off. Call 416-456-0777.
How do you know a flat roof is past repair?
Age alone proves nothing. These are the conditions that actually end a low-slope roof.
- Wet insulation across the field. Once water is in the insulation it spreads laterally and cannot be dried in place. An infrared or capacitance survey shows the extent. When the wet area is a large share of the roof, cutting it out stops being a repair.
- Field-wide membrane failure. Granule loss and exposed bitumen on an SBS or built-up cap sheet, widespread alligatoring, crazing or blistering, single-ply membrane that has shrunk and is pulling its base flashings down off the wall, or seams opening in more than one location.
- Repeat repairs. Three visits to the same quadrant means the cause is systemic, usually drainage, and the money is going into the wrong place.
- Deck deterioration. Corroded steel flutes under long-term wet insulation, fasteners that no longer hold, rotten wood decking at the perimeter or at drains, spalling concrete.
- Drainage that cannot be fixed from above. Deep, permanent ponding with no fall to the drains, which needs tapered insulation designed into a new assembly.
- A change to the building. New rooftop equipment, a change of use, a higher interior humidity load, or a solar or screen installation that the existing assembly was never designed to carry.
If only one or two of these are present and they are local, read flat roof repairs first. We would rather sell you a repair that works than a replacement you did not need.
What does the tear-off sequence look like?
A low-slope tear-off follows the same order on almost every building.
- Set up. Crane or hoist positioned, staging and dumpster located, ground protection down, perimeter guardrail or travel-restraint anchors installed, controlled access zones marked. Fresh-air intakes identified and protected.
- Strip. Membrane, cover board, insulation and vapour barrier removed down to the deck, in the area planned for that day and no more. Debris goes straight into the chute or bin rather than piling on the roof.
- Inspect and repair the deck. The exposed deck is checked for corrosion, rot, deflection and fastener holding. Repairs are made at the unit rate carried on the quote. Anything structural goes to a professional engineer.
- Vapour barrier. Installed and sealed at the perimeter and at every penetration, selected for the interior humidity load of the building.
- Insulation. Two or more staggered layers with offset joints, plus tapered insulation and crickets to drive water positively to the drains, then a cover board over the whole field.
- Membrane. The specified system installed with the fastening pattern increased in the perimeter and corner zones, where uplift is highest.
- Details. Cant strips at wall junctions, base flashing carried to an adequate height, new cap flashing over the parapet, drains reset and clamped with new flashings, scuppers lined, penetrations flashed properly rather than sealed with mastic.
- Close out. Water test where appropriate, final walk with your representative, photo record, warranty documents and a maintenance schedule.
The strategic decisions behind this, including recover versus tear-off and code-driven insulation upgrades, are set out on commercial roof installation and replacement.
When can flat roofing be done in Ontario?
Longer than most people assume, but not without rules. Weather windows shape both the schedule and the specification.
- Spring and fall are the busiest windows. Moderate temperatures suit adhered assemblies and torch work alike, but rain frequency is high, so daily watertight stopping points matter more than raw crew size.
- Summer gives the longest reliable dry runs and the strongest daily production on large open fields such as warehouses. Heat brings its own limits: membrane surface temperatures, adhesive open times and crew heat-stress management.
- Winter work is possible on many systems and is routine in this market, with conditions. The deck and substrate must be clean, dry and free of snow, frost and ice. Adhesives and fasteners are selected for low temperature, some materials need warming and staging indoors, and self-adhered products have minimum application temperatures. Where a full tear-off cannot be done safely, we phase smaller sections or dry the roof in and complete in the spring.
Frozen or snow-covered decks and high wind are hard stops. Membrane sheets act as sails, and no schedule justifies putting a crew at an open edge in a gale. Freeze-thaw cycling is also the reason we do not leave a partially completed assembly over a winter unless it has been properly terminated and made watertight.
Which system goes back on?
For most GTA commercial buildings the shortlist is SBS modified bitumen, TPO, PVC or EPDM. SBS gives a redundant two-ply build that tolerates heavy foot traffic and dense rooftop plant. TPO covers a large uninterrupted field quickly with heat-welded seams. PVC handles grease and chemical exhaust, which is why it appears on restaurants and food processing plants. EPDM stays flexible through cold weather. Built-up roofing is still repaired regularly but is specified less often on new work. Metal options are on commercial metal roofing, and the full comparison of composition, seaming, cold-weather behaviour and failure modes is on flat roof types.
Whatever goes on the surface, the assembly below it decides the outcome: the right vapour barrier for the building, insulation to current Ontario Building Code requirements for the project, tapered insulation that actually drains, a cover board, and a fastening pattern that respects the perimeter and corner zones. Detail work at parapets and walls is described on roof flashing and siding.
How do you plan and budget a replacement?
Start with a documented condition assessment. A roof condition report gives you a remaining service life opinion, deficiencies ranked by priority and photographs, which is exactly the material a capital plan or a board submission needs. Replacements are rarely urgent when they are planned and always expensive when they are not.
For a first budgeting pass, the flat roof cost calculator shows how system choice, tear-off versus recover, insulation, deck condition, access and drainage work move the range. It is a planning tool, not a quote.
Then book a free roof assessment. We attend, take test cuts, confirm the assembly and return a written quote with the scope described layer by layer. A workmanship warranty and a manufacturer system warranty are issued in writing with the job. WSIB coverage and certificates of insurance are provided on request. Call 416-456-0777 or use the contact page. Once the roof is new, keep it that way with a maintenance program.
Frequently asked questions
Do we have to move out during a flat roof replacement?
No. Almost all of our replacements run over occupied buildings. The roof is phased so only the area that can be closed in the same day is opened, hours are agreed with the property manager, and fresh-air intakes are protected during hot work. Noise during tear-off and fastening is the main disruption.
Can you replace a flat roof in winter in Toronto?
Often yes. Many membrane systems can be installed at low temperature when the deck is clean, dry and free of snow and frost and the adhesives and fasteners suit the conditions. What we will not do is build a new assembly over a frozen or wet substrate. Where conditions do not allow, we dry in and complete in spring.
What happens to the old roofing material?
It is removed to a regulated waste disposal facility. Tear-off generates a large volume, which is why the bin location, the chute and the truck schedule are planned before the first sheet comes off. Some materials can be diverted for recycling depending on the assembly and the receiving facility.
Will the new roof sit higher than the old one?
Usually a little, because rebuilding to current code requirements normally means more insulation, and tapered insulation adds height where it builds up. That is why base flashing heights, curbs, door sills and parapet caps are re-detailed as part of the work rather than reused as found.
Should we add tapered insulation, or keep the existing slope?
If your roof ponds, tapered insulation is the most valuable thing you can buy during a replacement. Standing water accelerates every failure mode through freeze-thaw cycling, adds load, and shortens the life of the membrane you just paid for. It is far cheaper to design drainage into a tear-off than to correct it later.
Ready for your free roof assessment?
Tell us the address, the roof size and what you are seeing. We will book a time to get up there and put it in writing.
Call 416-456-0777 for anything urgent. Our emergency line is answered around the clock.
Call 416-456-0777