Commercial Roof Replacement: How the Process Works
What happens before, during and after a tear-off on a building that stays open.
Call 1-833-775-7663 Oakville head office · Roof replacements for commercial, industrial and institutional buildings across Ontario
Karol Rajkowski
Vice President, OYH Roofing
Published October 19, 2026 · 14 min read
Short answer
A commercial roof replacement runs in three phases. Before work starts, the roof is surveyed, the scope is written, and permits, warranty registration and rooftop units are sorted out. On the roof, the old roof comes off one section at a time, the deck is checked, and each section is sealed every night. At the end, the maker inspects the roof and issues the warranty.
What are the stages of a commercial roof replacement?
Eight, and three of them happen before anyone tears off a single sheet. A commercial roof replacement removes the old roof down to the deck or the vapour retarder and builds a new roof system in its place. What makes it different from a house is that the building below usually stays open, so every stage is planned around the people, stock and equipment inside.
| Stage | What happens | Who leads it |
|---|---|---|
| 1. Survey | Moisture survey, core cuts, deck check from below, list of every curb, drain and penetration | Roofer or roof consultant |
| 2. Scope and design | New system, insulation, slope to drains, flashing heights; an engineer if the deck or loads are in question | Owner with the roofer or designer |
| 3. Approvals and paperwork | Building permit if needed, warranty registration, street occupation permit, gas and electrical trades booked | Owner and roofer |
| 4. Site setup | Hoist, bins or chute, fenced ground zone, edge protection, interior protection, notices to occupants | Roofer |
| 5. Tear-off in sections | Only what can be made watertight the same day comes off; debris goes down daily | Roofer |
| 6. Deck and insulation | Deck repairs, wet insulation replaced, vapour retarder, tapered insulation and crickets | Roofer, with the engineer for deck repairs |
| 7. New membrane and details | Membrane, flashings at parapets, curbs, drains and penetrations; units reconnected | Roofer, with gas and electrical trades |
| 8. Close-out | Maker's final inspection, punch list, warranty, roof plan and maintenance instructions | Roofer and the membrane maker |
If you have not yet decided between a repair, a recover and a full replacement, start with our repair or replace guide. This page assumes the decision is made and explains how the replacement itself runs. Our warehouse and apartment building guides add what is specific to those buildings.
What happens before a roof replacement starts?
Most of the decisions. A good replacement is settled on paper before the first bin arrives: what comes off, what goes on, who disconnects the rooftop units, and which days the noisy and smelly work happens.
- A survey that sets the scope. A moisture survey confirmed by core cuts shows how much insulation is wet. SOPREMA Canada, for example, asks for a moisture or thermographic survey done within 3 months before work whenever existing membranes are kept. Our repair or replace guide explains the survey methods.
- Permits. Whether a re-roof needs a building permit depends on the municipality and on what changes, such as structure, drainage or roof loads. Our roof replacement permit guide covers Toronto, Mississauga and Brampton, the Notice of Project and asbestos checks on older roofs.
- Warranty registration. IKO wants its Diamond Shield warranty application at least 14 days before the job starts, and SOPREMA Canada may refuse a warranty that was not pre-approved at least two weeks before work. Holcim Elevate logs each job as a Pre-Installation Notice. Only the maker's approved or certified roofers qualify.
- Rooftop units. A roofer does not disconnect gas. Under Ontario's fuel certificate rules, gas appliances are disconnected and reconnected by a TSSA-certified gas technician (a G.2 for units up to 400,000 Btuh, a G.1 for any size), working for a registered gas contractor. Electrical disconnects beyond an existing branch circuit are work for a licensed electrical contractor, and ESA says notifiable work must be filed before it starts. Our rooftop equipment guide covers curbs and lifts.
- The street. In Toronto, a bin, drop chute, material storage, roof hoist, mobile crane or boom truck on the boulevard, sidewalk, road or lane needs a Street Occupation Permit, plus a RoDARS road disruption report.
- Waste. Ontario's waste audit and source separation rules (O. Reg. 102/94 and 103/94) apply to construction or demolition of buildings with at least 2,000 m² of floor area, and must be in place before work begins. They do not name re-roofing, so ask whether your project falls in scope. Their lists include steel and clean wood, not asphalt or membranes.
- Notice to the people inside. The University of Waterloo's safety office asks for occupants to be told before roofing starts, and a U.S. federal re-roofing spec calls for at least 72 hours' notice before work that affects normal building operations.
How does the tear-off work, day by day?
One section at a time, and never more than the crew can close again before it leaves. CRCA's reference manual puts it on the supervisor: work out what the crew can do and make sure no more existing roofing is removed than can be made watertight at the end of each day. It goes further: no roofing should be removed at all unless the space below can be kept watertight.
A typical working day on a commercial roof replacement looks like this:
- Check the forecast and the night seals. Yesterday's temporary seal is cut free and trimmed away before the new section is tied in. Carlisle's instructions describe pulling the embedded membrane free and removing the seal material, so nothing temporary is left in the finished roof.
- Clear loose material first. CRCA advises removing loose gravel and debris before the membrane is ripped off; that part can be done on marginal weather days.
- Tear off the day's section. Old membrane, insulation and wet material come off down to the deck or a sound vapour retarder.
- Get the debris off the roof. Debris may not fall freely; it goes down by chute, container, crane or hoist and is removed at least once a day. CRCA warns that wet debris piles can overload a deck.
- Rebuild the same day. Deck repairs, vapour retarder, insulation and at least the base layers go down on the open area.
- Seal the edge. At the end of the day, the area to be tied in tomorrow is sealed to the finished membrane, so new insulation stays dry if it rains. Drain protection comes off and debris guards go back in, so the drains work overnight.
The night seal matters more than it looks. CRCA notes that insulation left open at a tie-in absorbs moisture that is only surface-dried, which can cause buckling and blistering at that spot years later. Carlisle (U.S.) seals the downslope edge so water cannot run under it, then checks for continuous contact along the whole seal.
What happens when the roof deck is exposed?
The deck gets its first full inspection in decades. CRCA notes that a complete tear-off lets the crew see the deck and parapets and correct any decking or structural defects, and its manual says plainly: do not proceed with the roof until every deficiency is corrected.
| Finding | What usually happens | Who decides |
|---|---|---|
| Deflection, bounce or movement in the deck | Work stops in that area until the deck is assessed and corrected | A professional structural engineer |
| Corroded steel deck | Light rust cleaned and treated; badly corroded or damaged panels replaced | Engineer of record for replacement panels and fastening |
| Wet or damaged insulation | Removed and replaced with new insulation of like thickness, never covered over | Roofer, per the specification |
| Missing or poor vapour retarder | New vapour retarder on top of the deck, below the insulation; a higher grade for humid buildings | Designer, from interior humidity and temperature |
| Water sitting at low spots | Tapered insulation and crickets to move water to the drains | Designer or roofer, per the specification |
| Thicker new insulation at the edges | Wood nailers raised so they are flush with the new build-up | Roofer, per the specification |
Two points are worth knowing. First, deck capacity is not the roofer's call: NRCA (U.S.) says judging whether a steel deck can resist wind uplift is beyond a roofing contractor's expertise and recommends a licensed design professional, and that pull-out tests alone are not an accurate indicator. Second, deck repairs are the one cost nobody can see before tear-off, which is why our guide to comparing quotes suggests asking every bidder for a unit rate for deck work.
Perforated acoustic decks need extra care. CRCA warns that asphalt or adhesive can drip through the openings, so the crew has to stop that before it reaches the space below. Drainage fixes found at this stage are covered in our drainage repair guide.
How do you keep a business running during a roof replacement?
By planning each kind of disruption on its own: smell, dust, noise, access and fire risk. CRCA's manual reminds roofers that on an occupied building the owner may have employees working and special equipment and processes set up inside, and that the crew should keep an open line of communication with the owner.
| Concern | What causes it | How it is usually handled |
|---|---|---|
| Fumes and odour | Hot asphalt, torch work, solvent adhesives and primers | Kettles kept away from and downwind of fresh-air intakes; low-odour or self-adhered products near intakes; closing intakes only as a last resort |
| Dust and debris inside | Tear-off and screws through a steel deck | Catch tarps under the deck, covered stock and machines; netting does not stop fine dust |
| Drips through the deck | Perforated acoustic decks | Openings protected before asphalt or adhesive is applied |
| Noise | Tear-off, hoisting, fastening | Work inside the municipal construction hours, noisy tasks scheduled with the occupants |
| Fire risk | Torches and hot work | Crew learns the fire safety plan, alarms and sprinklers; Fire Code firewatch for torching |
| Access and people below | Hoisting, bins, deck replacement | Fenced ground zone, protected entrances, no one under an area where deck is being removed |
Food, pharmaceutical and clean manufacturing plants need the most care in an industrial flat roof replacement. NRCA's magazine notes that customers in those sectors are especially sensitive to dust and that the owner has the final say on interior protection, so it belongs in the scope, not in a change order. For a plant that has to keep running, see our industrial roof guide, which sets out Ontario's Fire Code rules for torch roofing.
Noise hours differ by city. Toronto's noise by-law (Chapter 591) bars clearly audible construction noise from 7 p.m. to 7 a.m., until 9 a.m. on Saturdays, and all day on Sundays and statutory holidays. Mississauga's by-law bars construction equipment from 7 p.m. to 7 a.m. and all day on Sundays and statutory holidays where the sound reaches a residential area, and from 5 p.m. in a quiet zone.
Planning a roof replacement on a building that has to stay open?
We can walk the roof and the floor below with you and put the phasing, interior protection and schedule in writing before the work is priced.
Call 1-833-775-7663Request an assessmentWhat Ontario safety rules apply during a roof replacement?
Ontario's construction regulation, O. Reg. 213/91, has a short section written for roofing, and it sits on top of the general fall protection rules. The Ministry of Labour's guidance on flat roofs (March 2023) explains how they fit together.
| Topic | What the regulation says | Section |
|---|---|---|
| Barrier back from the edge | When a built-up roof is constructed, repaired or resurfaced, a barrier of weighted posts and a taut chain, cable or rope, 1.1 m high, at least 2 m from the perimeter | s. 207 |
| Work inside 2 m of the edge | The barrier alone is not enough; guardrails first, then travel restraint, fall restricting, fall arrest or nets | ss. 26, 26.1 |
| Openings and skylights | A guardrail or a secured, marked cover strong enough for the load; the Ministry treats every skylight as a fall hazard unless its strength is documented | s. 26.3 |
| Storage on the roof | No material stored within 1.8 m of a roof edge or opening | s. 40 |
| Debris and chutes | No free fall; chute, container, crane or hoist; removed at least daily; chutes fastened and closed when not in use | s. 35 |
| Roof hoists | Guardrails on both sides of the frame, cable kept vertical, competent operator; counterweights not made of roofing material | ss. 209, 210 |
| Hot bitumen | A guardrail at the edge where a hot bitumen pipe discharges within 2 m of it; competent kettle operator and a 4A40BC extinguisher | ss. 208, 211 |
For an owner, the practical point is that edge protection, hoisting and debris handling are visible on day one: if you see roofers working at an unguarded edge or debris thrown off the roof, something is wrong. Owners, staff and occupants should stay off the roof while the work is under way. Our guide to choosing a commercial roofing contractor lists the safety records to ask for before you sign.
How long does a commercial roof replacement take?
It depends on the roof, and no Canadian roofing body publishes a standard production rate. CRCA says roofing schedules are set by the weather, the condition of the deck and even the hours of daylight. The factors below decide most of the schedule:
- Roof area and layers. Two old roofs take longer to remove than one, and gravel has to come off before the membrane.
- Wet insulation and deck repairs. Both are only fully known once the roof is open.
- Rooftop equipment. Every unit that has to be disconnected, lifted or raised adds trades and scheduling.
- Access. A roof hoist versus a crane, street permits, and how far material has to travel.
- Weather. Rain stops tear-off, and the cold slows everything. IKO's December 2025 cold-weather bulletin defines cold weather as below 4°C, says adhesive and sealant drying times are much slower and production times will be longer, and lists 4°C as the minimum for its InnoviBond membrane adhesive.
- Working hours. Noise by-laws and the building's own operations set the length of the working day.
Ask each contractor for a written schedule that shows the order of sections, the expected working days and the assumptions behind them, such as the area of wet insulation. Our repair or replace guide explains when a replacement can wait for spring.
What should you receive when the roof replacement is finished?
A roof that has passed the maker's inspection, and a file to keep with it. The steps after the last sheet goes down are what turn a new roof into a warranted one.
- The maker's final inspection. SOPREMA Canada says a representative must, in all cases, make a final visit to accept the work before it issues a warranty. Carlisle says all warranted roofs are inspected by a field service representative, and IKO's Diamond Shield warranty requires a successful inspection by an IKO Field Services technician.
- A punch list, then the warranty. Defects found at the final visit are corrected before acceptance. IKO may withhold coverage until the punch list is resolved and the materials, installation and warranty are fully paid. Holcim Elevate calls these items Repairs for Warranty.
- A roof plan. SOPREMA requires a detailed roof plan showing the warranted area with every warranty request, and expects that request within 3 months of the end of the project.
- The contractor's own warranty. CRCA's standard form is a two-year workmanship warranty, with an inspection between 12 and 22 months after it starts and maintenance as set out in the maintenance manual.
- Maintenance instructions. Sika Canada's maintenance guide calls for inspections in spring and fall and after storms or new rooftop equipment, asks for leaks to be reported in writing within 7 days, and asks to be notified before anyone cuts into the roof.
Keep the warranty, the roof plan, the product data sheets, the inspection reports and photos together, and give a copy to whoever manages the building. Our warranty guide lists what voids them and our roof maintenance guide sets out a schedule for the years ahead.
How does OYH Roofing run a commercial roof replacement?
OYH Roofing is a flat roof replacement contractor for commercial, industrial and institutional buildings across Ontario, working from its head office in Oakville. We survey the roof and look at the building from inside, put the scope, phasing and schedule in writing, and plan the work around your operations, section by section, with every section sealed before we leave.
Recent replacements include the Devi Mandir in Pickering, where two layers of modified bitumen and a coating came off before a new 60 mil TPO roof went on, and the 50,000 sq ft Millomat Stampings roof in Mississauga, which combined a re-roof with a new addition under one TPO system. See also the 777 Niagara St roof in Welland, the City of Mississauga building, our modified bitumen installation guide and our other completed projects.
Frequently asked questions
What are the stages of a roof replacement process?
On a commercial flat roof there are eight: survey, scope and design, approvals and warranty registration, site setup, tear-off in sections, deck and insulation work, the new membrane and flashings, and close-out with the maker's final inspection. The first three happen before any roofing comes off, and each section is sealed watertight at the end of every working day.
How disruptive is replacing a roof?
Less than most owners expect, if it is planned. Most commercial buildings stay open, because the roof is replaced in sections and sealed every night. The real disruptions are noise during working hours, odour near fresh-air intakes, dust falling through steel decks, and hoists or bins at ground level. Each can be planned for with the occupants before work starts.
What are the steps involved in the roof tear-off process?
Check the forecast and remove yesterday's temporary seal, clear loose gravel and debris, tear off only the section that can be closed again the same day, lower the debris by chute, container or hoist, repair the deck and install new insulation, then seal the edge to the finished roof before the crew leaves. Ontario requires debris to be removed at least daily.
What does "tear off" mean in roofing?
A tear-off removes the existing roof covering, usually with its insulation, down to the deck or a sound vapour retarder, before a new roof is installed. The alternative is a recover, which installs a new roof over the old one. A tear-off lets the crew inspect and repair the deck and replace wet insulation, and it usually qualifies for a longer warranty.
What is the most expensive part of replacing a roof?
On a commercial flat roof, the biggest variables are usually the ones nobody can see until the old roof is off: how much insulation is wet and how much of the deck needs repair. Their extent decides how much material and labour the job needs, which is why a moisture survey before tendering matters. Rooftop equipment, cranes and access add to it on some buildings.
Ready to plan your roof replacement?
Tell us about your building and how it operates, and we will set up a roof assessment and a written scope with a schedule.
Call 1-833-775-7663Request an assessment
Karol Rajkowski
Vice President · OYH Roofing
Karol Rajkowski is Vice President of OYH Roofing, an Oakville-based ICI flat-roofing contractor whose projects include the City of Mississauga building at 3185 Mavis Rd and the Marriott Hotel in Vaughan. Learn more about the OYH team.
Sources
- CRCA, Canadian Roofing Reference Manual (2020): 3.1.3 inspection of roof deck, 3.1.6 vapour retarders, 3.2.1 re-roofing operations, 3.2.2 removal and disposal, 3.2.3 keeping the roof watertight, 3.2.5 fire protection, 3.2.6 additional precautions, 3.3.1 project schedule; standard form of warranty (2024).
- Ontario: O. Reg. 213/91, Construction Projects; Ministry of Labour, Immigration, Training and Skills Development, Construction projects on flat roofs (March 1, 2023) and Workers falling through skylights (2013); O. Reg. 102/94 and 103/94; O. Reg. 215/01 and 212/01 (TSSA); O. Reg. 570/05 and 164/99; Electrical Safety Authority, filing a notification.
- Municipal: City of Toronto, Municipal Code Chapter 591, Noise and Street Occupation Permit; City of Mississauga, Noise Control By-law 0360-1979.
- IKO: Diamond Shield warranty guide, Canada (February 2023); IKO Approved Applicator Program (September 2024); Innovi TPO cold weather guidelines (December 2025).
- SOPREMA Canada: General Conditions for roofing warranties (March 30, 2026). Holcim Elevate: warranty services update (Canada) and technical bulletin (January 2024, U.S.). Carlisle (U.S.): warranty services, A Daily Seal for Roofing Projects and Wood Nailers for Roofing (2020). Sika Canada: Sarnafil roof maintenance guide (2020).
- NRCA, Professional Roofing (U.S.): Low-slope reroofing guidelines (2005), Think inside the box (2005), Dealing with debris (2009), The situation with steel decks (2017). NIOSH: Reducing Roofers' Exposure to Asphalt Fumes (2003, U.S.). New York State Department of Health: indoor air quality during roofing projects (2023, U.S.). University of Waterloo Safety Office: roofing projects. U.S. Department of Veterans Affairs: specification 07 01 50.19, preparation for re-roofing (2021, U.S.).
- OYH Roofing project records: Devi Mandir, Pickering; Millomat Stampings, Mississauga; 777 Niagara St, Welland; City of Mississauga, 3185 Mavis Rd (October 2026).
OYH Roofing, 2120 Speers Rd, Oakville, ON L6L 2X8 · 1-833-775-7663 · info@oyhroofing.ca