Commercial Roof Storm Damage: What to Do After High Winds

Karol Rajkowski

Vice President, OYH Roofing

Published October 14, 2026 · 12 min read

How does wind damage a flat roof?

By suction. Wind passing over a flat roof pulls up on it, hardest at the corners and along the edges, so most commercial roof storm damage starts wherever something lets go first: edge metal, a seam or a row of fasteners.

The Canadian Roofing Contractors Association (CRCA) puts the highest uplift pressures at the corners, then the perimeter, then the field: corners take roughly three times the basic pressure and perimeters twice. A National Research Council Canada (NRC) paper calls the perimeters and corners the most vulnerable areas of a low-slope roof.

The edge is where most blow-offs begin. An NRC wind study reports that field data collected after four major hurricanes showed the majority of roof failures were due to the failure of metal roof edges. Once the edge lifts, wind gets under the membrane and peels it back.

• Mechanically attached single-ply (TPO, PVC, some EPDM): in NRC's tests the membrane billows at high uplift, bending the fastener plates until it tears around them.
• Ballasted roofs: the Single Ply Roofing Industry (SPRI) warns that ballast can scour away at the corners and perimeter in high winds.
• Every roof: the Roofing Contractors Association of British Columbia (RCABC) links storm leaks to punctures from wind-toppled equipment, missing hatch covers and loose guy wires.

Hail is the other storm hazard. An FM Global research scientist says most roofing systems start to fail when hail reaches 38 mm (1.5 in.), and NRCA's magazine notes that hail can knock the granules off modified bitumen and leave fractures that later leak.

What does commercial roof storm damage look like?

Mostly like trouble at the perimeter: bent or missing edge metal, loose membrane near the corners, and ballast or gravel blown out of place. Inside, it shows up as new stains near the outside walls and around rooftop units, sometimes days after the storm.

Not every leak after a storm comes through the roof: wind also drives rain into rooftop units, walls and windows. Our flat roof leak detection guide explains how roofers trace them, and ponding water on a flat roof covers drains blocked by debris.

How much wind can a flat roof withstand in Ontario?

As much as its design calls for, and in Ontario that design starts from the Building Code's local wind pressures. Across most of the GTA, the code's 1-in-50 design figure equals an hourly wind of about 94 km/h, and the Lake Ontario shore runs a little higher.

The code's Supplementary Standard SB-1 gives two hourly wind pressures for each place, with a 1-in-10 and a 1-in-50 chance of being exceeded in any year. They are one-hour averages at 10 m in open country, so short gusts are stronger, and the designer adjusts them for building height, surroundings and roof zone.

From those pressures, a structural engineer or architect calculates the wind load on each roof zone, CRCA explains, and the tested roof assembly must meet or exceed it. The Building Code requires the wind uplift resistance of membrane roofs to be determined under CSA A123.21, a test of the whole assembly; loose-laid ballasted roofs fall outside it.

Real storms go past design values. In the derecho of May 21, 2022, Environment Canada reports a downburst of 190 km/h in south Ottawa and a tornado with a maximum wind speed of 195 km/h at Uxbridge. The Insurance Bureau of Canada (IBC) put insured damage at $1 billion and said wind caused most of it. Ontario also leads Canada with 18.2 tornadoes a year over 1991 to 2020, according to Western University's Northern Tornadoes Project.

Environment Canada's warnings no longer use fixed wind speeds: since November 26, 2025, alerts are colour-coded by expected impact. Its Alert Ready alerts go out only for tornado warnings and for severe thunderstorm warnings with gusts of 130 km/h or more or hail of 7 cm or more.

What should you do in the first 24 hours after a storm?

Make the building safe, stop the water spreading inside, record the damage, and call your roofer, your insurer and, if the roof is under warranty, its maker. Nobody from the building should go up on the roof; that part belongs to a roofing crew once the storm has passed.

• Keep people off the roof and away from the walls below it. Ontario's construction rules require fall protection where a worker could fall more than 3 metres, and a damaged roof can hide wet, loose or torn areas. Loose edge metal can fall, so close off the ground below a damaged edge.
• Treat water and electricity as one hazard. RCABC tells crews to close off endangered areas and shut off power if necessary. The Electrical Safety Authority says not to use equipment that has been in contact with water, and to stay 10 metres back from a downed power line.
• Contain the water. Move stock and equipment and catch the drips.
• Document everything. IBC suggests photos, detailed notes and every cleanup receipt, and keeping damaged items unless they are a health hazard. Note when the storm hit and when each leak started.
• Report it early. IBC asks people to contact their insurer as soon as possible, and one roof warranty below gives you only one business day to report a leak.

How does emergency roof tarping work on a flat roof?

On a flat roof, emergency roof tarping usually means a cover or patch that a roofing crew holds down with ballast once it is safe to go up. The crew clears the drains first, then covers the torn area and weighs it down.

RCABC's manual lists the tools: battens, sandbags, concrete blocks, and tarps "applied and ballasted" where the membrane is gone; on steel or nailable decks, battens can be fastened through the membrane. It calls these repairs "temporary only", and says to record where each patch is and to check and redo them often if bad weather goes on for weeks.

The insurance side agrees. The statutory conditions in Ontario's Insurance Act ask the insured to take all reasonable steps to prevent further damage, and say the insurer contributes to reasonable expenses for doing so. Keep the invoice for the temporary work.

If the roof is under warranty, check who may do the work. Some makers allow emergency repairs and some do not:

Carlisle's owner's manual adds that an emergency patch should use its lap sealant or a good rubber caulk, never asphalt roof cement, and that Carlisle does not pay for emergency repairs. Our commercial roof warranty guide explains the warranty types.

Is commercial roof storm damage a warranty or an insurance claim?

Usually an insurance claim. Roof warranties cover defects in materials or installation, and most exclude strong wind, hail and other storms, as the table shows. Sudden storm damage is the kind of loss property insurance is written for.

IBC's guidance after the September 2, 2026 storms in the GTA says standard home and business policies cover hail damage, including water that gets in through openings the hail suddenly created, but not pre-existing damage. Wear and tear is the dividing line: a storm that tears off edge metal is sudden, a membrane cracking for years is not. Our roof leak insurance guide explains how insurers draw that line.

How soon must you report it? There is no single grace period. The Insurance Act's statutory conditions for fire insurance ask for written notice "forthwith" and proof of loss "as soon as practicable", and give one year to sue the insurer. Commercial policies set their own conditions for storm and water claims, so read yours and ask your broker. This is a summary, not legal advice. Report to the roof's maker at the same time, because warranty clocks are shorter.

Should a storm-damaged roof be repaired or replaced?

Repaired, when the damage is local: a lifted edge, a torn corner, a puncture, a loose flashing, with dry insulation around it. Replacement comes into the picture when large areas of membrane are loose or gone, the insulation is soaked, or the deck itself was damaged.

Wet insulation is the deciding factor, and it does not show from the surface. After a storm with leaks, ask for a moisture scan and core cuts before agreeing to a scope; our membrane roof repair and commercial roof inspection guides explain both.

If the roof is replaced, ask for the new assembly's wind uplift design, with its corners and perimeter designed for the higher loads they carry. NRC's study of hurricane damage found that roofs survived small initial failures when the peel forces were resisted. Check whether you need a permit with our roof replacement permit guide, and see the commercial flat roof lifespan guide if the roof was near the end of its life anyway.

How do you prepare a flat roof for the next storm?

Fix the edges, clear the drains, tie down what can move, and know who you will call. Most of it is ordinary maintenance, done before the windy months.

• Inspect in spring and fall, and after every major storm. Ask the roofer to check edge metal, copings and perimeter seams, where wind damage starts.
• Keep drains and scuppers clear. A storm that brings wind usually brings rain and debris too.
• Secure what is loose. Rooftop unit panels, hatch covers, guy wires, signs and anything stored on the roof. The Infrastructure Health and Safety Association (IHSA) tells crews to secure material so the wind cannot move it and to keep it at least 1.8 metres from an edge.
• Keep a roof access log. IKO's owner's guide recommends recording everyone who goes on the roof, which helps show whether damage came from the storm or from earlier work.
• Keep the paperwork in one place. Warranty number, installer, manufacturer contact, insurer and broker, so the first call after a storm takes minutes.

Our commercial roof maintenance guide sets out a yearly schedule, and preparing a flat roof for winter covers the fall visit.

How does OYH Roofing handle commercial roof storm damage?

Storm damage is one of the emergency calls OYH Roofing takes for commercial, industrial and institutional flat roofs across Ontario. A temporary seal stops the water first, the permanent repair follows once the surface is dry, and you get a written report with photos for your insurer and the roof's manufacturer.

For local leak service, see commercial roof leak repair in Toronto and in Mississauga. The roofs we install, such as the 6,500 sq ft TPO re-roof at Devi Mandir in Pickering and the 53,000 sq ft industrial roof recover in Mississauga, are on the completed projects page.

Frequently asked questions

What does wind damage look like on a roof?

On a flat roof, look for edge metal or coping that is bent, lifted or missing, membrane that billows, wrinkles or has come loose near the corners, ballast or gravel blown into piles, and punctures from debris. Inside, watch for new stains near outside walls and rooftop units. Wind suction is strongest at the corners and edges, so damage usually starts there.

How much wind can a flat roof withstand?

As much as it was designed and tested for. Ontario's Building Code requires membrane roofs to resist wind uplift tested under CSA A123.21, using local wind pressures. Around most of the GTA, the 1-in-50 design pressure equals an hourly wind of about 94 km/h, and up to 98 km/h along the lakeshore. Gusts are stronger than hourly averages, and corners carry the highest loads.

How fast does wind have to be to rip a roof off?

There is no single speed. A roof that is well attached at its edges can come through very strong winds, while a loose edge or a failed seam can let go at much lower speeds, and the damage spreads from there. Field data reviewed by the National Research Council Canada showed that most commercial roof failures in major hurricanes started at the metal roof edge.

How long can a tarp protect a roof?

Only until a permanent repair can be made, and it needs checking. The Roofing Contractors Association of British Columbia treats storm patches as temporary and says to check and redo them often if bad weather continues. In the United States, the Army Corps of Engineers describes its Operation Blue Roof tarps on homes as a 30-day temporary fix.

How long after a storm can you claim damage?

Report it as soon as you can. The statutory conditions in Ontario's Insurance Act for fire insurance ask for written notice forthwith and proof of loss as soon as practicable, and roof warranties set their own limits, from the next business day to 30 days. Your commercial policy has its own wording, so check it with your broker.

What is the best roofing material for high winds?

No membrane wins on its own. CSA A123.21 tests a whole roof assembly, not a material, and Canadian research shows a roof fails at its weakest link, whether that is the fasteners, the seams, the adhesive or the edge. A tested assembly with well secured edges and corners matters more than the choice between TPO, PVC, EPDM and modified bitumen.

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

• 2024 Building Code Compendium, Volume 1 (5.2.2.2, wind uplift of membrane roofing) and Volume 2 (Supplementary Standard SB-1, Table 2), January 16, 2025 edition, Publications Ontario.
• CRCA: Wind Uplift for Roofing Systems: Understanding the wind uplift standard, Canadian Roofing Expo, April 16, 2025.
• National Research Council Canada: Baskaran, Molleti and Roodvoets, Understanding Low-Sloped Roofs Under Hurricane Charley, Journal of ASTM International, 2007; Roof Edge Blow Offs: NRC wind study, December 2015; Baskaran, Ko and Shyti, How to Optimize Wind Resistance for Low Slope Membrane Roofing Systems, CRCA, 2023.
• SPRI: ANSI/SPRI RP-4 and RP-14, Wind Design Seminar, October 2024.
• RCABC Roofing Practices Manual: Roof Maintenance, sections 4.1 to 4.3 (British Columbia).
• FM Approvals: New Tools Can Help Reduce Losses from Hail, October 18, 2018. NRCA, Professional Roofing: Standing up to hail, June 2007.
• Environment and Climate Change Canada: Canada's top 10 weather stories of 2022; Weather alerts; new weather alert system, November 26, 2025.
• Insurance Bureau of Canada: derecho storm ranks 6th largest, June 15, 2022; severe weather in 2022, January 18, 2023; Severe storms hit the GTA and southern Ontario, September 3, 2026.
• Western University, Northern Tornadoes Project: A new 1991 to 2020 Canadian tornado database, 2024.
• Sika Canada: Sarnafil System Warranty, sample, March 2022, and Warranty Selection Guide, March 2023. IKO: BUR Limited Labour and Material Warranty and Roofing System Building Owner's Guide, February 2026.
• Johns Manville: Peak Advantage Guarantee, sample (U.S.). Elevate: Red Shield 100 MPH Limited Warranty, form 1003.001.2026 (U.S.). Carlisle: Owner's Manual, March 2025.
• Ontario: Insurance Act, s. 148, statutory conditions 6, 9 and 14; O. Reg. 213/91, Construction Projects, s. 26.
• Electrical Safety Authority: flooding hazards, March 4, 2025, and flooding safety tips. IHSA: Falling-object hazards safety talk.
• U.S. Army Corps of Engineers: Operation Blue Roof hitting its stride, December 19, 2017 (U.S.).
• OYH Roofing project records: Devi Mandir, Pickering; Mississauga industrial recover (October 2026).

OYH Roofing, 2120 Speers Rd, Oakville, ON L6L 2X8 · 1-833-775-7663 · info@oyhroofing.ca