Snow on Flat Roofs: Myths and Reality in Ontario’s Winter Conditions
Many of our clients ask: Is it safe to leave snow on a flat roof, and should it be cleared? The answer is simple: with proper design and adherence to building codes, flat roofs are completely safe even in Ontario’s harsh winter conditions. In fact, snow often does not accumulate on such roofs due to wind load. In this article, we will debunk myths about flat roofs and explain how snow and wind interact with them.
Flat Roofs and Snow: What the Building Codes Say
In Canada, all roofs are designed according to the National Building Code of Canada and regional standards. These codes account for not only snow load but also wind load. A flat roof is designed to:
- ✔ Withstand the maximum snow load typical for the specific region.
- ✔ Effectively handle wind gusts.
In Ontario, the standard calculated snow load is 2.5–3.5 kPa (kilopascals), equivalent to a layer of dense snow 50–70 cm thick.
Why Flat Roofs are Safe Even in Winter
- Strong Structure: Flat roofs are designed to evenly distribute the load from snow and wind, making them more stable than pitched roofs, where snow can accumulate in certain areas.
- Wind Cleaning Effect:
- Wind plays a key role in removing snow from flat roofs. Wind gusts, especially in open areas, blow snow off the roof, preventing accumulation.
- For example, with an average wind speed of 20 km/h during the winter, in open areas such as suburbs or fields, only a minimal layer of snow may remain on the roof.
- Natural Melting:
- Heat from the building warms the roof, causing slow snow melt.
- Water is drained through internal or external downspouts.
Example Calculation: How Snow and Wind Work on a Flat Roof
Ontario Site, Roof Area of 100 m²:
- Calculated load: 2.5 kPa (equivalent to 25 tons of snow).
- Average wind speed in winter: 20–30 km/h.
- With wind load, about 30–50% of the snow is blown off the roof within the first 24 hours after snowfall, leaving only a safe layer.
In contrast, snow on pitched roofs can accumulate at the lower sections, creating excessive loads on the structure.
Advantages of Flat Roofs Over Traditional Pitched Roofs
- ✔ Even load distribution: Flat roofs handle snow loads better due to even weight distribution.
- ✔ Reduced risk of sudden snow collapse: On pitched roofs, snow may slide off suddenly, posing a danger to people and property.
- ✔ Wind resistance: The design of flat roofs allows them to efficiently deal with wind flows that naturally remove snow.
- ✔ Energy efficiency: Snow on flat roofs creates an additional layer of insulation, reducing heat loss.
When Should You Clear a Flat Roof?
Under normal conditions, flat roofs do not require snow removal. However, in rare cases, it may be necessary:
- Atypical snowfalls: If the amount of snow exceeds the calculated load, which is extremely rare.
- Drainage issues: If the downspouts are clogged, meltwater may not drain in time.
Important: Cleaning should be performed by professionals to avoid damaging the roof’s waterproofing.
Conclusion: Flat Roofs – A Reliable Solution for Ontario’s Winters
Flat roofs are a proven and reliable solution that fully meets the requirements of Ontario’s winter climate. They are safe, energy-efficient, and resistant to snow and wind. Additionally, wind often blows snow off flat roofs, reducing the likelihood of accumulation.
If you want to build a house with a flat roof that meets all building codes, contact us. We will design a project that ensures maximum safety and comfort under any conditions!
