Ontario Snow Loads Calculator

Ontario follows the Ontario Building Code, which uses the National Building Code method. The specified snow load is S equals Is times the quantity Ss times Cb times Cw times Cs times Ca, plus Sr. In plain terms, you start with the ground snow load for your location, adjust it for the roof with a basic factor, a wind exposure factor, a slope factor, and an accumulation or shape factor, add an associated rain load, and then scale the whole thing by an importance factor that reflects how critical the building is. The ground snow load and the rain load come from the code's climatic data in Supplementary Standard SB-1 and vary significantly across the province. The result is the specified snow load in kilopascals, which is what a designer uses to size the roof structure. For designing individual structural members under Part 4, that specified load is then multiplied by a load factor of 1.5. This calculator performs the same arithmetic for a simple roof so you can see each factor and understand what drives the number, but the sealed value for a permit must come from a licensed engineer for your exact site and roof.

Cb is the basic roof snow load factor, and it accounts for the fact that a roof does not hold the full ground snow load uniformly. Under the code, Cb is 0.8 for the great majority of roofs, including essentially all houses and small buildings, and it only begins to increase toward 1.0 for very large roofs where snow has more room to accumulate. This is a common point of confusion, because some simplified calculators use much lower numbers like 0.45 or 0.55, or assume the factor drops on narrow roofs. That is not how the code works. For a typical residential or small commercial roof you should use Cb equal to 0.8, and using a lower value understates the load, which is the dangerous direction to be wrong in a province with real winters. If your roof has a plan dimension larger than roughly seventy metres, the basic factor is calculated with a formula and the result should come from an engineer. This calculator uses the correct 0.8 for typical roofs so its estimates do not read low.

Steeper roofs shed snow more readily, so the code reduces the load through the slope factor Cs. For a standard roof surface such as asphalt shingles or wood, Cs stays at 1.0 up to a thirty degree slope, then decreases linearly to zero as the slope approaches seventy degrees. For a slippery roof such as smooth metal or glass, snow slides off sooner, so Cs stays at 1.0 only up to fifteen degrees and then decreases to zero by sixty degrees. That is why a metal roof at a given pitch can carry less design snow than a shingled roof at the same pitch. It is important not to over-rely on this, however, because snow does not always slide cleanly, snow guards or obstructions can hold it in place, and sliding snow has to land somewhere, potentially overloading a lower roof. The calculator applies the correct slope formulas for both surface types, but the decision about whether a slippery-roof reduction is appropriate for your specific design is a judgment an engineer should confirm.

This calculator is a planning tool, and it is genuinely useful for understanding the drivers and having an informed conversation with your designer, but it is not a substitute for engineered design. You need a licensed professional engineer whenever a building permit requires structural sign-off, whenever the roof is anything beyond a simple single-slope or gable shape, and whenever there are multiple levels, valleys, dormers, parapets, large skylights, rooftop equipment, or any situation where snow can drift off a taller surface onto a lower one. Those drift and sliding loads can be much larger than the uniform load this tool estimates, and they are exactly where roofs get into trouble. An engineer also confirms the correct ground snow and rain values for your precise location from SB-1, checks unbalanced and partial loading, and seals the calculation so it satisfies the building department. The most cost-effective path is to use a tool like this to plan, and then bring a professional in early so the structure is designed once, correctly, rather than revised after a plan reviewer flags it. We coordinate that engineering as part of our design and build service.

Yes, and it is one of the most important local points for anyone building in the region. The Georgian Bay shoreline and the highlands behind it, including areas around Collingwood, the Blue Mountains, and parts of Grey and Simcoe, sit in a well-known snowbelt where lake-effect snow produces some of the heaviest accumulations in southern Ontario. That means the ground snow load used for design can be higher than the value for a nearby larger city, and using a too-low number is a real risk. The building code's SB-1 climatic data assigns specific ground snow and rain values by location precisely so that design reflects these local differences, and a competent designer will look up the correct value for the exact municipality rather than assuming. This calculator lets you pick a range of Ontario locations to see how much the load changes, but for a snowbelt site the safe approach is to confirm the SB-1 value for your specific address, because in this part of the province the difference between guessing and checking can be the difference between a roof that lasts and one that does not.

The specified snow load is the realistic design snow load itself, expressed in kilopascals, and it is what this calculator reports. The factored snow load is that value multiplied by a load factor for strength design. Under Part 4 of the building code, the load factor for snow at the ultimate limit state is 1.5, so when an engineer sizes beams, rafters, trusses, and their connections for strength, they design for one and a half times the specified load. The specified value, without the factor, is what is used for serviceability checks such as deflection, and it is the number people usually mean when they ask what the snow load is. The reason for the factor is safety margin, ensuring the structure has reserve capacity beyond the expected fifty-year snow event. This calculator shows both the specified load and, in the breakdown, the factored value at 1.5 times, so you can see the number a structural designer will actually design members for. Both come from the same starting calculation, just at different stages of the design process.

It can, and it does happen, most often on roofs that were under-designed, poorly maintained, or subjected to loads the original design did not anticipate. The classic failure situations are not simply a big snowfall on a well-designed new roof, but rather drifting snow piling deep against a wall or a taller adjacent roof, sliding snow dumping onto a lower roof, rain falling on an existing snowpack and adding weight while blocking drainage, and older or altered structures whose capacity was reduced by a renovation. Long-span roofs, flat and low-slope roofs, and additions like sunrooms and carports are more vulnerable than a typical steep house roof. The protective steps are to design for the correct code load in the first place, to account for drift and sliding where taller surfaces are nearby, to keep roof drains and low areas clear, and to take deep or crusted accumulations seriously, removing snow safely when it builds up beyond what the roof was designed for. If you are worried about an existing roof or an addition, a structural assessment by an engineer is the right call, and it is something we can help arrange.