Metal buildings are designed to accommodate snow build-up on roofs based on the local snow load requirements. While under typical snow storms, the roof is well equipped to handle the weight of the snow/ice build-up, some conditions can cause the load to exceed the capacity the roof was designed for. It is important to understand what your roof is designed to handle, and what to do if you find yourself in a situation where you believe the snow build-up on the roof is becoming an issue.
Metal building roofs are designed to handle a snow load. The snow load the roof is designed for is based on your geographic area’s ground snow load. The ground snow load is reduced dependent on building-specific conditions to arrive at a roof snow load. Factors that affect the degree the ground snow load is reduced include:
If the building is heated, it is assumed that there will be some snow melt and the roof snow load will be less than a completely unheated building. The building insulation system also comes into play because with a very well-insulated roof system, there will be less snow melt.
Metal roofs are inherently “slippery,” and if there are no roof obstructions, the snow load can be reduced by a larger degree than if the roof is not slippery or obstructed. An example of a non-slippery roof would be a traditional shingled roof. An example of a roof obstruction would be a snow retention system. Because snow retention systems are often added after the initial design, we design all of our roofs as obstructed/not slippery.
In the Niagara & Erie Counties of Western New York, the ground snow load is 50 psf (pounds per square foot). Without getting into the math, a standard occupancy heated structure with a roof insulation system greater than R-30 and an obstructed/not slippery roof will have a roof snow load of 31.50 psf.
If you are constructing a new building and hiring a local metal building contractor, odds are your new roof will be designed accordingly. The more significant concern will be if your building will be an expansion off of an existing building. While this article’s aim is not to go into detail about snow load design, you should know that if you are expanding off of an existing building, the existing building needs to be looked at and analyzed for how snow drift caused by the new building will affect it. This is not the responsibility of the metal building manufacturer; it is the responsibility of the project’s architect or engineer of record.
Like all conditions designed for, you will not have to worry about your roof collapsing under the weight of snow in most normal snowfalls. It is hard to know the exact weight of snow/ice on your roof because the condition of the snow can significantly affect the weight of the snow.
If you have a fresh snowfall of 1′-0 depth on your roof, it can weigh as little as 3 psf or as much as 50 psf. If it is ice, the weight is even higher. This makes determining if a specific snowfall will cause an issue a guessing game. Often building owners realize the snow accumulation is becoming an issue because they recognize warning signs like unusual cracking sounds and major deflection of purlins.
The number one thing you can do to prevent excessive snow accumulations is some basic preventative maintenance. And the most important preventative maintenance activity is to ensure the gutters and downspouts are clear and free of debris. Doing so will allow snow melt to run off the roof. If the snow melt cannot get off the roof, it can end up icing up and compounding the weight of the snow.
In addition to preventative maintenance, you can install heat tracer cables in the gutters and downspouts to prevent ice build-up. Heat tracer cables are great, but if they are not installed correctly, they waste time and money. Heat tracer cable systems should be designed and installed by a licensed electrician.
If you suspect the snow accumulation on your roof is becoming an issue, and you have observed some warning signs that the roof is under stress, removing snow from the roof may be necessary. Anytime you work on a roof, you must take safety precautions, such as appropriately being tied off. Working on slippery snow and ice makes this even more critical.
When you remove snow from the roof, you want to do it as uniformly as possible. You want to keep the snow evenly distributed, so you don’t compound the issue with an imbalanced snow load. If you shovel snow downhill from the ridge and pile it up halfway down the roof because you can’t push it any further, that will cause an issue. So take it slow and remove snow in layers if necessary. Start with the snow drift areas by bringing the level down the meet the rest of the roof.
As for tools, make sure you use plastic shovels or brooms that will not damage the roof.
You can analyze the snow condition of your roof and even remove the snow on your own, but I would not recommend it. The safest bet is to contact a professional – the contractor who constructed your building.
First and foremost, then can assess the situation and determine if the snow accumulation is even an issue. If snow accumulation is an issue, they have trained personnel that can safely remove the snow from the roof. And in the rare situation of an actual roof failure, they have a direct line of communication with the building manufacturer to document the issue properly and expedite the appropriate remedy.