WHY SO MANY QUESTIONS – Dave Kowch February 2019
Snow Guard design is dependent on a multitude of variables which need to be taken into consideration when designing and installing a snow guard system which is why we ask so many questions.
“Snow loads on roofs vary according to geographical location (climate), site exposure, shape and type of roof, and also from one winter to another. To account for these varying conditions, NBC Subsection 4.1.6. expresses the specified snow load, S, on a roof or other surface as the sum of two components—one being the product of a series of factors—multiplied by an importance factor:”
“Structural Commentaries (User’s Guide – NBC 2015: Part 4 of Division B)”
where
Is = importance factor for snow load, Ss = ground snow load, in kPa, with a 1-in-50 probability of exceedance per year,
Cb = basic roof snow load factor, Cw = wind exposure factor,
Cs = roof slope factor, Ca = accumulation factor, and
Sr = associated rain load, in kPa, (however, the rain load at any location on a roof need not be taken greater than the load due to snow, i.e., Sr ≤ Ss(CbCwCsCa)).
“Structural Commentaries (User’s Guide – NBC 2015: Part 4 of Division B)”
WHAT WE NEED TO KNOW
These factors are not the only ones to consider when it comes to designing proper snow guards;
ONLINE FORM
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- Roof panel manufacturer and model example – Vicwest Tradition 100
- Pitch or slope of roof
- Ridge to eave distance
- Assembly length or eave length looking to cover
- Architectural designs such as dormers and valleys created by dormers
- Spacing between standing seams
- Vent stacks, sky lights, air conditioning units
- Upper roofs shedding snow down to lower roofs
- Snow pile up due to adjacent roofs or roof top units
- Unbalanced snow loads on roof
- Single slope roof, Gable roof, Curved roof
Once Sky Products Ltd has all the information to the questions we can begin the design process by cross referencing the clamp to seam load test from S-5! based on the roof panel manufacturer and produce a load calculation.
SAMPLE LOAD CALCULATION
VENTS AND OTHER ROOF TOP EQUIPMENT
Often times we also receive questions about stack vents and other protrusions and should they be protected; according to the National Building Code of Canada 2015… “When a large amount of snow is deposited on slippery sloping roofs, it has been known to shear off vents, chimneys, aerials, wiring, stacks, skylights and ventilators when it slides. It is a menace to people and things below when it falls. In addition, it may creep off the roof, rotating slowly at the eaves, and may even break windows if it hits the side of the building. Protrusions through the roof should be located at the ridge or be especially protected against the shearing forces of sliding snow.” Structural Commentaries (User’s Guide – NBC 2015: Part 4 of Division B)
As you can see from the image below, the protection of vent stacks need to be considered. A small amount of investment can save a large amount of replacement and labour costs.
GUTTERS
What about gutters… “we have heavy duty gutters installed that should stop the snow.” Depending on the calculated snow load sliding down the roof a gutter may not be able to withstand the force and can be sheared off and need to be replaced. Gutters are not designed to be snow guards and is a common misconception. A simple search on the internet will provide image results of gutters being ripped off under load.
WHAT IS AN EFFECTIVE SNOW GUARD
“For a guard to be considered effective in retaining snow, it must be constructed of horizontal rails with a minimum diameter of 25 mm spaced no more than 100 mm apart.” Structural Commentaries (User’s Guide – NBC 2015: Part 4 of Division B)
IMPORTANCE OF PROPERLY DESIGNED SNOW GUARDS
Now that the NBC 2015 specifically addresses the factors which go into the proper design of snow guards; roof geometry, roof type (gable, single slope, curved) climatic conditions across Canada, uneven snow loads, upper roofs and many other factors, it is important to work with a company such as Sky Products Ltd who has the proper resources to provide a properly designed snow guard.
GUTTERS
