A few years back, my group of engineers got a question from the shop floor: “We’re seeing the conduits from the ceiling snaking back and forth a lot, is the roof okay?” This was amid some of the heaviest snow our area had experienced in decades, and another building in our industrial park had recently had a roof cave-in. After checking through the building plans and scooping square foot areas of snow from several different spots on the roof into buckets to weigh, we did determine we were pushing the allowable loads but were still under. Thankfully the weather turned nicer and it all melted away, but it sparked my interest in the code snow load provisions (and some of their potential flaws).
If you’ve ever had to shovel a driveway, you probably know that in the right conditions, snow can be extremely heavy. For those from warmer climates: depending on the weather conditions, snow can fall as anything from a light, loose, fluffy cloud that can be blown away with a weak leaf blower to a wet, sloppy, slushy mess that will cause more than a few thrown-out backs. The latter kind unfortunately does cause all too many heart attacks each year.
That same heavy snow can wreak havoc on structures, and the traditional architecture of many snowy areas of the world has evolved steep, strong, peaked roofs to help bear the weight.
When it comes to structural design, we engineers work up most of our loads from the ASCE 7-16 (or relevant edition for your area) “Minimum Design Loads and Associated Criteria for Buildings and Other Structures”. To work up a snow load, we use ASCE 7-16 to look up the appropriate ground snow load for the area and apply several adjustment factors to account for site exposure, roof insulation, roof slope, and other influences to produce the balanced snow load. This is the load used for most designs, though further consideration of the potential for snow drifts to accumulate may also be necessary.
What is the Balanced Snow Load?
Balanced Snow Load is the weight of snow buildup on a structure, neglecting any drifting that may occur. Weather conditions can greatly influence how heavy snow is, but ASCE offers us an equation for snow density as a function of depth, up to 30 lbs/ft³, per ASCE 7-16 eq 7.7-1.
The actual unit weight doesn’t matter for balanced design loads though, as these come from a mapped ground snow load (already in pounds per square foot) manipulated by the use of a few adjustment factors. You can freely look up the ground snow load for any address in the US at https://asce7hazardtool.online/
How do you Calculate the Balanced Snow Load in ASCE 7-16?
ASCE 7-16 provides a whole chapter, Chapter 7, laying out all the various snow load provisions, but the brief overview for balanced snow load is as follows:
- Look up Input Values
- Snow Importance Factor, I.s, from Table 1.5-2, based on the building’s Risk Category
- Check out my article on How Risk Categories Are Assigned
- Ground Snow Load, p.g, as determined from Fig. 7.2-1 (or use the ASCE 7 Hazard Tool) and Table 7.2-1; or a site-specific analysis, in lb∕ft2 (kN∕m2)
- Exposure Factor, C.e, as determined from Table 7.3-1
- Thermal Factor, C.t, as determined from Table 7.3-2
- Snow Importance Factor, I.s, from Table 1.5-2, based on the building’s Risk Category
- Calculate the Flat Roof Snow Load, p.f, per Equation 7.3-1
- Determine the Slope Factor, C.s, from Fig. 7.4-1
- Multiply the Flat Roof Snow Load by the Slope Factor to obtain the Balanced Snow Load
Calculating the Flat Roof Snow Load
The Flat Roof Snow Load is either the final output (for roof planes with less than 15° slope or curved roofs with less than 10° between eaves and crown) or an intermediate step in developing the sloped roof snow load.
Note: §7.3.4 specifies a minimum required flat roof snow load, essentially limiting the amount of reduction allowed from favorable adjustment factors. Where ground snow loads are less than 20 psf, the minimum required load is the importance factor times the ground snow load (I.s * p.g), otherwise the minimum is 20 psf * I.s where the mapped ground snow load exceeds 20 psf.
The major inputs for the calculation of the Flat Roof Snow Load (eq 7.3-1 below) are the Snow Importance Factor (which adjusts for the building Risk Category), Mapped Ground Snow Load (from Fig & Table 7.2-1), site Exposure Factor, and roof Thermal Factor:
Snow Importance Factor
Importance factors come from Chapter 1 of ASCE 7-16, table 1.5-2, with selected values presented below:
| Risk Category | Snow Importance Factor – I.s |
|---|---|
| I | 0.80 |
| II | 1.00 |
| III | 1.10 |
| IV | 1.20 |
Ground Snow Load
Ground snow loads come from either the Fig 7.2-1 map (also available for digital interpolation by address at ASCE7Hazard) or the tables 7.2-1 through 7.2-8, depending on the state.
Note that there are “Case Study” regions in Fig. 7.2-1 that do not provide mapped ground snow loads due to extreme local variation in snow loading. For those regions, local climactic data will be needed, which can often be had from the building inspector in larger municipalities. Note that in ASCE 7-22, all of these “CS” regions go away, thanks to the better resolution of the updated digital data.
Exposure Factor
Wind tends to blow snow off of more exposed roofs, while a roof surrounded by taller buildings or trees can be sheltered from such effects. To capture this, we use the Exposure Factor from Table 7.3-1. This table makes use of the same surface roughness/exposure definitions as the wind loads in Chapter 26.
It also categorizes the degree of shelter the particular roof (each individual roof plane can technically receive its own exposure classification if so desired) can be expected to receive over the lifespan of the structure from surrounding features. Bear in mind that small trees grow into big trees, so a roof that’s exposed when built may become sheltered over time.
A couple of definitions for use with the chart below:
- Obstructions within a distance of 10*h.o provide “shelter,” where h.o is the height of the obstruction above the roof level.
- If the only obstructions are a few deciduous trees that are leafless in winter, the “fully exposed” category shall be used.
- Note that these are heights above the roof.
- The various exposure categories are defined in footnotes of the table as follows:
- Fully Exposed: Roofs exposed on all sides with no shelter afforded by terrain, higher structures, or trees.
- Roofs that contain several large pieces of mechanical equipment, parapets that extend above the height of the balanced snow load (h.b), or other obstructions are not in this category.
- Sheltered: Roofs located tight in among conifers that qualify as obstructions.
- Partially Exposed: Roofs that are neither Fully Exposed nor Sheltered.
- Fully Exposed: Roofs exposed on all sides with no shelter afforded by terrain, higher structures, or trees.
| Fully Exposed Roof | Partially Exposed Roof | Sheltered Roof | |
| Surface Roughness B | 0.9 | 1.0 | 1.2 |
| Surface Roughness C | 0.9 | 1.0 | 1.1 |
| Surface Roughness D | 0.8 | 0.9 | 1.0 |
| Above the tree line in windswept mountainous areas | 0.7 | 0.8 | N/A |
| In Alaska, in areas where trees do not exist within a 2-mi (3-km) radius of the site | 0.7 | 0.8 | N/A |
Thermal Factor
Heat transferring through the roof from conditioned spaces below can serve to melt away some of the roof snow accumulation. The 0.7 coefficient in equation 7.3-1 partially serves to capture the relationship observed between ground and roof snow loads, and reflects “ordinary” heated structures which are insulated to try to prevent heat from escaping through the ceiling. As such, adjustment factors are necessary to adjust for any deviation from that default condition.
Table 7.3-2 gives the user an appropriate thermal factor based on the “anticipated conditions during winter for the life of the structure”, meaning if a structure is planned to be heated for the first several years, but then may well be left unheated for a while, it’s best to use the more strenuous condition of unheated.
| Thermal Condition | Thermal Factor, C.t |
| All structures except as indicated below | 1.0 |
| Structures kept just above freezing and others with cold, ventilated roofs in which the thermal resistance (R-value) between the ventilated space and the heated space exceeds 25°F × h × ft² ∕ Btu (4.4 K × m² ∕ W) | 1.1 |
| Unheated and open air structures | 1.2 |
| Freezer Buildings | 1.3 |
| Continuously heated greenhouses with a roof having a thermal resistance (R-value) less than 2.0°F × h × ft² ∕ Btu (0.4 K × m² ∕ W) | 0.85 |
Also, note that there are requirements on the “continuously heated greenhouse” condition to ensure that any issues with heating can be promptly noticed and dealt with. Be sure to review footnote b thoroughly before taking advantage of the reduction offered by this condition.
Calculating the Sloped Roof Snow Load
For sloped roofs (the cutoff for what qualifies as “sloped” varies by the thermal coefficient of the roof), an added factor gets worked in, creatively named the “Slope Factor”, C.s.
Figure 7.4-1″Graphs for Determining Roof Slope Factor, Cs, for Warm and Cold Roofs” (I was unable to reproduce this one well here, you’ll have to actually crack open the book for this one!) consists of three separate graphs to give you the Slope Factor. The thermal factor in the preceding section determines which of the three graphs to use, and then the combination of roof slope and “slipperiness” can be used to extract the correct Slope Factor.
Multiplying the Flat Roof Snow Load by the Slope Factor (per eq 7.4-1 above) gives the Sloped Roof Snow Load, which is applied as a vertical, gravity load on the horizontal projection of the roof.
Caution on Very Large Roof Areas
A word of caution: Remember that ASCE 7-16 is the “Minimum Design Loads and Associated Criteria for Buildings and Other Structures”.
Consider the consequences of snow loads while designing. Many northern homeowners will shovel excess snow load off their house roof if there’s an extraordinary snow storm. When the roof at my 8-acre factory started deflecting enough to worry people, it hit us pretty quickly that there’s no way to shovel off a roof that large. In my personal opinion, I believe a roof that large should be treated as if it’s the ground, removing the 0.7 factor from the flat roof snow load, and only including relevant increases from the factors, not taking in any credits.
You, as the engineer, are always required to exercise your judgment to “first and foremost, protect the health, safety, and welfare of the public“.
Snow Drifts and Other Considerations
The Flat Roof and Sloped Roof Snow Loads only reflect the uniform loads on our roofs. Any discontinuity, such as a ridge, hip, or step in a roof has the potential to cause an aerodynamic drift which must be considered in design.
Watch for the next article soon on unbalanced snow loading, but for now, make sure to read up on §7.5 through 7.9 in ASCE 7-16 on your own.
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