Ground vs. Roof Snow Load

Many don’t understand that the snow load that’s rated for an area typically refers to the snow accumulating on

the ground surface.  There are many variables that come into play when converting from ground to roof snow load.  This article explains those factors for uniform snow-loading conditions.

Ground Snow Load Defined

Ground Snow Load (p_G)

The ground snow load a.k.a. P_G is used in the determination of the design roof snow load of a site. This number is based

on the historical measurements taken at various weather stations in the U.S. It can be found either in the ASCE 7 (chapter 7 – Fig 7.2.1, ASCE 7-16/22) or on ATC website (https://hazards.atcouncil.org/).

Case Studies

However, in some areas designated as “CS” in ASCE 7, site-specific case studies are required to determine the ground snow load. In fact, these areas face extreme variations in ground snow loads which make difficult the mapping considering the scale use in the Code. For these areas, ground snow load shall be approved by the Authority Having Jurisdiction. For these areas, some more research is necessary even though it can be mentioned on ATC website.

Takeaway:  Always contact your local building department to determine the local ground snow load.

So what about the Roof Snow Load?

It is important to notice that the ground snow load is not the actual load that is applied on the roof structure being designed, it is just a datum, given resulting of historical measurements and statistical calculations. It does not consider either the characteristics or the purpose of the structure.  Let’s examine the various types of roof snow loading.

Flat Roof Snow Load (p_f)

In most cases of patio enclosure designs (such as carports, louvered roofs, and monoslope sunrooms), the structure will have a flat roof (roof slope ≤ 5°).

Formula 7.3-1 of ASCE 7 (2016/22 versions) gives us the flat roof snow relation:

p_f=0.7×C_e×C_t×I_s×p_g

Were
Were
C_e=Exposure Factor

C_t=Thermal Factor

I_s=Importance Factor

p_g=Ground Snow Load  (per the ASCE-7 snow map)

The Exposure Factor C_e gets higher the more the structure is sheltered and close to other buildings. It is driven by 2 factors: the Terrain Category (“Surface Roughness”), and the Exposure of Roof. C_e value is determined using Table 7.3-1 of the ASCE 7-16/22 as summarized herein:

–          Fully Exposed – Roof exposed on all sides with no shelter afforded ny terrain, higher structures, or trees.

–          Sheltered – Roofs located tight in among conifers that qualify as obstructions.

–          Partially Exposed – Most frequent, all other roofs than those indicated above.

–          B – Urban and suburban areas, wooded areas, or other terrain with numerous, closely spaced obstructions.

–          C – Open terrain with scattered obstructions that have heights generally less than 30 ft

–          D – Flat, unobstructed areas and water surfaces.

The Thermal Factor C_t is supposed to reflect, at some point, the warmth inside the structure. This number can be determined using Table 7.3-2 of the ASCE 7 (2016). It will also affect the Sloped Roof Snow Load, which is discussed below. Roofs can be divided into two categories depending on their Thermal Factor:

• Warm Roofs: C_t ≤ 1  (typically used for sunrooms and used for habitable structures)
• Cold Roofs: C_t > 1 (typically used for open pergolas & canopies)

The Importance Factor I is not limited to the Snow. In fact, it is a set of 4 numbers (Snow, Ice-Thickness, Ice-Wind and Seismic Importance Factor) that depend on the Risk Category (Table 1.5-1) of the structure. They can be determined using Table 1.5-2 of the ASCE 7 (2016/2022). In our case, only the Snow Importance Factor I_s matters.

• All structures other than below  = 1.0Structures just above freezing (such as a pergola or patio enclosure) = 1.1
  Open Air Stru

The Sloped Roof Snow Load p_s (where roof slope > 5°)

A roof with a slope ≥ 5° is considered a sloped roof, and the Snow loads acting on a sloping surface are assumed to be acting on the horizontal projection of the surface. Hence, using trigonometry reasoning, we can easily understand that p_s must be inferior or equal to p_f  i.e., a slopped roof faces less snow loads than a flat roof.

The sloped roof (balanced) snow load is given by the Formula 7.4-1 of the ASCE 7-16:

p_s= p_f C_s

Where:

The Roof Slope Factor C_s relies on 3 Factors: The Thermal Factor C_t, The slope of the roof, and the (un)obstruction of the surface. It can be determined using the Table 7.4-1 of the ASCE 7-16/ ASCE 7-22.

General Comparison:

Here are some general comparisons for your understanding of the above when comparing a ground snow load to a roof snow load.  THESE ARE NOT the design loads you should use.  See the section below for more factors.  Factors other than snow such as wind, seismic, minimum code loading requirements, and site-specific conditions all factor in to the final load condition to be used for design.  It’s complicated and a task for a licenced design professional.  We recommend you Contact Engineering Express to request a service to run these factored load calculations for your condition to obtain a proper design and certified plan for permit.

Using a 20psf Ground Snow Load:

Commentary

It can be seen that generally, the roof snow load is 33%-84% of the ground snow load in these examples.  To be conservative, the ground snow load can be used as the roof snow load, but sometimes this can be excessive.

Other Factors To Consider:

Snow Drift

On the other side of the load calculation is snow drift. this is the further accumulation of snow due to wind collecting on a rooftop due to the collection of snow against a verticle surface.  This is an in-depth calculation and beyond the scope of this explanation.  We recommend you Contact Engineering Express to request a service to run this calculation for your condition.

Snow Build Over

Another factor to consider when reviewing snow loading on open trellises and pergolas is the snow build-over effect.  That was covered in another article in this knowledge base.   Click here to read that article.

Icing

Another related condition is the accumulation of ice on structures. This is caused by the melting and re-freezing of snow or freezing rain that builds on surfaces.  ASCE-7 has an icing chart and methodology for calculating this force.

The total load applied to a structure due to snow & ice is a factored load combination of the above elements.  Again, out of the scope of this article, but important to take note of.  We recommend you Contact Engineering Express to request a service to run these factored load calculations for your condition to obtain a proper design and certified plan for permit.

References:

IBC 2018 Chapter 16 section 1608.2