Risk Management Series

Snow Load Safety Guide 

FEMA P-957 / January 2013

Risk Management Series

Snow Load Safety Guide

FEMA P-957 / January 2013

FEMA nor any of its employees makes any warrantee, expressed or implied, or assumes any legal liability or responsibility for the 

accuracy, completeness, or usefulness of any information, product, or process included in this publication. Users of information from 

this publication assume all liability arising from such use.

SNOW LOAD SAFETY GUIDE i

CONTENTS

Contents

Chapter 1. Introduction.......................................................................1-1

1.1 General Objective.......................................................................... 1-1

1.2 Objective and Scope...................................................................... 1-1

1.3 Intended Users of this Document................................................ 1-2

1.4 Reasons for Preparing this Document.........................................1-3

1.5 Limitations.....................................................................................1-3

Chapter 2. Snow Loads ......................................................................2-1

2.1 Building Code Definitions............................................................2-2

2.2 Snow Types.....................................................................................2-2

2.2.1 Range in Snow Weight......................................................2-3

2.2.2 Ice ......................................................................................2-3

2.2.3 Regional and Local Considerations................................2-3

2.3 Other Variables..............................................................................2-3

2.3.1 Unbalanced Snow Load ...................................................2-3

2.3.2 Snow Fall Rate...................................................................2-6

2.3.3 Ambient Temperature......................................................2-6

2.3.4 Rain-on-Snow Load ..........................................................2-6

2.3.5 Snow Melt between Storms ..............................................2-6

Chapter 3. Building Basics.................................................................3-1

3.1 Building Characteristics and Shapes ...........................................3-1

3.1.1 Use Type............................................................................3-1

3.1.2 Roof Geometry and Roofing Material ............................3-2

3.1.3 Exposure to Wind.............................................................3-4

3.1.4 Insulation ..........................................................................3-4

3.2 Roof Conditions ............................................................................3-5

 3.2.1 Short Spans .......................................................................3-6

3.2.2 Long Spans ........................................................................3-6

3.2.3 Secondary and Other Structures .....................................3-6
ii SNOW LOAD SAFETY GUIDE

3.3 Common Roof Framing Materials...............................................3-6

3.3.1 Wood Construction ..........................................................3-7

3.3.2 Heavy Timber Construction ............................................3-7

3.3.3 Steel Construction............................................................3-8

3.3.4 Pre-Engineered Metal Building Systems ........................3-8

3.3.5 Cold Formed Steel Truss Construction...........................3-8

3.3.6 Reinforced Concrete Construction.................................3-8

3.4 Additions and Modifications to Roof Areas ...............................3-9

3.4.1 Rooftop Equipment..........................................................3-9

3.4.2 Reroofing ..........................................................................3-9

3.4.3 Additions and New Neighboring Buildings....................3-9

3.5 Age and Building Code Considerations .....................................3-9

Chapter 4. Monitoring a Building Structure....................................4-1

4.1 Pre-Season Inspection and Condition Assessment.....................4-1

4.1.1 Key Building Information................................................4-2

4.1.2 Items to Inspect ................................................................4-2

4.1.3 Repairs, Corrective Action, and Mitigation....................4-3

4.2 Warning Signs of Overstress Conditions During 

a Snow Event .................................................................................4-3

4.3 When to Contact a Professional Structural Engineer ...............4-4

Chapter 5. Measures to Reduce the Potential of Snow 

Load-Induced Structural Failures .................................5-1

5.1 Pre-Season Preventative Measures and Planning .......................5-2

5.2 Snow Event Response Strategies...................................................5-3

5.2.1 What to Do During a Significant Snow Event.................5-3

5.2.2 What to Do After a Major Snow Event.............................5-3

5.2.3 What to Do Before Another Snow Event.........................5-3

5.3 Removal of Snow from Roof.........................................................5-4

5.3.1 When to Initiate Snow Removal ......................................5-4

5.3.2 Safety Measures for Snow Removal .................................5-4

5.3.3 Method of Snow Removal.................................................5-5

Chapter 6. Bibliography......................................................................6-1
SNOW LOAD SAFETY GUIDE iii

CONTENTS

List of Figures

Chapter 2

Figure 1a: Unbalanced Snow Load from Drifting and Sliding 

Snow on Typical Commercial or Industrial Building .........2-4

Figure 1b: Unbalanced Snow Load from Drifting and Sliding 

Snow on Residential Structure .............................................2-4

Figure 2: Snow Guards or Snow Cleats................................................2-5

Chapter 3

Figure 3a: Flat or Low-Slope Roof With or Without Roof Drain.........3-2

Figure 3b: Stepped Roof.........................................................................3-2

Figure 3c: Saw-Tooth Roof......................................................................3-2

Figure 3d: Mono-Slope Roof ..................................................................3-2

Figure 3e: Gable/Multi-Span Gable Roof .............................................3-2

Figure 4: Insulation – Ice Dam Effect..................................................3-5

Chapter 5

Figure 5: Snow Removal Diagram........................................................5-6

SNOW LOAD SAFETY GUIDE 1-1

Snow Load Safety Guide

RISK MANAGEMENT SERIES

1Introduction

1.1 General Objective

T he objective of the Risk Management Series Snow Load Safety Guide

is to inform building stakeholders about the risks a snow event 

poses to their buildings, provide them with information about 

preventative measures to take before the snow season, and inform them 

of actions that should be taken before, during, and after a snow event.

1.2 Objective and Scope

Most buildings are not at risk of snow induced failure. More 

often than not, attempting to remove snow from a roof is more 

hazardous than beneficial, posing a risk to both personnel and 

the roofing structure. However, snow accumulation in excess of building 

design conditions can result in more than a temporary loss of electrical 

power and inaccessible roads. Buildings may be vulnerable to structural 

failure and possible collapse if basic preventative steps are not taken in 

advance of a snow event. Knowledge of the building roof framing system 
1-2 SNOW LOAD SAFETY GUIDE

and proper preparation in advance of a snow event is instrumental in 

reducing risk to the structure.

Structural failure due to roof snow loads may be linked to several possible 

causes, including but not limited to the following:

n Actual snow load significantly exceeds design snow load

n Drifting and sliding snow conditions

n Deficient workmanship

n Insufficient operation and maintenance 

n Improper design

n Inadequate drainage design

n Insufficient design; in older buildings, insufficient design is often 

related to inadequate snow load design criteria in the building 

code in effect when the building was designed

This document is not intended to provide a comprehensive discussion of 

the underlying issues or forensics of snow-induced structural failure. The 

purpose is instead to:

1. Inform building stakeholders of susceptible snow loading 

conditions

2. Identify potentially vulnerable roof framing systems

3. Outline a general methodology to monitor buildings for signs of 

potential failure so that steps can be taken to reduce the potential 

risk of snow-load-induced structural failure

1.3 Intended Users of this Document

n The intended audience of this guidance document is building 

stakeholders. Stakeholders may include:Property owners

n Building and facility managers

n Homeowners

n Emergency managers

n Other decision-makers
SNOW LOAD SAFETY GUIDE 1-3

INTRODUCTION 1

1.4 Reasons for Preparing this Document

T he northeastern part of the United States suffered multiple major 

snow storms in the winter of 2011 that resulted in numerous 

building failures. Unlike national safety and response guidelines 

for flood and seismic events, currently there does not exist a unified 

document that discusses mitigating damage to roof systems resulting 

from excessive snow loading.1

 State governments and local jurisdictions 

throughout the United States may publish snow safety guidelines, but 

the guidelines might not be issued until a snow event has occurred. 

1.5 Limitations

T his document is not a commentary on existing building codes 

nor a guidance manual on how to compute snow loads. It is not 

intended to be a technical reference for the design of the building 

structural roof system.

For concerns about the safety of a building or for information about 

structural engineering, contact a local building official or design 

professional. 

1 Snow load design information is available in ASCE 7-10, Minimum Design Loads for 

Buildings and Other Structures (ASCE, 2010), and in Snow Loads: Guide to the Snow Load 

Provisions of ASCE 7-10 (O’Rourke, 2010).

SNOW LOAD SAFETY GUIDE 2-1

2

Snow Load Safety Guide

RISK MANAGEMENT SERIES

Snow Loads

A snow event is not necessarily a single large snow storm. A snow 

event can be a series of storms that result in additional snow loads 

on a building. No two snow events are identical, and the resulting 

snow loads on nearby buildings from one snow event may be different. 

One foot of snow on the ground does not necessarily equal 1 foot of 

snow on a roof. Further, differing snow load conditions are a function of 

the variables associated with an individual building. The characteristics 

of snow can differ significantly from snow event to snow event. 

This chapter contains the building code definitions of snow load, types of 

snow, the variables that factor into roof snow loads, and the risks various 

snow conditions pose.

2.1 Building Code Definitions

Structural engineers use building codes to determine design snow 

loads on building structures. Currently, the International Building 

Code (IBC) is used throughout the United States for snow loads. 
2-2 SNOW LOAD SAFETY GUIDE

States and/or local jurisdictions may amend or supplement the IBC or 

adopt their own code. The building code identifies the ground snow 

load, which building designers use as the starting point to calculate the 

uniform design snow load on a building roof.

Ground snow load is defined as the weight of snow on the ground 

surface (IBC, 2012). Ground snow load values are established using data 

collected by the National Weather Service. Maps of ground snow loads in 

IBC and in ASCE 7 indicate a 2 percent probability of the indicated load 

being equaled or exceeded in any given year. Ground snow loads do not 

discount that actual snow loads may exceed them, only that the risk of 

snow-load-induced failure is reduced to an acceptably low level. 

Roof snow load is defined as the weight of snow on the roof surface used 

in design of the building structure (IBC, 2012). It is determined based 

on multiple factors, including:

n Ground snow load value

n Importance, occupancy, and use of the building 

n Wind exposure of roof

n Roof slope

n Roof shape

n Roof obstructions

n Thermal condition of the building

Before acceptance of the IBC as a national code, multiple building codes 

were used throughout the United States. The Building Officials Code 

Administrators (BOCA) was used on the East Coast and throughout much 

of the Midwest, used. In parts of the Midwest, the Standard Building 

Code was referenced. In the West and Alaska, the primary building code 

used was the Uniform Building Code (UBC). Drifting loads were first 

incorporated into BOCA in 1975. Unbalanced roof snow loads were 

not introduced in UBC until 1988. Only relatively recently have drifting 

and sliding snow loads been addressed in building codes. A building 

constructed 40 years ago may not have been designed for snow loads as 

they are understood today.

2.2 Snow Types

S now comes in many forms and is affected by numerous 

atmospheric and geographic conditions. Regional differences in 

season, altitude, humidity, and other variations result in a range 
SNOW LOAD SAFETY GUIDE 2-3

SNOW LOADS 2

of snow densities. For example, the weight of 1 foot of snow in Utah does 

not necessarily equal the weight of 1 foot of snow in Vermont.

2.2.1 Range in Snow Weight

The weight of 1 foot of fresh snow ranges from 3 pounds per square 

foot for light, dry snow to 21 pounds per square foot for wet, heavy snow 

(Gooch, 1999). 

2.2.2 Ice

One inch of ice weighs a little less than 5 pounds per square foot, and 1 

foot of ice weighs approximately 57 pounds per square foot. Ice weighs 

significantly more than heavy, wet snow per inch depth. In part, this is 

why it is important to prevent ice buildup on a roof structure.

2.2.3 Regional and Local Considerations

Snow in the western part of the United States is typically lighter and less 

dense than snow on the East Coast, which tends to be wetter and denser. 

There are exceptions based on locality and unique weather conditions, 

such as the Pacific Northwest around Seattle, WA, and the southwestern 

coast of Alaska. Local authorities and structural engineers are most 

familiar with regional snow characteristics.

2.3 Other Variables

T he uniform roof snow load, as determined by the factors discussed 

above, is a value determined for a flat, wide open roof free of 

obstructions and protrusions. Rarely is a roof such. Variables from 

the ideal condition are accounted for by designers as dictated in the 

building codes. These refined design loads are predicated on real world 

snow conditions. Although the process for determining these special 

load conditions is beyond the scope of this document, awareness of 

what these circumstances are will increase understanding of potentially 

vulnerable areas of the roof. This section addresses unbalanced snow 

loading resulting from drifting and sliding snow, as well as other 

environmental factors involved in managing snow.

2.3.1 Unbalanced Snow Load

Unbalanced snow loading is the condition in which snow accumulates at 

different depths in different locations on a roof, resulting in differential 

snow load. Unbalanced snow load poses a greater risk to the roof 

structural system than a uniform snow load. Hence, the danger of 

drifting and sliding snow is that both create an unbalanced snow load 
2-4 SNOW LOAD SAFETY GUIDE

condition. Figures 1a and 1b illustrate various unbalanced snow load 

scenarios resulting from drifting and sliding snow. Notice the snow is 

deeper at roof protrusions, obstructions, and elevation changes than in 

Figure 1a. Unbalanced snow load from drifting and sliding snow on typical commercial or industrial building

Figure 1b.

Unbalanced snow load 

from drifting and sliding 

snow on residential 

structure
SNOW LOAD SAFETY GUIDE 2-5

SNOW LOADS 2

more open sections of the roof. The influence of these conditions on 

snow load is discussed in further detail in Chapter 3.

2.3.1.1 Drifting Snow

Drifting snow is the result of wind transporting snow from one portion 

of the roof to another. Snow drifts often form on a lower roof in the 

wind shadow of the higher portions of the building. Snow can also be 

blown up against and accumulate next to an obstruction (e.g., high 

roof framing, rooftop equipment, parapet, adjacent building, dormer 

windows). The snow depth at this obstruction is greater than the overall 

roof snow depth and therefore creates a larger load on the roof structure 

at the location of the drift. 

2.3.1.2 Sliding Snow

Unless a sloped roof has snow guard or cleats (Figure 2), a sliding snow 

condition can occur. Snow from a higher roof can become unstable and 

slide onto a lower roof where it accumulates. Common instances of this 

are on porch/sunroom roofs and entrance canopies beneath gabled 

roofs of residential buildings (Figure 1b). Consequently, the snow depth 

on the lower roof within the fall zone of the sliding snow is greater than 

the overall roof snow depth, an unbalanced load condition. Again, this 

creates a larger load on the structure at the location where the snow fell 

Figure 2.

Snow guards or snow 

cleats
2-6 SNOW LOAD SAFETY GUIDE

from the upper roof. Furthermore, the dynamic force of the sliding snow 

onto the lower roof may produce a significant impact force on the lower 

roof framing, which can potentially overload the roof structure. 

2.3.2 Snow Fall Rate

Snow fall rate is not a direct variable of the calculated roof snow load. 

Snow fall rate does not influence the snow load. Instead, the rate of 

accumulation factors into when to begin monitoring a structure during 

a snow event. A faster snowfall rate means the stakeholder should start 

monitoring the snow condition earlier. A faster snowfall rate also means 

less time to remove the snow before it reaches the critical threshold.

2.3.3 Ambient Temperature

Temperatures fluctuating above and below freezing can produce 

hazardous conditions. Snow melts and then refreezes, creating ice and 

ice dams and resulting in higher concentrated loads at roof low points 

(if not properly drained) and at eaves on sloped roofs. Also, when 

temperatures hover around freezing, precipitation tends towards a 

“wintery mix” that can result in a rain-on-snow condition, which current 

building codes address.

2.3.4 Rain-on-Snow Load

Building code provisions for design snow loads incorporate light rain on 

snow, but heavy rainfall is not included. Therefore, this additional load 

must be taken into account separately by the structural engineer. Factors 

influencing the rain-on-snow load include rain intensity, roof geometry, 

and drainage characteristics of the roof. Duration is also considered 

because continuous rain can wash away snow, effectively reducing the risk 

of snow-induced collapse. Conversely, a period of short rain may cause 

snow to melt and become further saturated, significantly increasing the 

load on the roof structure.

2.3.5 Snow Melt between Storms

Snow melt between snow events may be beneficial or potentially 

hazardous depending on weather conditions. Snow melt between storms 

on properly designed, functioning roof drainage systems will reduce 

snow load. However, if the roof drainage system is blocked, improperly 

designed or maintained, snow melt may pose risks. Ice dams may 

form, which creates a concentrated load at the eaves and reduces the 

ability of sloped roofs to shed snow. This topic is discussed further in 
SNOW LOAD SAFETY GUIDE 2-7

SNOW LOADS 2

Chapter 3. On flat or low slope roof systems, snow melt may accumulate 

in low areas on roofs with poorly designed or blocked drainage systems. 

This condition is referred to as ponding. Ponding creates a concentrated 

load on the roof structural system and a potential hazard.

SNOW LOAD SAFETY GUIDE 3-1

Snow Load Safety Guide

3Building Basics

T he risk of structural failure from snow load is influenced by the 

characteristics of the building. Some roof structures and materials 

are more susceptible to snow-induced collapse than others. 

Building configurations create conditions in which drifting and sliding 

snow may pose a risk. 

This chapter discusses building characteristics, materials, structural 

systems, and their respective vulnerabilities.

3.1 Building Characteristics and Shapes

As noted in Chapter 2, roof snow loads differ from ground snow 

loads. The variables in roof snow load are roof geometry and 

roofing material, exposure to wind, and insulation.

3.1.1 Use Type

Building structures are categorized as residential, commercial, industrial, 

institutional, or agricultural.

RISK MANAGEMENT SERIES
3-2 SNOW LOAD SAFETY GUIDE

3.1.2 Roof Geometry and Roofing Material

Roof construction comes in many forms. The geometry has an influential 

role in the distribution of roof snow loads. Snow loads on an open flat roof, 

free of obstructions and rooftop equipment tend to be uniform, whereas 

roofs with defining geometric irregularities and obstructions accumulate 

snow in an unbalanced pattern. Intuitively, steep roofs are more likely to 

shed snow through sliding than flat and low slope roofs, although other 

variables factor in. Commercial, industrial, and agricultural buildings 

typically use one roof configuration throughout the structure, sometimes 

with varying roof and ridge elevations. Architecturally influenced design, 

such as residential, often incorporate multiple roof geometries.

Figures 3a through 3e illustrate common roof configurations:

Figure 3a. Flat or low-slope roof with or 

without roof drains

Figure 3c. Saw-tooth roof

Figure 3e. Gable/multi-span gable roof

Figure 3b. Stepped roof

Figure 3d. Mono-slope roof
SNOW LOAD SAFETY GUIDE 3-3

BUILDING BASICS 3

Roof geometric characteristics provide an opportunity for snow 

accumulation and drifting resulting in an unbalanced loading condition, 

including:

n A geometric feature that blocks wind from blowing snow, known 

as an aerodynamic shade, creates an opportunity for snow to drift. 

This commonly occurs at the leeward side of the ridge on gable 

and mono-slope roofs (Figures 3d and 3e) or rooftop protrusions 

such as mechanical penthouses and stair towers.

n Parapets on flat and low slope roofs and at changes in roof 

elevation of stepped roofs (Figures 3a and 3b) are common 

locations for snow drifts

n Valleys of saw-tooth roofs (Figure 3c) and at the intersection of 

sloping residential roofs (Figure 1b) accumulate greater snow 

depth than elsewhere on the roof

n Roof impediments, such as mechanical screen walls, rooftop vents, 

and skylights collect snow drifts (Figure 1a)

n Roof top equipment, such as heating and cooling units or solar 

panels, provide a place for snow to drift (Figure 1a)

n On residential roofs irregularities such as chimneys, dormer 

windows, porch roofs, and skylights may collect snow drifts 

(Figure 1b)

Roof geometry and roofing material influence the tendency of snow to 

slide from a roof as follows:

n Low slope roofs retain snow more so than pitched roofs. However, 

roof pitches as low as 10 degrees have been observed to shed snow.

n Steeper roof slopes shed snow more effectively. Thus, greater roof 

slopes are common on buildings in the northern States and in 

mountainous snow-prone regions.

n Roof pitch that exceeds the angle of repose of snow results in snow 

sliding; the angle of repose is the maximum angle at which snow 

will not slide, approximately a 30 degree roof slope, often referred 

to as 6:12 or 7:12. This is not to say that snow on roofs with a 

shallower slope will not slide.
3-4 SNOW LOAD SAFETY GUIDE

n More tactile, abrasive roofing materials are less slippery and 

do not shed snow as easily as a slippery surface. Tactile roof 

materials include asphalt shingles and aggregate surface built-up 

membranes. Slippery roof materials include metal roof panels and 

single-ply membrane roofing.

n The presence of snow guards or snow cleats will inhibit snow from 

sliding off the roof (Figure 2 in Chapter 2).

3.1.3 Exposure to Wind

A building’s exposure to wind greatly influences the snow load on a roof. 

In fact, wind exposure has the largest effect on roof snow of the variables 

that are discussed. A building constructed in an open area is less likely to 

retain snow on the roof than a building in a sheltered location. However, 

wind also dictates snow drifting. If a building in an open area has varying 

roof elevations, parapets, or rooftop equipment, snow drifting may occur 

at these locations resulting in an unbalanced loading condition. 

3.1.4 Insulation

A roof assembly’s thermal properties affect roof snow load in several ways. 

A well-insulated or well-ventilated roof typically retains more snow than a 

poorly insulated roof or a roof over a poorly ventilated attic. Well-insulated 

roofs do not permit heat from within a building to melt roof snow from 

beneath. Similarly, a well-ventilated attic space has a temperature similar 

to ambient air; therefore, as illustrated in the bottom graphic of Figure 

4, heat within the building does not cause roof snow melt. This condition 

is analogous to the roof of an open carport, where both the top and 

underside of the roof have the same temperature. Because there is no 

temperature difference, the rate of snow melt is unaffected.

For gabled and sloped roof systems, uninsulated heated attics increase 

the propensity for snow to melt or slide. However during freezing 

temperatures, discontinuous thermal characteristics at roof eave 

overhangs can inhibit snow shedding by causing ice dams, as shown in 

the top graphic of Figure 4. Melt water flows down the roof slope and 

freezes at the eave. The problem propagates as the ice dam becomes 

bigger, resulting in more melt water blockage at the eave.

Ice dams create several problems. First, they prevent snow from sliding off 

the roof. Snow and ice accumulation at the eave creates an undesirable 

unbalanced snow loading condition. A second (and potentially larger) 

problem that may arise from ice dams is water infiltration into the 

building interior.
SNOW LOAD SAFETY GUIDE 3-5

BUILDING BASICS 3

A properly designed, internally drained flat or low slope roof is not 

affected by building insulation characteristics to the degree sloped 

structures are. In fact, on an uninsulated low slope roof, there is the 

benefit of reduced snow load through melting. Melt water will not 

encounter a temperature differential that leads to freezing (ice dams) 

and pose the problem which occurs on sloped roofs with cold eaves.

3.2 Roof Conditions

S pan length of the primary roof framing members is also important 

in determining whether a roof structure may be susceptible to 

excessive deflection or failure from snow loading. Additionally, 

what structure the roof is covering should be accounted for when 

determining susceptibility to snow.

Figure 4. 

Insulation – Ice Dam Effect
3-6 SNOW LOAD SAFETY GUIDE

3.2.1 Short Spans

In general, short-span roof structures are less susceptible to excessive 

snow loading failure than long-span roof structures. Short-span roofs 

are less prone to deflection and therefore less at risk to ponding and 

improper roof draining. 

3.2.2 Long Spans

Long-span roof framing systems may have less structural redundancy 

than short-span roof framing systems, which can make failure more 

catastrophic in a long-span system. Long-span systems typically consist of 

wood or steel truss construction. It is imperative that adequate bracing 

of long-span systems is properly installed and maintained. Many longspan roof failures can be attributed to poorly performing or inadequate 

bracing.

3.2.3 Secondary and Other Structures

Secondary structures include canopies, porches, carports, detached 

garages, sheds, agricultural buildings, and other usually uninhabited 

spaces. According to building code, only agricultural buildings and sheds 

are permitted to be designed to a lower life-safety importance factor than 

occupied building structures. However, it is important to be aware that 

secondary structures do not typically perform as well as primary buildings 

during snow events (SEAW, 2009). Entrance canopies and porch roofs 

are particularly susceptible to drifting and sliding snow because they 

are adjacent to the main building structure. These structures are often 

building additions in which design considerations for the main structure 

may not have been accounted for. 

Agricultural buildings are typically long-span wood-framed or preengineered metal buildings. Many older agricultural structures were not 

required to be designed to a specific snow load.

3.3 Common Roof Framing Materials

Roof construction material is an important factor in determining 

a structure’s susceptibility to failure. To clarify, engineers design 

structures for a total load comprised of dead load (permanent 

load) and temporary loads (includes snow load). The weight of the 

structure itself is a dead load; therefore heavier structural materials have 

heavier dead load. Buildings constructed of heavier materials are typically 

less susceptible to snow-induced structural failure. This is because snow 

load in excess of the design snow load constitutes a smaller percentage 
SNOW LOAD SAFETY GUIDE 3-7

BUILDING BASICS 3

increase of the actual total load above the total design load in heavier 

structures. A smaller variation from the total design load means less 

likelihood that a structure will fail from the excess load. Another way 

to think of this is dead load to snow load ratio. Therefore, a good rule 

of thumb is that the higher the dead load to snow load ratio, the less 

susceptible a structure is.

3.3.1 Wood Construction

Wood-framed structures come in a variety of types and are used in the 

range of small-to-medium sized building applications. Typically, wood 

structures are relatively lightweight and may be more susceptible to 

excess snow loads. Beam and joist framing systems are often referred 

to as “stick framing” and are used in residential construction, although 

they are also found in other buildings such as warehouses and churches. 

Sloped roofs, such as gabled or hipped configurations in residential 

and small commercial buildings, are often framed with wood trusses or 

rafters.

Wood trusses consist of sawn lumber for both the vertical and diagonal 

members. Newer wood truss members are typically connected with metal 

plate connectors. Wood trusses commonly built today are engineered 

structural elements that are manufactured offsite, transported as bundled 

trusses, and installed onsite.

Possible failure modes of wood-framed structures are member fracture, 

joint failures, and lateral buckling from improper or inadequate 

installation of lateral bracing.

Wood trusses often have a tag that identifies the manufacturer and size 

designation, and the tag is important if the member capacity needs to be 

evaluated.

3.3.2 Heavy Timber Construction

Timber-framed structures typically consist of large timber members 

oriented vertically (post) and horizontally (beam). Timber trusses consist 

of heavy timber sections for vertical and diagonal members.

Although referred to as heavy timber, this type of construction has a low 

dead load and often long spans. In a report conducted by the Structural 

Engineers Association of Washington for the 2008–2009 snow season, 

heavy timber accounted for the second largest number of failures only 

to wood trusses.
3-8 SNOW LOAD SAFETY GUIDE

Possible failure modes of timber-framed roof structures are split or 

broken truss members and failure of truss connections.

3.3.3 Steel Construction

Common types of steel-framed buildings include beam and girder 

systems, open web steel joists, steel joist girders, and steel trusses. Steel 

beam and girders are typically hot-rolled sections, typically in an I-shaped 

configuration. Structural members in older buildings may be cast iron 

rather than steel. Open web steel joists are manufactured framing 

members that consist of small steel angles and solid steel bars arranged 

in a truss-like configuration. Similarly to wood trusses, steel joists often 

have a tag that identifies the manufacturer and size designation, and the 

tag is important if the member capacity needs to be evaluated. 

3.3.4 Pre-Engineered Metal Building Systems 

Pre-engineered metal building (PEMB) systems are typically constructed 

of lite-gage steel members. The primary roof framing and wall system 

consists of widely spaced rigid frames with steel purlins or steel joists 

supporting the metal roofing. These systems are used in single-story 

warehouses, gymnasiums, and similar open-use commercial and 

industrial buildings. PEMBs are highly engineered with little redundancy 

or reserve capacity. PEMB may therefore be susceptible to excessive snow 

loading, but if properly designed, collapse potential is low. If one roof 

framing member collapses, the failure can propagate because of a loss 

of integrity of the whole building system. Older pre-engineered metal 

building systems designed before drifting snow loads (unbalanced loads) 

were addressed in the building codes may be problematic. 

3.3.5 Cold Formed Steel Truss Construction

Cold formed steel (CFS) trusses consist of prefabricated members made 

up of light gage metal sections. CFS trusses are engineered structural 

systems that are manufactured offsite, transported, and installed onsite. 

CFS trusses require proper lateral bracing for bucking stability. CFS 

systems, similarly to PEMB systems, have a relatively low self-weight.

3.3.6 Reinforced Concrete Construction

Concrete construction can vary widely. Structural systems can be castin-place, precast, or post-tensioned. Because of the relatively high 

self-weight (dead load) of concrete systems, concrete structures have a low 

susceptibility to snow-induced failure. Historically, concrete structures 

have performed well under high snow accumulation conditions.
SNOW LOAD SAFETY GUIDE 3-9

BUILDING BASICS 3

3.4 Additions and Modifications to Roof Areas 

3.4.1 Rooftop Equipment

Rooftop equipment acts as an obstruction that can collect drifting snow. 

Typical rooftop equipment items include solar panels, heating/cooling 

units, and ventilation units and ducts. The equipment must be accounted 

for when assessing snow load. Rooftop equipment on older buildings 

is of special interest because many of these buildings were constructed 

before drifting snow was addressed in building codes. Furthermore, the 

structural adequacy of the roof structure may not have been checked for 

rooftop equipment added during a renovation or retrofit. The additional 

weight of equipment on the rooftop increases demand of the structure 

system. Without structural modifications to the existing roof, the available 

capacity to support additional load, such as snow, is reduced. 

3.4.2 Reroofing

The building structure typically outlasts the roofing material. When 

buildings are reroofed, a roof covering is often placed over the existing 

roof. This approach increases the weight of the roof assembly, thus 

reducing the structural capacity to carry the snow load. When buildings 

are re-roofed, the thermal efficiency of the roof system is often increased 

by using thicker insulation or a modern, more efficient insulation that 

may increase snow retention. Additionally, new roofing material can 

weigh more than the original roofing system, reducing the load carrying 

capacity. For example, changing from shingles to slate or tile increases 

the roof weight significantly.

3.4.3 Additions and New Neighboring Buildings

If a roof addition to an existing building is higher than the existing 

building roof, drifting of snow on the existing lower roof must be 

considered. Thus, determining whether the existing roof structure has 

been checked for drifting snow is important. 

A new building can act as an aerodynamic shade to the adjacent existing 

building if the new building’s roof elevation is higher than the existing 

roof, reducing the propensity for shedding snow. Similarly, wind exposure 

characteristics may also change.

3.5 Age and Building Code Considerations 

K nowing the age or approximate age of a building is valuable in 

determining the structural basis of design for the roof framing 

with regard to snow loads. As noted in Chapter 2, building codes 
3-10 SNOW LOAD SAFETY GUIDE

only began to address non-uniform snow loads in the 1970s and 1980s. 

In addition, even though codes are updated, there is typically a lag 

between the code revision and code adoption. If the original drawings 

are available, the building code used to design the building should be 

identified. If the drawings are not available but the age of the building 

is known, the local building department should be able to provide the 

code edition that was in place when the building was constructed.

Some building materials deteriorate over time. Steel members are 

susceptible to corrosion, which can reduce the load-carrying ability of 

the framing members. Wood members can split or rot, which can reduce 

the load-carrying capacity of the framing members. 
SNOW LOAD SAFETY GUIDE 4-1

4

Snow Load Safety Guide

RISK MANAGEMENT SERIES

Monitoring a Building 

Structure

A wareness of the warning signs a structure exhibits during a snow 

event can be the difference between getting safely through the 

snow event and experiencing structural failure. Property owners 

should have a contractor or licensed professional engineer conduct a 

building inspection and vulnerability assessment before the snow season. 

During a snow event, there are common signs that indicate a building 

structure is under duress and action is required. In some cases, the 

skills of a licensed Professional Engineer are required for an assessment 

before, during, or after a snow event.

4.1 Pre-Season Inspection and Condition 

Assessment

Familiarity with the building structure before the snow season begins 

is important in determining whether any changes occurred during 

a snow event. Knowing a building’s characteristics and structural 

system are part of establishing the baseline condition. In addition to the 
4-2 SNOW LOAD SAFETY GUIDE

as-built knowledge, the current condition of the structure needs to be 

known. 

This section discusses what to be aware of and the steps that should be 

taken to prepare for a major snow event.

4.1.1 Key Building Information

The following baseline information should be collected for the building:

n Applicable building codes

n Design snow load 

n Structural framing system

n Thermal properties

n Renovation history

4.1.2 Items to Inspect

Although all of the roof structure and building should be inspected, 

closer inspection is warranted at particular roof areas. As detailed in 

Chapter 3, Roof valleys, low points of saw-tooth roofs, abrupt roof 

elevation changes, parapets, and other obstructions and protrusions are 

locations that accumulate greater amounts of snow. Structural members 

in these locations therefore experience greater snow loads. Roof framing 

should be checked for deterioration, weakness, damage, or modification. 

In addition to the building structure, other items that should be inspected 

for proper function include:

n Gutters and downspouts for disrepair and free of debris

n Seals around rooftop penetrations are intact

n Openings around exhaust vents

n Internally drained roof downspouts are clear of debris

n Flashing around connections of rooftop equipment

n Roof soffit and ridge ventilation

n Cold eave electric heaters are properly functioning

n Vertical position of trusses are not leaning out-of-plane 

n Metal plates connecting truss member chords 

n Lateral braces are firmly connected to roof structure and do not 

show signs of over-stress or disrepair

n Attic areas are dry and free of excess moisture
SNOW LOAD SAFETY GUIDE 4-3

MONITORING A BUILDING STRUCTURE 4

Many of the items on the list above do not require the services of a design 

professional for routine inspection. However, for a comprehensive 

inspection or if there is a question that any of these items are deficient, 

professional services should be retained.

4.1.3 Repairs, Corrective Action, and Mitigation

Managing deficiencies ahead of a snow event is important in ensuring the 

integrity of the structure during the event. Small disrepairs can propagate 

into much larger issues. For commercial and industrial buildings, many 

of the issues can be handled internally through the skilled maintenance 

personnel. For residential properties, minor mitigation or corrective 

actions like ensuring gutters and downspouts are not blocked at the 

discharge point can be handled by the homeowner. It is imperative that 

safety measures be taken and equipment warnings are heeded. However, 

for structural concerns such as out-of-plane trusses or corroded metal 

brackets used in wood construction, the skills of a professional are 

required. An attempt to mitigate or repair building components vital 

to the proper function that are beyond the ability of the stakeholder or 

homeowner can cause additional damage.

4.2 Warning Signs of Overstress Conditions During 

a Snow Event 

Overstressed roof deck or framing typically displays some warning 

signs. Wood and steel structures may show noticeable signs of 

excessive deflection before failure. The following warning signs 

are common in wood, metal, and steel constructed buildings:

n Sagging ceiling tiles or boards, ceiling boards falling out of the 

ceiling grid, and/or sagging sprinkler lines and sprinkler heads

n Sprinkler heads deflecting below suspended ceilings

n Popping, cracking, and creaking noises

n Sagging roof members, including metal decking or plywood 

sheathing

n Bowing truss bottom chords or web members

n Doors and/or windows that can no longer be opened or closed

n Cracked or split wood members

n Cracks in walls or masonry
4-4 SNOW LOAD SAFETY GUIDE

n Severe roof leaks 

n Excessive accumulation of water at nondrainage locations on low 

slope roofs

4.3 When to Contact a Professional Structural 

Engineer 

I

f any of the warning signs identified in the previous section are 

observed, the building should be promptly evacuated and a 

qualified design professional should be contacted to perform a 

detailed structural inspection. A qualified design professional, such 

as a Professional Engineer, has the experience to make an assessment 

of structural integrity of the building and identify steps to make the 

building safe.
SNOW LOAD SAFETY GUIDE 5-1

5

Snow Load Safety Guide

RISK MANAGEMENT SERIES

Measures to Reduce 

the Potential of 

Snow Load-Induced 

Structural Failures

Similar to monitoring a building structure, preventative measures 

should be taken in advance of the snow season and not only 

before an actual snow event. The pre-season building inspection 

and vulnerability assessment should be followed by implementation 

of mitigation measures for structural vulnerabilities identified during 

the investigation. Steps should be taken during a snow event to reduce 

the risk of snow-induced structural failure. Formulating a plan and 

delegating responsibility prior to the snow season will make dealing with 

a snow event more manageable and help eliminate confusion about 

responsibilities during and immediately after the event. A Snow Event 

Response Plan should address both preventative measures and actions 

to be taken during the storm. This chapter discusses these two topics 
5-2 SNOW LOAD SAFETY GUIDE

and describes the fundamental items that need to be included in a Snow 

Event Response Plan.

5.1 Pre-Season Preventative Measures 

and Planning

E nsuring the building structure and roof are in good repair prior 

to the snow season will prevent possible problems and potentially 

save repair and cleanup costs. Developing a Snow Event Response 

Plan ahead of the snow season is imperative for major facilities, such 

as manufacturing buildings, warehouses, and big box retail facilities 

with multiple work shifts. Assigning tasks/roles in advance will help 

reduce confusion during a snow event and help ensure that nothing is 

overlooked, such as determining whether snow removal will be done by 

in-house crews or by a contractor on retainer.

Prepare an Occupational Safety and Health Administration (OSHA)-

compliant safety plan to ensure worker safety during snow removal 

work and inspections. The means of roof access and egress should be 

identified and all edge fall protection requirements reviewed.

Measures should include locating areas where snow removed from a roof 

can be stored on the ground away from entrances, exits, canopies, and 

building equipment. Accessibility for snow removal vehicles should be 

checked. Necessary snow removal equipment should be acquired and an 

inspection schedule for the equipment should be prepared.

Snowfall can hide rooftop hazards. Conduits, gas lines, vents, equipment, 

lightning protection, and skylights should be marked with flags or a similar 

system to prevent injury to staff and unnecessary damage to these items. 

Skylights are of particular concern because personnel clearing snow 

may fall through them if not properly identified. If possible, locations 

of columns and primary structure members should be marked to help 

develop a snow removal sequence. Roof drains, gutters, downspouts, and 

vents should be checked to make sure they are free of debris and other 

obstructions. 

Structural deficiencies identified in the pre-season inspection should be 

prioritized and addressed by severity and the risk posed. If the construction 

drawings do not clearly identify the design snow load or the information 

is not available, contact a licensed Professional Engineer. Familiarity with 

the roof design and the areas that require closer observation is critical 

information before a snow event.
SNOW LOAD SAFETY GUIDE 5-3

MEASURES TO REDUCE THE POTENTIAL OF SNOW LOAD-INDUCED STRUCTURAL FAILURES 5

5.2 Snow Event Response Strategies

5.2.1 What to Do During a Significant Snow Event

The Snow Event Response Plan should be reviewed. The Plan should 

define a methodology to determine an approximate snow load and at 

what point snow removal should be initiated. Snow removal will prevent 

overstressing of the roof structure. Having an in-house plan for snow 

removal in place or a contractor on retainer is imperative. Contractors 

will be in high demand and difficult to find once a snow storm begins.

5.2.2 What to Do After a Major Snow Event

Even if snow accumulation during an individual snow event approaches 

but does not surpass the threshold of building safety, removal of snow 

from the roof may still be in order. Roof snow that is exposed to sunlight 

can soften, become denser, and then harden when the temperature 

drops below freezing. As noted in Chapter 2, melt water may pool and 

subsequently freeze, creating a concentrated area of loading on the roof. 

If subsequent snow events are anticipated, removing snow from the roof 

will minimize the risk of accumulating snow causing structural damage. 

One benefit of immediate snow removal is that the effort required to 

remove the snow from the rooftop is reduced. However, when snow 

accumulation is minor, the likelihood of damaging the roofing material 

or risk of being on a roof outweighs the benefits of removal.

5.2.3 What to Do Before Another Snow Event

The steps taken in advance of a subsequent snow event vary only slightly 

from the approach in advance of any snow event. Key steps include 

inspection of drains, gutters, downspouts, and vents for snow or ice 

blockage from the earlier snowfall.

Designated areas for removed rooftop snow must have sufficient capacity 

to accommodate additional snow. After a major storm, snow may need to 

be removed from the premises to create space for subsequent snow. For 

example, if snow is stored in the parking lot, but the amount of snowfall 

is such that it occupies too much of the lot area, the snow may need to 

be transported offsite.
5-4 SNOW LOAD SAFETY GUIDE

5.3 Removal of Snow from Roof

5.3.1 When to Initiate Snow Removal

A number of factors influence when snow 

removal should begin including, but not 

limited to, the pre-snow condition of the 

building structure, snow from a previous 

snow event still on the roof, the rate of 

snowfall, and/or the need to keep ahead of 

additional snow if there is a valid concern 

of a snow-induced structural failure.

Knowing the design snow load of the structure is key in determining 

when to begin removal. Design snow load can be determined from 

construction drawings. If there are no drawings, retain the services of 

a licensed design professional before the snow season to determine 

structural capacity. A local Professional Engineer will be familiar with the 

snow conditions of the region. Their knowledge of the pervasive snow 

type can help the stakeholder formulate some benchmarks to when snow 

accumulation at the building is approaching the snow load capacity. As 

noted earlier, roof snow loads vary from ground snow based on multiple 

factors. Therefore, attempting to measure ground snow weight and 

assuming that it is the same as the roof snow load is incorrect.

If the existing snow load is close to the capacity of the roof structure, 

snow removal is prudent. If the existing snow load is somewhat close to 

capacity and additional snow is forecast that could come close or exceed 

the capacity, snow removal is also prudent.

Snow removal should be performed by a licensed, insured professional 

roofing contractor who has experience in removing snow from roofs. 

Using a professional roofing contractor is highly recommended because 

of the contractor’s familiarity with safety protocols.

WARNING

If there is any concern 

that snow loads may 

cause a collapse of the 

roof structure, cease 

all removal activity and 

evacuate the building.

5.3.2 Safety Measures for Snow Removal

n Any roof snow removal should be conducted following proper 

OSHA protocol for work on rooftops. Use roof fall arrest harnesses 

where applicable. 

n Always have someone below the roof to keep foot traffic away from 

locations where falling snow or ice could cause injuries. 
SNOW LOAD SAFETY GUIDE 5-5

MEASURES TO REDUCE THE POTENTIAL OF SNOW LOAD-INDUCED STRUCTURAL FAILURES 5

n Ensure someone confirms that the area below removal site is free of 

equipment that could be damaged by falling snow or ice.

n Whenever snow is being removed from a roof, be careful of 

dislodged icicles. An icicle falling from a short height can still cause 

damage or injury.

n When using a non-metallic snow rake, be aware that roof snow can 

slide at any moment. Keep a safe distance away from the eave to 

remain outside of the sliding range.

n Buried skylights pose a high risk to workers on a roof removing 

snow. Properly mark this hazard as well as other rooftop hazards 

prior to snow events.

5.3.3 Method of Snow Removal

n Removing snow completely from a roof surface can result in 

serious damage to the roof covering and possibly lead to leaks and 

additional damage. At least 2 inches of snow should be left on the 

roof. 

n Do not use mechanical snow removal equipment. The risk of 

damaging the roof membrane or other rooftop items outweighs the 

advantage of speed.

n Do not use sharp tools, such as picks, to remove snow. Use plastic 

rather than metal shovels.

n Remove drifted snow first at building elevation changes, parapets, 

and around equipment. 

n Once drifted snow has been removed, start remaining snow 

removal from the center portion of the roof.

n Remove snow in the direction of primary structural members 

(Figure 5). This will prevent unbalanced snow loading.

n Do not stockpile snow on the roof.

n Dispose of removed snow in designated areas on the ground.

n Keep snow away from building exits, fire escapes, drain 

downspouts, ventilation openings, and equipment.

n If possible, remove snow starting at the ridge and moving toward 

the eave for gable and sloped roofs.
5-6

n Use a non-metallic snow rake for steep roof slopes if possible. Metal 

snow rakes can damage roofing material and should be avoided. 

Snow removal can be conducted from the ground, removing the 

risk to people on a hazardous roof.

n Upon completion of snow removal, the roofing material should be 

inspected for any signs of damage. Additionally, a quick inspection 

of the structural system may be prudent after particularly large 

snow events.

SNOW LOAD SAFETY GUIDE

Figure 5. Snow Removal Diagram
SNOW LOAD SAFETY GUIDE 6-1

Snow Load Safety Guide

6

RISK MANAGEMENT SERIES

Bibliography

American Society of Civil Engineers / Structural Engineering Institute 

(ASCE/SEI). 2010. Minimum Design Loads for Buildings and Other 

Structures. ASCE 7-10. 

Building Officials Code Administrators (BOCA) Code

Buska, J. and Tobiasson, W. 1993. “Standing Seam Metal Roofing Systems 

in Cold Regions”. 10th Conference on Roofing Technology. National 

Roofing Contractors Association.

Buska, J. and Tobiasson, W. 2001. “Minimizing the Adverse Effects 

of Snow and Ice on Roofs.” International Conference on Building 

Envelope Systems and Technologies. Ottawa, Canada.

Gooch, C.A. 1999. “Heavy Snow Loads.” Dairy Facilities/Environmental 

Management Engineering, Cornell University. Ithaca, NY.

Gooch, C.A. and Gebremedhin, K.G. 1999. “Assessment of Failures of 

Post-Frame Buildings in New York State Due to Snow Load” Dairy 

Facilities/Environmental Management Engineering, Cornell University. 

Ithaca, NY.
6-2 SNOW LOAD SAFETY GUIDE

International Code Council. 2012. International Building Code.

Liel, A.B., Jackson, K.A., and Geis, J.M. A Framework for Performance 

Based Design and Assessment of Buildings Subjected to Extreme Snow 

Loads. Department of Civil, Environmental and Architectural 

Engineering, University of Colorado, Boulder, CO.

O’Rourke, M. 2008. “Structural Collapse from Snow Load” Structure 

Magazine: November.

O’Rourke, M. 2010. Snow Loads: Guide to the Snow Load Provisions of ASCE 

7-10. Reston, VA: ASCE Press.

O’Rourke, M. 2011. “Snow & Rain Provisions in ASCE 7-10.” Structure 

Magazine: February.

Schriever, W.R. 2005. “CBD-193. Estimating Snow Loads on Roofs” 

Canadian Building Digests. NRC-IRC Publications.

Structural Engineers Association of Washington (SEAW), Spokane 

Chapter. (2009). Study of Structural Failures Associated with the 

Winter 2008-2009 Snow Event in the Spokane/Coeur d’Alene Area. 

Uniform Building Code (UBC). 

U.S. Army Corps of Engineers (USACE). (1998). TI 809-52 Commentary 

on Snow Loads. Washington, DC.

Zallen, R.M. 2001. “Snowdrift Causes Roof Collapse.” Forensic Engineering 

in Construction, 4. Framingham, MA.

FEMA P-957

Catalog No: 13052-4