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Adverse weather conditions can af fect the structural integrity of building roofs and

cause partial or complete collapse. The potential for roof collapse or structural dama

increases as the weight of accumulated snow and ice exceeds the snow load capac

of the roof. Rain falling on accumulated snow is especially dangerous because snow

covered roofs do not drain well and accumulating water, ice and saturated snow can

quickly exceed the design limits of the roof.

There are several factors that can inuence the expected snow-load intensity on roo

such as drifting and roof geometry.

Approximately 35% of all structural failures are roof failures. Of roof failures, 36% ar

caused by snow and ice, and 75% of these snow-related failures are caused by snow

drifting. Snow drif ting can easily result in additional loading of a localized portion of

the roof. Snow drifts accumulate on roofs (even sloped roofs) in what is known as th

aerodynamic shade or wind shadow of the following:

1. Higher portions of the same structure.

2. Nearby (within 20 ft.) higher structures or terrain features.

3. Roof projections, such as parapets, solar panels, penthouses and mechanical

equipment.

It is important to consider the localized load intensities produced by drifts.

The geometry of roofs also inuences snow accumulations. Every roof shape has

its own susceptibilities to snow, ice or rain loading. The three most susceptible roof

shapes are:

Multi-level Snow drifts build on multi-level roofs at the intersection of high and

low bays. Winds blow snow from higher bays onto lower ones, or across lower ba

to create drifts against higher bays. If the upper bay is a sloped or a gabled roof, t

snow load will drift onto the lower bay.

Curved On curved roofs, an unbalanced snow load can collect on the leeward

side at the eaves. Even if the roof sustains the unbalanced snow load, ice forms

easily at the eaves in a freeze-thaw cycle that prevents ice and snow from droppi

off normally. This also creates the danger of ponding water loads on curved roofs

Valley Snow collects in the valleys formed by gable, saw-toothed and barrel-va

roofs. The snow load in these valleys can be as much as three times the balance

snow load. In extreme conditions of sustained high winds, even single gabled roo

sloped about ve degrees (or one inch per foot) can accumulate snow drifts.

Snow Loading

Winter Weather Advisory

Are You Prepared?

Do you have an emergency

response plan?

Have you inspected your

roof for existing damage?

Are your roof drains and

downspouts clear?

Do you know what to

do when it snows?

2011 Liberty Mutual Agency Corporation All Rights Reserved June 2011

LOSS PREVENTION REFERENCE NOTE :from Ohio Casualty

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Metal roong systems and steel decks and boards on joist construction are the roof types most susceptible to collaps

from snow or rain loads. These systems have little rescue capacity in their ability to withstand large live loads of snow

rainfall. This is due to their lighter construction and dead load capacity.

Loss Prevention Recommendations:

Prior to the Start of Winter

Keep an updated winter emergency response plan in effect, especially for snow removal. Keep workers trained and

the proper equipment available.

Determine the maximum safe snow depth for the roof based on its load capacity as indicated in the building plans aspecications or in an engineering analysis of the roof design.

Inspect the roof structure for damage or deterioration and repair or reinforce as necessary.

Inspect and clean all roof drains and downspouts to prevent clogging of the drainage system.

Look for evidence of past water ponding and eliminate the causes.

When it Snows

Monitor the snow depth on the roof, paying close attention to areas where snow tends to drift and accumulate;

areas such as valleys, low roof sections next to higher roof sections and areas adjacent to roof-mounted structures

(penthouses, tanks).

Remove snow accumulations from the roof before the snow reaches 50% of the safe maximum depth (Table 1) and

use safe practices. Do not send employees onto the roof once the snow load approaches the load capacity. Removsnow during a storm only if the forecast indicates the total snowfall will result in dangerous accumulations.

Remove snow in layers uniformly across the roof to prevent unbalanced loads that may cause a collapse. Avoid

making snow piles or creating snow drif ts by moving snow from one roof area to another.

Clear snow and ice from storm drains and catch basins. Periodically inspect the roof drainage system to make sure

is not clogged with ice or debris.

Use care with snow removal equipment to prevent roof cover damage. It is not necessary to c lean completely down

the roof surface as long as melting snow and water can ow freely to the drain.

Measuring the Snow Load

Table 1 is a guide that combines live load design (lbs/sq. ft.) and the density (lbs/cu. f t.) of accumulating snow, ice or

water to determine when to take corrective action.

Example:A roof designed to handle a snow load of 20 psf could possibly stand 11.5 inches of heavy wet snow.Therefore you should remove it from the roof (if safe to do so) when it reaches approximately 6 inches.

Table 1: Equivalent Snow Load Table

Density Information Light/Dry Snow Heavy/Wet Snow Ice Water

density (lbs/cu. Ft.) 3.12 20.81 57.25 62.43

% of water weight 5% 33% 92% 100%

Equivalent inches of Precipitation

Design Load (lbs/sq. ft.) Light/Dry Snow Heavy/Wet Snow Ice Water

depth clear at depth clear at depth clear at depth

5 19.2 in. 10 in. 2.9 in. 1.5 in. 1.0 in. 0.5 in. 1

10 38.4 in. 19 in. 5.8 in. 3.0 in. 2.1 in. 1.0 in. 1.9

15 57.7 in. 28 in. 8.6 in. 4.0 in. 3.1 in. 1.5 in. 2.9

20 76.9 in. 38 in. 11.5 in. 6.0 in. 4.2 in. 2.0 in. 3.8

25 96.1 in. 48 in. 14.4 in. 7.0 in. 5.2 in. 2.5 in. 4.8

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Safe Work Practices on Snow Covered RoofsWorking on snow-covered roofs poses signicant dangers, including slips, falls and exposure to extreme weather

conditions. To minimize the risk of working on snow-covered roofs, follow the guidelines in LP Reference Note 5171,

Safe Work Practices on Snow-Covered Roofs.

References:

Liberty Mutual Property, Winter Threats: Freeze-Ups and Snow Loading, PE1029, Weston, MA, 03/07.

Insurance Services Ofce, Inc., Roofs- Snow Loads,EngineeringandSafety@iso.com , 2000.

The illustration, instructions and principles contained in the material are general in scope, and to the best of our knowledge, current at the time of publication. No attempt ha

been made to interpret any reference codes, standards or regulations. Please refer to the appropriate code-, standard-, or regulation-making authority for interpretation or

clarification. Provided that you always reproduce our copyright notice and any other notice of rights, disclaimers, and limitations, and provided that no copy in whole or in pa

transferred, sold, lent or leased to any third party, or may make and distribute copies of this publication for your internal use.

2011 Liberty Mutual Agency Corporation. All r ights reserved.

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