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What snow load capacity does the building code require?

A commonly asked question in recent weeks has been, "How much snow can my roof carry?" or, "When do I have to worry about having too much snow on my roof?"

Older buildings constructed in the early part of the 20th century or before don't appear to be having as much of a problem as more recently constructed buildings. This observation applies to both residential and commercial buildings.

First, let's examine what snow load really is. Snow is frozen water in a crystalline form, but what matters most is its water equivalency. Water has a weight density of approximately 62.4 pounds per cubic foot. One inch of water causes a downward pressure, or load, of a little more than five pounds per square foot (5.2 psf). That same inch of water is equivalent to about 10 inches of dry, cold snow. This means each inch of dry, cold snow creates about half a pound per square foot of vertical loading.

With melting and evaporation, some of the snow pack is lost resulting in a likely reduced load. If rain falls on the snow, some of it will run through and off the roof and some will be captured by the snow pack and held in place.

For this reason, it is important to measure the total weight density of the material on the roof--ice, snow and water--not just the thickness.

What snow load capacity does the building code require?

For many years, builders and building code officials assumed that the ground snow load for the entire state of New Hampshire was 60 pounds per square foot. The only town or city in the state that had a National Weather Service station was Concord, and the 60 psf applied only there.

But in February 2002, the U.S. Army Cold Regions Research and Engineering Laboratory and the Structural Engineers of New Hampshire collaborated on the publication of a definitive method of determining snow loads throughout the state, "Ground Snow Loads For New Hampshire."

The new building codes specifically give precedence to this method of determining snow loads, and this publication is referenced by them. The ground snow load is further modified in the building code to account for variables like roof slope, shape complexity (especially those causing drifts), surface conditions (smooth vs. rough), temperature (cold or heated) and the surrounding terrain conditions. The final result is the snow load for which building roofs should be designed. This number explicitly includes the effect of rain on the snow.

'Accident waiting to happen'

Prefabricated roof trusses are commonly and frequently involved in roof failures and have been since they were first introduced into the building industry nearly 40 years ago.

Prefabricated roof truss assemblies have been shown to be very strong and reliable when properly assembled and braced. The Wood Truss Council of America has published design guidelines for bracing and the manufacturers typically place markings on every truss showing the best locations for bracing attachment.

Nonetheless, roof failures continue to occur. The typical failure of prefabricated roof truss assemblies is the direct result of contractors' failure to provide the required lateral bracing necessary to prevent buckling and collective collapse, as is frequently found during forensic examination of collapsed structures.

Contractors commonly use nothing or one-by-three strapping in lieu of the full-sized lateral and cross-bracing required to provide stability and structural capacity to these assemblies. Without adequate bracing, pre-fabricated roof truss assemblies cannot provide the load-carrying capacity for which they have been designed.

In fact, without the required bracing, prefabricated roof truss assemblies are "an accident waiting to happen."

Residential building contractors are not regulated in New Hampshire, and residential construction is monitored inconsistently from one town to the next.

A few of our larger cities and towns have fulltime building code enforcement officers who are familiar with construction details and armed with regulatory authority over compliance and certificates of occupancy. However, the majority of towns have only part-time code enforcement officers or utilize the thinly stretched fire department or volunteer members of the board of selectmen to examine buildings under construction.

How much snow will any roof hold? That question can only be answered on a case-by-case basis. Properly designed and constructed roofs will hold more snow than we're likely to see in a lifetime. The larger question is, "Is my roof properly constructed?"

Until the insurance companies and the state and local governments come together to properly enforce building code requirements, failures are likely to continue to occur.

Timothy L. Grant, regional manager for Provan & Lorber Inc. engineers, is a licensed professional engineer with expertise in structure/failure forensics end is one of the founders of Structural Engineers of New Hampshire.
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Author:Grant, Timothy L.
Publication:New Hampshire Business Review
Geographic Code:1U1NH
Date:Mar 14, 2008
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