Snow and Ice Load Roof Collapse

Post date: Feb 18, 2014 3:17:25 PM

Dear Clients and Friends:

This winter, the tri-state area has experienced significant snow and ice precipitation that has not been seen in many-many years. One of the effects of the snow and ice precipitation is roof failures. The property owners attribute the failures of their roofs to the snow and ice load.

Metropolitan Forensic Engineering (METRO) is often asked to address property defects and causes of collapses. Our experience is that most of the roof failures occur due to rafter framing failures, truss failures, other faulty design or construction, foundation settlement, and so on. The first part of METRO’s analysis requires assessing snow accumulation by obtaining nearby weather stations pertinent records. The next step is to assess if there are any special considerations, such as, the potential for drifting, any special loading situations, etc. While the weight of snow varies and changes with temperature, it has been determined through various studies by METRO and other professionals that heavy wet snow can weigh from 12 pounds per cubic foot (pcf) to 20 pcf. Most design codes in the region require at least a 30 psf ground snow live load. This would suggest that the roof should be capable of handling a snow pack of 30” (2.5 feet) of light snow (30 psf/12 pcf) and 18” (1.5 feet) of heavy snow (30 psf/20 pcf).

METRO typically has found a range of snow and ice accumulation loads in the tri-state ranging from 10 psf (pounds per square foot) to 25 psf. Normally this loading would cause bending of the rafters and the roof ridge but typically does not break anything. The design code tolerance allows for a specified amount of bending at the design load. Typically, a 1" in 20' is the tolerance for most roof structures.

The amount of snow we had in most of our area is still under the design capacity of a typical roof and therefore the weight of snow and ice cannot be the root cause of the roof failure. Many of the rafter problems observed this winter were due to the lack of lateral restraint. That is, the horizontal load from the rafter pushed on the top of the wall and the wall leaned out. In yet other case we found that the rafters weren't connected to the ceiling joists, or ceiling joists ran the wrong way. METRO has found in several cases that the rafter size and spacing is under-designed to support this type of loading we had so far this winter, as 2” x 6” rafters typically should not exceed a 12’ span, but unfortunately they do. The rafters are not however what typically fail although they sometimes appear to have a slight creep deformation.

METRO has also found a significant number of truss failures. The snow and ice loads we have experienced thus far this winter approached the design load but rarely exceeded it. Even with the loads exceeded, the margins of safety would have prevented a collapse or member failure. METRO forensic investigations found manufacturing defects, design defects and installation defects. One very common reason for truss failure was the lack of bracing per manufacturers specifications.

Whether you are an insurance adjuster, contractor or a property owner, we can quickly and accurately assess damages.

What Metropolitan can provide for you:

• Roof Condition Assessments
• Cause of roof leaks and water penetration
• Cause of chimney discoloration
• Cause of blistering, cracking and/or separation of rooftop coverings or membranes
• Evaluation of damage to roof components such as HVAC units, skylights, and gutters
• Itemized reports of necessary repairs for cost effective remediation
• Structural assessments of roof framing and ceiling rafters

Storm Damage Analysis

• Snow, hail, wind and ice build-up loss claim evaluations
• Analysis and assessment of roof damage and/or roof replacement due to storms
• Assessment of roof failure or collapse as a result of accumulated loads, additional weight or snow drift

Civil and Structural Engineering

Metropolitan has extensive experience in civil and structural engineering evaluation. Our Registered Professional Engineers have evaluated thousands of structures since 1976. We investigate failures and accidents, provide oral or written reports of findings, and provide courtroom testimony when required. We have investigated damage caused by hurricanes, tornados, floods, fires, construction defects, soil subsidence and long term neglect.

Our experience includes evaluations of homes, commercial buildings, industrial facilities, marine terminals and historic structures. Our areas of specific expertise include residential and commercial roofing, foundations, building envelopes, water intrusion, construction defect evaluation, and premises liability evaluations.

• Moisture Intrusion
• Roof Collapse
• Pool Failures
• Humidifier Failures
• Mold Assessments in Attic and other spaces
• Storm Damage Assessments
• Hurricane Damage
• Tornado Damage
• Construction Defects
• Thermal Imaging
• Slip and Falls
• Construction Vibration
• Hail Damage
• Flood Damage Assessments
• Resident Inspection Services
• Foundation Damage Assessment
• Retaining Wall Damage Assessment
• Stucco Damage Assessment
• Roadway and Sidewalk Claims
• Interior Finishes
• HVAC
• Plumbing
• Marine Facilities
• Basement Evaluation

A comprehensive investigation is crucial to good claims handling. Our clients require prompt and thorough claims investigation and fact finding and Metropolitan delivers high quality services at a very competitive price.

We are here to serve you 24/7. Give us a call or email. An assignment form is attached.

Sincerely,

Bill Stephan, P.E.

METROPOLITAN PROFESSIONAL ENGINEERING AND FORENSIC SERVICES

P.O. Box 520

Tenafly, NJ 07670-0520

Tel.: (973) 897-8162

Fax: (973) 810-0440

E-mail: metroforensics@gmail.com

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