Case of the Month: A Sagging Roof

Sep 1, 2014 by

The insured claimed roof damages due to recent high winds. GEI was hired to examine the damages and to identify and determine the predominant cause and their approximate age. The structure was a single-story, single-family residence, with an attached garage, built circa 1956. The insured purchased the home in January of 2014.

The structure was timber framed with a stucco exterior finish and plaster interior finishes under a composition shingle roof. The garage roof had a distinct sag or bow downwards in the center, when viewed from the outside.

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On initial review of the garage roof interior, our expert immediately saw the cause of the garage roof bowing. The ridge board was cracked and sagging. He advised the insured that immediate temporary structural reinforcements should be inserted under the cracked ridge board to relieve the load on the broken ridge board and rafters. The homeowner agreed.

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The ridge board, which spanned the width of the garage, was a nominal 2-inch by 8-inch timber of unknown grade. Attached to that ridge board were 2-inch by 6-inch rafters at approximately 20-inches on center. The rafters were tied at every other pair at the top and near the springing (the point where an arch or roof rises), with a vertical hanging tension member between the two, all ties and tension members of 2-inch by 4-inch ungraded lumber, fastened with 16d nails. All connections of rafters, ridge board and ties were nailed. There were no other undue loading of the rafters and ties, other than the weight of the garage door and its electric garage door opener.

The manner of connecting the garage door and its opener was to support the drive motor in the center from two metal straps. The straps were fastened to the ridge board with lag bolts, which did not appear to penetrate the timber through. There were both old and new lag bolts screwed into the ridge board.

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The position of one of the lag bolts, was exactly coincident with the position of the crack in the ridge board. Two new lag bolt fasteners, evidenced from their lack of patina of oxidation, were recently added.  The edges of the garage door were supported on door tracks, wherein the guide wheels ran in a closed track attached to the overhead and to the front of the garage. The north end door support track was supported from one of the under-tie members, which was not intended for vertical loading, being fastened only using nails. That tie member was separating from the rafter tie.

In addition, it was being placed under a cantilever loading, multiplying the force on the nailed junction. The south end of the door track support was secured to a diagonal strut, which strut had a knot defect that resulted in the strut being bent at that knot and was defective in accepting loads from the movement of the door in the track. The center door support also relied partly on a secondary but defective vertical support.

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The possibility that wind loads may have caused excessive deflection of the roof, and subsequent cracking and damage, presupposed that winds were of a sufficient strength and duration to impose excessive loading. Since there was no instrumentation mounted on the residence, our expert looked at measured wind speeds in the surrounding area. The nearest three municipal and international airports were from four to seven miles away from the residence. During the month of April 2014, reports indicated wind speed gusts (3-second duration) of 61 mph. Maximum sustained wind speeds (1 minute duration) of 40 mph were recorded. Neither the sustained nor the gusting wind speeds exceeded the criteria set out in the California Building Codes for wind speeds to be resisted by residential buildings (80 mph base speed). Therefore he concluded that excessive wind speeds did not occur at the residence.

Based on the foregoing, the expert’s opinion was that the predominant cause of the damage observed was long-term failure of an overloaded and poorly constructed garage door suspension system that relied on the roof structure improperly for support. The approximate age of the cause of those damages was at least one year prior to the date of inspection.

 

Expert of the Month: Lindsey Philpott

 

Mr. Philpott is a Registered Civil Engineer in the State of California. His education includes study at Bristol Polytechnic in the UK, a Master of Science Degree in Civil Engineering from the University of Arizona and a Master of Civil Engineering from Norwich University, Vermont. His areas of expertise include residential construction issues, water process system safety, and potable water treatment systems including matters relating to reclaimed water. He is also versed in issues relating to sludge composting and trash compacting. He is licensed by the US Coast Guard as a Captain, and he teaches rigging and rope work, as well as volunteering as a rigger and mate on many of the square-rigged vessels in southern California. He is a published author, and is president of the Pacific Americas Branch of the International Guild of Knot Tyers. He has also provided forensic knot analyses in criminal cases.

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