The fallen flue

Aug 1, 2009 by

The insured owned an unoccupied, three story rental mountain cabin.   A second story chimney fell, hitting the guardrail of a balcony located over the garage.  This damage reportedly allowed water intrusion into the roof of the first story garage.   The insured said the chimney fell as a result of wind.   The client said that the cause was snow load.   GEI was assigned to inspect the insured’s cabin to identify and determine the cause and approximate age of the damage.

The building was a single-family, three-story home of wood frame construction on a steep slope.   The roof consisted of sheet metal on plywood sheathing.   The exterior walls were of 2x stud construction with exterior plywood sheathing on the outside and gypsum wallboard on the inside.   The foundation consisted of conventional concrete spread footings.   Some snow still remained on the balcony at the time of our inspection.

The chimney that fell onto the balcony deck and impacted the guardrail was found to be a metal fireplace flue that was 10 inches in diameter and 7 feet long.   The weight of the flue, along with the components that fell with it, was 193 pounds – the weight of a reasonable size male adult.

The chimney fell off the furnace and onto the balcony.   Determining the cause of this event was not part of this evaluation, but it was reported to  be due to some combination of high winds and excessive snow loads.   The possibility that the wind alone launched the chimney into the guardrail was considered.   This was dismissed because, short of a tornado, wind forces do not launch the equivalent of a 200-pound man.

It is presumed that once the chimney fell, it then slid approximately 12 inches and came to rest against the steel framed portion of the guardrail.   The slide of the chimney was likely facilitated by the snow accumulation on the balcony, which sloped towards the balcony guardrail.

The steel framed portion of the balcony guardrail was leaning outward, and the broken mounting surface allowed water intrusion into the balcony framing.

Lag bolts of different lengths connected the chimney steel straps to the roof framing (prior to the fall).   The lag bolt non-uniformity was evidence of a faulty installation.

We observed water damage to the roof plywood sheathing where the chimney mounting straps had been embedded in the roof sheathing.   This led to a combined reduction in the holding capacity of the wood and corrosion of the lag bolts.   The long-term intrusion of water and deterioration of the sheathing in the area of the connectors contributed to the collapse of the chimney.

Both welds that connected one of the steel straps to the chimney had been severed before the collapse, as evidenced by the significant amount of corrosion present in the sheared faces of the welds.   The collapse of the chimney was the result of a combination of construction and maintenance issues.

The outward tilting of the steel framed portion of the guardrail intermediate posts damaged the balcony waterproofing immediately around the posts.   This then allowed ambient water to enter and deteriorate the balcony floor/garage roof framing.   Paint was chipped off of a small area of the steel framed portion of the balcony guardrail, where the chimney struck it.  This was a recent event.   Although paint had been chipped off, there was no kinking in the steel surface that would be consistent with a substantial impact.   In addition, there was no paint transfer from the guardrail to the chimney.   This indicated that, at most, the impact was minor.

Our expert computed the horizontal thrust that the balcony guardrail would be required to resist to be in compliance with the California Building Code as 320 pounds.   The horizontal thrust, due to the impact of the chimney was determined to be 88 pounds.  The computations illustrated what was intuitively apparent.   A man – or the equivalent thereof – leaning on, or even running into, an adequately constructed balcony guardrail would not cause it to fail.

There was a separation between the wood framed portion of the guardrail facing east and the wood framed guardrail facing north.   This separation was not the result of the chimney impact.   The ends of the balusters were painted over indicating the gap was present and pronounced at the time the guardrail was last painted.

The base of the balcony guardrail was observed through the garage ceiling.   One of the two lag bolts that connected the guardrail post to the garage wall framing was missing.   It was this deficiency that was the principal cause for the outward tilting of the balcony guardrail.

In conclusion, the primary cause of the distress observed was deficient construction (lag bolts) and maintenance of the structural elements of the balcony guardrail.   This was exacerbated by a combination of snow forces and impact from the fallen chimney.   The distress was likely to have occurred within the last three months before the date of the inspection.