Dear Editor,
As reported in SN of Feb 9, a site inspection team from the Ministry of Education concluded without checking other possible contributory causes that high wind loads on the morning of Feb 6 created pressures or suction on the sloping windward roof of the library building of the Cyril Potter College of Education (CPCE) causing about 75 % of its zinc sheets to be blown off.
It is worth noting that no wind related damage to other buildings at this location was reported, although localized high wind forces seem to have been a factor which inflicted the damage on the CPCE library roof.
Reports elsewhere have stated that sub-standard work carried out by the contractor who built this roof was the main cause why the structure could not withstand the wind loads imposed upon it, and since the materials and the methods used to construct this roof were claimed to be of suspect quality the site inspection team should have broadened its investigations.
The evidence suggests that there are several possible causes other than wind-generated suction for this catastrophe, and these should have been carefully examined before a firm conclusion was reached as to the reasons why the sheets were blown off the roof. Firstly, the weight and thickness of the zinc sheets should have been determined to ascertain that their rigidity or stiffness met design requirements. The crumpled nature of the blown-off sheets indicate they were very flexible and probably of a very high gauge (30+). It should be noted that these parameters are usually stamped on the sheets by reputable manufacturers since the gauge of a corrugated metal sheet which is a measure of its stiffness should be stated before it is zinc coated, and not after. The zinc coatings on a corrugated steel sheet increase its thickness and protect it against corrosion, but do not increase its rigidity. Secondly, the type of fasteners used to properly anchor the sheets to the wooden roof frame should have been checked for adequacy. Were they corrosion treated nails or screws of the required lengths and with special heads to resist pull-out forces and water leaks? Thirdly, the spacing of the fasteners to ensure that they adequately anchored the sheets to the wooden roof frame to prevent separation should have been checked.
It is only after the site inspection team had checked what the contractor built against the requirements of the drawing and specifications for the roof and with the foregoing as a guide as to what is expected for a properly constructed roof, that a determination could have been made as to whether the roof was poorly designed and/or the contractor used sub-standard materials and methods in his construction.
The Ministry of Education site inspection team cannot come to a superficial conclusion that wind forces were the only cause for the separation of the sheets from the roof of the CPCE. After all, other buildings in the vicinity withstood these wind forces without damage, and only a thorough investigation will reveal in no uncertain way what the causes really were for failure of this recently renovated roof to withstand a squall of moderate proportion.
Yours faithfully,
Charles Sohan
