The recent devastating earthquake in Christchurch New Zealand has revealed some major differences in the way roofs react during a seismic event. The type of roofing used being the difference between minor damage and total collapse. It would be useful to look at the reasons roof structures fail during ground movement and what can be done to improve roof security.
Designers building earthquake resistant buildings have a saying, "Earthquakes do not kill people, buildings do". At present buildings designed in earthquake regions must fulfill two criteria. Minimize the structural damage during a moderate shake and to preserve life during a more serious Earthquake.
Chimneys are one of the first things to fall during an earthquake. An estimated 10,000 chimneys collapsed during the 1942 Wairarapa earthquake. The chimneys strength lies in the bond between brick and mortar. Once this bond is broken all you have is a few tons of stacked bricks. It is advisable to remove the chimney completely and replace it with a lighter chimney flue. An aesthetically pleasing result can be achieved by the use of imitation bricks formed around a secure structure.
The kinetic energy released during a seismic event can exert massive stress on roof structures. The horizontal stresses imposed on the building structure can cause the beams supporting the roof to collapse. They simply lose support. The lateral forces imposed are increased with heavy roof coverings. Heavy roofs exert higher lateral loads on the supporting structure. The supporting walls are constantly being pushed outwards. Consequently, lighter roofs exert less pressure and reduce the chances of building collapse. Some examples of this can be seen in Christchurch. Houses in close portfolio suffered different levels of damage depending on the roof mass.
Modern metal roofing components are three times lighter than Concrete and clay roofing tiles. Steel tile roofing weighs around 8 pounds per square meter. The tiles cover a larger area and are fitted to the same strength batons as concrete tiles. When you consider that a single concrete tile weighs in at 5 pounds, it's clear to see which is more likely to come crashing through the ceiling. Because of the weight to area ratio, sheet metal roofing, corrugated and shingles reduce the risk of building penetration even further.
At present methods of seismic control use kinetic energy reduction to help buildings absorb the forces created during a shake. Helical steel springs, dampers and oil filled pistons have all been used to help structures "flex". More recently rubber bearings have been installed at foundation level. These huge rubber pads sit at the base of structural pillows and were injected by Bill Robinson to dissipate seismic energy.
Could an adapted version of these seismic dampers be used at roof level? A rubber bearing placed under the roof plate would allow the roof structure a degree of lateral movement. This would reduce the likelihood of the roof losing its bond with the supporting structure. Moreover, if the roof is allowed to flex without losing support it will help retain the stability of supporting walls. The roof causes the majority of the damage during a collapse so it would seem wise to give more thought to its stability.
Legislation and building codes have been in place for many years to protect our homes from the dangers of fire damage, damp, electrocution, and structural requirements. In areas of seismic activity, it is my belief we need to give some serious consideration to introducing legislation for acceptable roof coverings. Five tonnes of concrete above your head is to my mind, completely unacceptable.