Storm Damage to Buildings: Act of God or Oversight By Man?

Following the recent spate of storm damage to buildings we believe it is necessary to review the possible contribution of the various components of the cladding systems to the apparent failure of roofs and to a lesser extent side cladding. All too often the damage is claimed to be as a result of extraordinary weather conditions, an Act of God, when in reality it is due to an oversight by man especially when a single structure in a group is damaged. In this article we review the part played by flashings. The main function of flashings is to weatherproof the junction between sections of cladding, intersection between cladding and other parts of a building such as walls and items of plant. The secondary function is aesthetics. The most vulnerable

flashings are ridge (including hip), barge/gable, external corners and to a lesser extent back flashings. These flashings are subjected to the most severe wind loading, invariably applied as a suction, imposed on a building which can be as high as 2.8 times that of other parts. The loss of one of these types of flashing can precipitate the loss of adjacent cladding. Their ability to resist these forces is in the main dependent on the fixings attaching them to the cladding and to a lesser extent geometry and thickness of material. It’s for this very reason that rivets are not considered to be suitable fasteners.

Possibly the most vulnerable is the ridge flashing where the minimum girth is 230mm (±430mm between fasteners) to 600mm or more with just the centre fold providing any form of stiffness (flatter the roof the lesser the stiffness). Ridge flashings should be fixed to every fourth rib of corrugated iron, every second rib of box rib profiles and every rib on concealed fix profiles. Whilst the fasteners on the leeward slope are subjected to a pull over force those on the windward slope are mainly in shear with the risk of sideward tear-out.

Barge and corner flashings have to resist similar loading patterns but are stiffer by virtue of the right angle profile but need to be fastened at no more than 600mm centres. On structures with an eave height in excess of twelve metres barge flashing may need to be fastened to the roof cladding at 450/500mm centres in the high suction zone in the corner where the eave and gable intersect.

Back flashing in excess of 700mm wide needs to be attached to roof cladding at not more than 600mm centres at each purlin line for pierce fixed cladding or between 100-150mm from the purlin line for concealed-fix profiles(so as not to impede the free movement of the cladding over the anchor clip or cleat).

These flashings should be fixed to the cladding with stitching screws, complete with 19mm diameter bonded washers, which are specifically designed to fix thin gauge materials together. The use of connector brackets in lieu of fixing directly into the cladding profile is preferable on large buildings. Conventional roofing screws are not suitable. Steel flashings with a girth of 600mm or less should be a minimum of 0.58mm thick and flashings of greater girth 0.80mm thick. Flashings with a girth in excess of 1.0m require special consideration.
Another consideration is thermal movement. Ridge flashings move transversely across cladding, barge flashings move longitudinally with roof cladding but transversely across gable cladding. We recommend an expansion joint at not more than 12.0m centres.