Although hail damage to buildings is usually associated with large hail, the damage from a deluge of small hail can be even more damaging, warns Dennis White, director of the South African Metal Cladding and Roofing Association (SAMCRA).
The summer rains in the inland regions of South Africa are often accompanied by high winds and hail, which can vary in size from small (6 to 15mm) to large (45 to 60mm) and everything in-between.
“Large hail can dent metal cladding and shatter less robust materials, but the storms are generally of short duration and modest volume, while those of small hail can be of an extended duration and the volume huge,” he states. “A storm can begin with a brief barrage of large hail and then progress to a deluge of small interspersed hail with heavy falls of rain.”
The threat explained
According to White, accumulated large hail is porous and therefore allows rain and melted water to flow into gutters and downpipes, whereas large volumes of small hail tend to refreeze, particularly when the storm is accompanied by wind.
Another problem is that small hail tends to accumulate in drifts behind parapets or where a low-level roof adjoins a wall etc., thereby blocking the gutters, downpipes and flooding of the building.
Design to mitigate hail damage
To prevent this unnecessary damage as a result of small hail, White points out that the insertion of suitable hail guards will prevent the blocking of gutters and downpipes.
“Although common until the early 1970s, hail guards are seldom specified for new buildings today. Another benefit is that they help to prevent the accumulation of debris in gutters and downpipes. It is not uncommon to find plastic bags, sheets of bubble wrap and bottles blocking rainwater systems to roofs of shopping centres and buildings with low-pitch roofs,” he adds.
“When designing hail guards, it is important to remember the following,” says White:
• The guard should be located above and extend over the end of the cladding where it projects into the gutter. If it is located beneath the cladding, there is the danger of water from the melting hail breaching the lip of the gutter and leaking into the building particularly, if the roof has been fitted with insulation material that projects into the gutter. Blanket-type insulation will become saturated and if damp for an extended period of time, accelerate corrosion of the cladding. Triangular-shaped guards are not suitable for this reason.
• Preferably the upslope edge of the guard should follow the profile of the cladding, but with a 5 to 10mm clearance. The guard must not be in direct contact with the surface of the cladding. On residential buildings, where the guard also acts as a leaf guard, the gap should be to the lower tolerance.
• In order to reduce the risk of galvanic corrosion of the coating to the cladding, the guard must be manufactured from electrochemically compatible material. The same criteria will apply to the interface between the gutter and supports for the guard.
• The guard will need to be capable of supporting the mass of the accumulated hail (± 500kg/m³).
• Plastic-type guards will need to be of sufficient mass in order not to be damaged or made ineffective by wind action.
• Mini guards fitted across gutter outlets into downpipes are not an effective option, as they are easily clogged by debris and don’t prevent the accumulation of hail blocking the gutters.
Full thanks and acknowledgement are given to SAMCRA for the information provided.
To prevent damage as a result of small hail, the insertion of suitable hailguards will prevent the blocking of gutters and downpipes.
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