Hailguards: Design Considerations to Mitigate Hail Damage

Frequently the late arrival of the summer rains in the inland regions of South Africa is accompanied by high winds and hail. The hail can vary in size from small (6-15mm) to large (45-60mm) and everything in between. Although most damage to buildings and their contents is associated with large hail, damage from a deluge of small hail can be even more damaging.

Whilst large hail may dent metal cladding and shatter less robust materials the storms are generally of short duration and modest volume, those of small hail can be of an extended duration and the volume huge. A storm may begin with a brief barrage of large then progress to a deluge of small interspersed with heavy falls of rain as happened in Klerksdorp in November 2014. Accumulated large hail is porous allowing rain and melt water to flow into gutters and downpipes whereas large volumes of small hail tend to refreeze, particularly when the storm is accompanied by wind. (refer to related article in January 2015 edition of STEELSPEAK pertaining to sidelap leaks). Another problem is that small hail tends to accumulates in drifts behind parapets or where a low level roof adjoins a wall, etc. thereby blocking gutters and downpipes and flooding of the building.

The insertion of suitable hailguards will prevent the blocking of gutters and downpipes. Although common until the early seventies hailguards are seldom specified for new buildings. 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.

When designing hail guards it is important to remember the following;

  • The guard is to 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 5-10mm 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 leafguard 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 so as 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.