Cladding systems – Design working life

When it comes to choosing a cladding system there is often a misunderstanding between design working life and service life. The design working life of a cladding system is the fixed period of time during which it will meet prescribed design parameters, subject to specified maintenance, without abuse or repair whereas the service life is the period of time during which it can be used economically, without abuse, with or without maintenance and repair.  Service life can, therefore, be longer or shorter than design working life.

Factors that influence the design working life are;

  • probability of wind and rain damage
  • environment
  • ancillary items
  • fasteners
  • workmanship
  • temporary conditions during construction
  • maintenance

Probability of wind and rain damage:

The longer the time period the greater the risk of exposure to abnormal storms.

SANS 10160-1 provides indicative periods for four different categories varying from 10 to 100 years for temporary to essential buildings respectively with a mean of 50 years and 25 years for replaceable structural parts. SANS 10400-B calls for a minimum of 30 years for the structural system and non-accessible components but reduces the period to 15 years for repairable or replaceable components and materials, such as cladding, roofing materials, exterior trims and integrated components, such as windows and doors. We are of the opinion that a return period of 15 years should be limited to residential and low rise (10 m) buildings only. Regarding wind load the difference between a 15 and 30 year return period is in the region of 27% and for rainfall 15%. For internal gutters we recommend a minimum return period of 50 years.

Environment:

Often the micro environment and/or the internal environment can be more severe than the macro environment. This is an important consideration when selecting the protective coating/s including those to the fasteners. The inclination of a roof in a highly corrosive environment can also impact on the durability of coatings.

Ancillary items:

The design, sealing and method of attachment of flashings and translucent cladding needs to be carefully considered particularly in the areas around the periphery that are subjected to the highest wind loading. Differential thermal movement between flashings, ventilators and translucent cladding must be taken into account. This aspect is commonly overlooked with long length concealed-fix cladding. Failure to consider these factors will invariably compromise the weatherproof effectiveness of the cladding envelope.

Fasteners:

It is important to match the mechanical properties and durability of protective coatings on both the primary (attach cladding to supporting structure) and secondary (attach ancillary items to cladding plus tie side laps) fasteners to that of the cladding material, likewise the durability of the sealing gasket with pierce-fix cladding. The performance of secondary fasteners is one of the most overlooked considerations. Using inadequate fasteners frequently negates warranties.

Workmanship:

Installation of a cladding system by inadequately trained workmen frequently reduces the design working life of a cladding system. The shortcomings of their work may manifest after a short period or only become apparent when the system is subjected to the more severe performance requirements. We recommend the use of contractors approved by the manufacturer of the system.

Temporary conditions during construction:

Wind loading on partially clad buildings and buildings with temporary openings can exceed that on the completed building. The degree of vulnerability is linked to the duration of the installation of the cladding system.

Maintenance: There is a correlation between initial purchase price and cost of maintenance. What may initially appear fit for purpose invariably has a far higher cost of maintenance. Remember there is no substitution for quality in the long term.