Quick Dips

Fatigue under the Floor

The Building Research Association's 10-year Atmospheric Corrosion Survey has produced information on how common building metals corrode when fully exposed to the atmosphere, and is now looking at the microclimates that occur under houses.

The study's national coverage is unmatched in the world, according to BRANZ materials manager Adrian Bennett. He notes that it has been immensely useful for putting a particular location in perspective in terms of corrosivity, and also for predicting the lifespan of exposed components such as roofs.

"However, it is of limited use when predicting the behaviour of items that are sheltered from the rain but still exposed to wind-blown salt and other airborne contaminants."

These microclimates occur in places such as under floors of houses. BRANZ is looking at these finer details of atmospheric corrosivity using a specially constructed test enclosure at Oteranga Bay, near Wellington, a severe marine environment.

Because of the unique nature of New Zealand's Building Code, and its requirement that structural fasteners last at least 50 years, researchers need to know in greater detail the factors that lead to the corrosion of fasteners, especially under houses. Bennett says research so far strongly suggests that the more ventilation, the more corrosion.

"However, there appears to be a threshold value for ventilation levels below which corrosion will also increase because of the humidity levels resulting from low levels of ventilation. This means further investigations are needed to establish a range of under-floor ventilation designs that will provide low levels of chloride and still maintain adequate ventilation under the floor."

This information has already been used to update NZS3604, the standard to which many New Zealand houses are built.