Alan Edwards, of Dunedin, asks :-

As the owner of a hundred-year-old villa I am aware the nails in it are rusted, and presumeably the older the building the more the nails are rusted. How would such a building stand up to an earthquake?

Bruce Deam, a timber and earthquake engineer at the University of Canterbury, responded.

In my experience, it is normally only surface rust. However a period of dampness sometime during the life of the building could make this far more significant. The steel purity (particularly the amount of and potential concentration of carbon) would also affect its decay rate. I don’t know of any studies on the rate of decay, although retired CSIRO researcher Bob Leicester may have considered this in his durability studies a decade or so ago.

The rust would have some effect on the seismic response but it would be pure speculation to quantify this without knowing exactly where the nails were and what they do. Surface rust is often beneficial in providing higher withdrawal strength, but, conversely it weakens the shank strength. Rust makes the steel fracture more readily if it is bent back and forward during an earthquake.

For general enquiries regarding houses I recommend the Building Association of New Zealand (BRANZ) Helpline (0900 5 9090, $2/min). For investigating a particular house, then only a qualified building engineer could comment after a thorough inspection.