Case Study - Cashmere Flat

Designed and constructed using reinforced concrete slab-on-ground and reinforced concrete masonry systems, this Cashmere flat emerged unscathed from the Canterbury earthquakes – unlike the weatherboard house on the same section.

Built five years ago by Firth Industries’ Southern Regional Manager, Dominic Sutton, for his mother-in-law, the two-storey flat makes extensive use of Firth 20 series (reinforced) masonry blocks with a concrete mid floor and a RibRaft® foundation.

The Firth RibRaft® system is an innovative method of concrete floor construction that is quick and efficient to lay and offers great strength and insulation benefits.

The system uses polystyrene pods, steel reinforcing rods, plastic spacers and RaftMix™ concrete. Each of the components fit simply together, dramatically reducing labour time and costs.

RibRaft® complies with the specifications of the Department of Building and Housing approved flooring solutions for the Christchurch rebuild.

The evidence of just how well concrete systems performed during the recent Canterbury earthquakes is apparent not only in the uncompromised flat, but also in the Sutton Family’s severely damaged weatherboard home that sits only metres away on the same property.

Built in the late 1940s on a more traditional unreinforced foundation, the Sutton Family home did not stand up to the second big earthquake to hit the region, unlike the flat.

Come February 22 our house exploded and the foundations are now broken and ruptured in multiple places,” says Sutton.


The Cashmere flat sustained no structural damage during the Canterbury earthquakes.

The traditional Sutton Family home suffered extensive damage during the earthquakes.


Basically the house twisted and vibrated on itself and has now come off its foundation and we have not been able to live in it since then.”

The Sutton family home is in the white zone – which is an undecided hill zone.  It is likely to require either significant repair or a total rebuild.

The reinforced concrete flat had no structural damage at all despite the fact that the two dwellings are immediately next door to each other and would have been hit by the same the shock wave.

“There is some minor cosmetic damage - a small amount of broken GIB board - but my mother in law is still living there,” says Sutton.

“The home acted like a bunker - the RibRaft® would have shaken but all the steel in it would have held everything tight.”

“As the Rib Raft system floats on top of the ground when the ground starts shaking it can largely mitigate the effect of lateral spreading.  It’s very strong compared to the old system. The current building code is far more earthquake specific than it was in 1948.”

Firth was contacted after the first earthquake in September by research engineers who wanted feedback as to the performance and location of existing RibRaft® floors in the Christchurch and Kaiapoi regions.

The September and February quakes have caused significant ground failure around the region, and some homes have sunk by more than 200mm.

Many of the worst affected areas have now been red zoned.

Sutton says he has spoken with many of the occupants of these homes with RibRaft® floors and they are very pleased with their purchasing decision to pay a bit more for this type of floor.

“The DBH foundation solutions guide suggests six or so ways of building foundations to cope with liquefiable soils and the RibRaft® system complies with their requirements,” says Sutton.