Update 10 clarifies how the Residential Guidance can be applied to poorer quality concrete perimeter foundation walls that are sometimes referred to as 'rubble' foundations.
63. How does the Residential Guidance apply to 'rubble' concrete foundation walls?
The Canterbury rebuild has shown that houses in Canterbury have a wide variety of perimeter foundation walls. These range from reinforced concrete perimeter foundations that are compliant with the standard for building timber-framed buildings (NZS 3604), through concrete walls with nominal reinforcement, to unreinforced walls with large elements of loosely cemented stone or rock.
Section 1.4.3 Technical Scope of the Residential Guidance states, "The document focuses principally on one- and two-storey timber framed dwellings (ie houses built to NZS 3604 or its predecessor Standard)." This does not mean that every component or element of the house has to comply with current NZS 3604 requirements for the guidance to apply. Rather, it is intended that the scope of buildings covered by the guidance is similar to the scope of those covered by NZS 3604, ie one- and two-storey timber-framed dwellings.
It is worth noting that NZSS 95, a predecessor Standard to NZS 3604, stated that the brick, stone and concrete foundation walls shall have adequate bearing area to safely support the imposed loads. The reference to brick and stone means that masonry could be used as long as it supported the vertical loads, which was the principal function of earlier foundation construction.
The guidance does not contain specific repair solutions for 'rubble' foundations, and is not mandatory for developing a repair solution. However, information in the guidance will be useful for developing repair solutions to reinstate the original function of a damaged foundation. The repair work must meet the performance requirements of the Building Code, which have remained the same since the Canterbury earthquakes. Some of the methods in the guidance to repair cracks and repair or replace perimeter concrete foundations are applicable across the range of existing foundations, (whether they be considered as "rubble foundations", or they comply with NZS 3604) provided that careful consideration is given to the nature and condition of the foundation.
Applying the damage assessment methods and repair solutions provided in the guidance requires a good understanding of the overall performance of affected houses. In addition, the development of any repair solution requires case-specific consideration and professional engineering advice. Regardless of whether the Residential Guidance is used to develop a repair solution, all repair work must comply with the Building Code.