The causes of leaky homes can be split into three categories; Poor design and/or materials, poor construction, and poor quality control.
Lack of eaves, over-complicated shapes and junctions for the technology and expertise of the day, flat roofs and a complete lack of detail on design drawings led to homes that leaked like sieves when cobbled together by the poorly skilled and supervised builders of the day.
As a countermeasure, building regulations were fine tuned with the updated Building Act in 2004, and Councils now require much more design detail in Building Consents before approval to build will be granted.
Lack of skill, but also lack of knowledge were contributing factors here.The final 'nail in the coffin' was lack of professionalism. When 'tradesmen' didn't know how to produce the result the designer was looking for, they would just wing it. Ironically, there was a lot of information available in the 90's regarding 'good practice', many builders and budding developers simply chose not to read it.
In 2007 the Licensed Building Practitioner scheme was introduced by the government so that a register of all tradespeople with appropriate qualifications for specific tasks of building could be formed. From 1 March, 2012 only builders with the appropriate Licensed Building Practitioner rating will be allowed to carry out critical parts of construction, in particular in relation to weathertightness and durability.
Poor Quality Control
Developers and City Councils are ultimately responsible for a building's compliance to building regulations. Of these regulations, Clause E2 for External Moisture and Clause B2 for Durability of the Building Code are the most important. It is vital that during the construction stages sufficient inspections are carried out to guarantee moisture will be held at bay, and the building will last the required 50-years.
Developers who payed no attention to the construction of their projects, and Council and private inspectors who were often not qualified enough to conduct accurate inspections, or were simply lazy, led to sub-standard workmanship and design slipping through the net.