Agents of change

This Issue This is a part of the Uniquely NZ feature

By - , Build 142

New Zealanders are a resourceful lot, always exploring new and better ways of doing things. We look back in time at two distinctly Kiwi twists on ways of building.

Figure 1: Shaw’s camerated concrete used in an Upper Hutt house built in 1912.
Figure 2: 1950s precast concrete ventilator and positioning. Illustration from Carpentry in New Zealand, 1958.

IN BUILD 118, pages 58–60, we looked at three New Zealand inventions. Here, we examine two ways of building things that have been different in New Zealand – camerated (vaulted) concrete and subfloor ventilation.

Camerated concrete walls

In 1905, Henry Arthur Goddard of Sydney patented his camerated concrete wall system. He patented it in New Zealand in 1906, with a further patent in 1910 for improvements to the collapsible core box.

The camerated concrete system involved pouring concrete into formwork of about 2 ft (0.6 m) in height with internal collapsible cores. When the concrete had set, the inner cores and exterior boards were removed, leaving a hollow concrete wall ready to be finished.

A 1911 camerated concrete cottage built in Nelson had concrete walls 8¾ in (220 mm) thick with a 5 in (127 mm) cavity and a solid concrete tie every 18 in (460 mm). The wall rose at the rate of a foot a day (0.3 m) to the final height of 11 ft (3.4 m).

Camerated concrete was represented in New Zealand by H Leslie Friend and actively promoted in newspapers. Local authorities included it in their building controls, with calls for tenders for buildings using camerated concrete reported throughout New Zealand from 1908 to 1912.

Local improvement

Yet even camerated concrete could be improved. In 1912, Herbert Walter Shaw, builder of Upper Hutt, patented his improved means for building concrete walls, promoting ‘the Shaw “improved” camerated concrete patent’. Shaw also patented a better core design – a reinforcing medium based on two or three twisted wires – and a complex internal wall design.

According to local newspapers, Shaw built a shop for a Mr Gibbs and a dwelling for Mr H R Pearson (see Figure 1). Unfortunately, Shaw’s own home was destroyed by fire in August 1913, but he quickly announced he was to rebuild in camerated concrete.

Newspaper reports of camerated concrete stopped in 1920, although it is not clear whether this was due to the method becoming so well known it no longer require promotion or the closing in 1919 of Friend’s Auckland-based Camerated Concrete Co.

Subfloor ventilation

The area underneath a suspended floor has a foundation to raise the floor above the ground and protect the timber joists and flooring. There is a long tradition of recommending adequate ventilation for the subfloor space, initially for health reasons rather than durability.

In 1875, the UK Public Health Act set out mandatory minimum requirements for timber ground-floor structures, requiring that timber boards be supported on joists on a damp-proof course (DPC) with a ventilated airspace beneath the floor to prevent the penetration of cold and damp. DPC materials included slate, asphalt and cement.

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NZ early adopter of subfloor ventilation

The first New Zealand attempt to establish minimum subfloor ventilation requirements appears in a background paper prepared for the Building conference relating to the use of timber in building construction held in Wellington in June 1924.

The paper recommended a minimum floor clearance of 12 in (30 cm) with 7% of the floor area provided as cross-ventilation. This recommendation had come from the 1923 report of the US Bureau of Standards Building Code Committee, which, in the words of the New Zealand conference organiser, was ‘an inspiration to all concerned with the Building Code problem’. This appears to be the first specific numerical requirement for subfloor ventilation in the world.

The conference attendees rejected 7% as excessive, noting that a 1600 ft² (149 m²) house would require 28 ft² (2.6 m²) of ventilation on each 40 ft (12.2 m) side. The conference finally recommended ‘one square inch to one square foot of floor area’ (0.7% of the floor area).

Figure 1: Shaw’s camerated concrete used in an Upper Hutt house built in 1912.

The original source of the 0.7% has not been found, but the two speakers in favour represented architects – Mr FE Greenish, Wellington – and engineers – Mr J Maxwell, Auckland City Council. It appears that this recommendation was not widely used, if at all. For example, the City of Auckland City bylaw Number 1, passed on 15 April 1925, required only 9 in (230 mm) clear between the ground and the underside of the joist and this space to be ‘thoroughly ventilated by means of suitable and sufficient air-bricks, or some other effectual method’.

When, the following year, the Christchurch City Engineer submitted the proposed Christchurch building bylaws to the government’s Special Timber Committee, the Forest Service comments noted no specific details were provided in the proposed bylaw and suggested 7% of the floor area, but this does not seem to have been implemented.

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Specified in 1944

It was not until NZSS 95:1944 Part IX that specific requirements found their way into the bylaws. Clause 920 Foundation ventilation required vents of ½ in² per ft² (1:288 or 0.35%) to be spaced a minimum of 6 ft (1.8 m) apart and 2 ft 6 in (0.76 m) from corners (see Figure 2).

Vents had to be as close to the bottom plate as possible and vermin proof to exclude animals and poultry.

The Army Education Welfare Service’s (AEWS) 1944 Carpentry Part 1 study course stated the usual practice was to space 9 × 4½″ (230 × 110 mm) ventilators at 8 ft (2.4 m) centres, the same ventilation area per square metre as NZS 95, although in both cases, the reduction in ventilation area due to the grating or grill was not considered.

NZS 1900:1964 Chapter 6.1 Clause 6.1.18 Subfloor ventilation retained the same vent areas and spacing up to the 1978 revision. The requirement was then passed to NZS 3604 and converted to metric, requiring 3,500 mm²/m² (0.35%).

Figure 2: 1950s precast concrete ventilator and positioning. Illustration from Carpentry in New Zealand, 1958.

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US specified in 1942

The first US-specific requirement came in the Federal Housing Administration’s 1942 minimum construction requirements – ‘total ventilating area equivalent to ½% of the enclosed area plus ½ ft² for each 25 lineal feet of wall enclosing that area’ with a minimum of two vents located to provide cross-ventilation (0.9% for the same 149 m² house).

In 1948, the ½% was decreased to ⅓%, (0.74%), which could be further reduced with a suitable ground cover. By 1958, the requirements had been revised to require ‘at least four foundation wall ventilators … with an aggregate net free ventilating area of not less than 1/150 of the area of the basementless space’ (0.67%) although 1/1500 of the area (0.067%) was acceptable if the ground was covered with a vapour barrier. These requirements remained virtually unchanged until 1984.

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UK specified in 1961

The first UK specific requirement came from the Building Research Establishment in 1961, when the standard was given as 1½ in² of open area for every foot run of external wall (0.21% for 149 m² floor area). It was not until the 1985 regulations that there were specific mandatory requirements for ventilation. The ground was required to be covered, a ventilated space provided between the cover and the timber and a DPC between the timber and any ground moisture.

The 1985 ventilation requirement was 3,000 mm² of free openings per metre run of wall (0.10% of floor area), and as well, ‘there should be a free path between walls’. The current UK subfloor ventilation requirement is the greater of 1,500 mm²/m run of external wall (0.05%) or 500 mm²/m² of floor area (0.05%), although again both assume a DPC.

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Australia in 2003

In Australia, the Building Code of Australia has three subfloor ventilation zones, with the more humid coastal areas having a higher requirement. The use of a ground-sealing impervious membrane permits a halving of the requirements in each zone.

Table 1 summarises the New Zealand, US, Australia and UK subfloor ventilation recommendations or requirements from 1923 to 2004 based on a 149 m² plan house.

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BRANZ’s role

New Zealand was an early adoptor of subfloor ventilation requirements, albeit based on best practice and committee discussions.

Today, BRANZ is actively researching to establish a scientific moisture balance basis for these ventilation requirements.

Table 1 SUBFLOOR VENTILATION REQUIREMENTS
YEARSOURCECOUNTRYFLOOR AREA
1923 Recommended minimum requirements USA 7.00%
1924 Paper for Timber Building Conference NZ 7.00%
1924 Recommendation of Timber Building Conference NZ 0.70%
1942 Federal Housing Administration USA 0.70%
1944 NZSS 95 NZ 0.35%
1944 AEWS Carpentry (9 × 4½" at 8 ft centres) NZ 0.35%
1948 Housing and Home Finance Agency (HHFA) USA 0.53%
1958 FHA minimum property standards USA 0.67%
1961 Principles of modern building UK 0.10%
1964 NZS 1900 NZ 0.35%
1978 NZS 3604 NZ 0.35%
2003 Building Code of Australia Zone 3 AU 0.20%
2004 Building Regulations Part C (with DPC) UK 0.05%

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Articles are correct at the time of publication but may have since become outdated.

Figure 1: Shaw’s camerated concrete used in an Upper Hutt house built in 1912.
Figure 2: 1950s precast concrete ventilator and positioning. Illustration from Carpentry in New Zealand, 1958.

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