Asbestos-cement cladding was available in sheets or planks. It was popular due to ease of installation, its low cost and because it was considered to be fireproof. During the 1970s asbestos-cement sheets were New Zealand made.

Though 1970s houses are usually weathertight, there may be a risk of moisture getting in through asbestos-cement claddings if they have suffered impact damage. See moisture and weathertightness.

Asbestos-cement sheets

Locally-made asbestos-cement sheet sizes were commonly 1.8-3.0 mm in length and 900-1200 mm in width, and were available in 5, 6 and 9.5 mm thicknesses.

Sheets were supplied smooth or with a low profile moulded surface.

They were fixed over building paper to the framing that was spaced at 450-600 mm centres to suit the sheet width as the sheet edges needed to be fully supported.

Joints

Edges could either use a jointer strip (Figure 1, left hand side), be open jointed (Figure 1, right hand side), butt-jointed with a cover batten, or sheets could have moulded edges to form joints.

Figure 1: Vertical joints in flat asbestos-cement sheet. Building paper omitted to show framing location.

A strip of bituminous fabric was laid under the joint in both directions, or joints were sealed with a bituminous compound. Vertical joints were then covered by asbestos-cement D mouldings or timber battens, while horizontal joints had a Z- shaped metal flashing inserted in the joint before the vertical moulding was fixed (Figure 2).

Figure 2: Horizontal flashed joint in asbestos-cement sheet. Building paper omitted to show framing location.

Finishing

Sheets were commonly left unpainted, but readily accepted water-borne paint coatings.

Corners

External corners could have a boxed timber detail that was the same as that used for horizontal weatherboards, and proprietary asbestos-cement corner mouldings were available. Internal corners were flashed with 75 mm metal flashing in each direction for the full height of the corner.

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Asbestos-cement planks

Asbestos-cement planks made in New Zealand were readily available during the 1970s. They were considered a lower cost utilitarian cladding.

They were installed as overlapping units directly to the framing. Soakers were used over joints and at corners.

Planks were up to 5 m long and became available with a wood grain finish.

Asbestos-cement was often left unpainted (something not recommended for its replacement material cellulose reinforced fibre-cement, which replaced asbestos-cement in the later 1980s).

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Safety

Cement-based sheets and planks that are known to be pre-1988 and have a corrugated profile or a dimpled back surface are likely to contain asbestos. Where the cladding is in good condition and the surface or coating is in good condition, there is little risk to health. Serious health risks can arise when asbestos materials are damaged or being removed or broken up, allowing fine asbestos particles to become airborne where they can be breathed in. The removal of materials containing asbestos is a carefully regulated area. You can read more on the page Health risks: Asbestos.