Original roofs will need maintenance and may need replacement.
Asbestos-cement materials will typically last about 50-60 years. Any asbestos-cement roofing that shows evidence of damage such as broken roofing or noticeable surface weathering may need to be replaced.
However, if the roof is in good condition, it may not need to be disturbed so long as it is properly and regularly maintained.
Asbestos materials become health hazards when asbestos fibres are released and can be breathed in. The asbestos fibres in asbestos-cement roofing are not readily released unless the material is disturbed through actions such as cutting, waterblasting or aggressive cleaning.
As asbestos-cement products age, they become less impervious to water. The absorption of water results in moss growth (see ‘maintenance’ below) and sheets are susceptible to breakage in colder climates when water absorbed by the asbestos-cement freezes and expands.
Note: Corrugated asbestos-cement roofing is likely to be brittle and must not be walked on. If you must access the roof, planks or a roof ladder should be used to avoid standing directly on the asbestos sheets. Alternatively, consult with a professional person experienced in dealing with asbestos removal.
Removal and replacement
If the asbestos-cement is extremely weathered and can be crushed by hand, it must be removed professionally. Work with asbestos (including removal work) is covered by the Health and Safety at Work (Asbestos) Regulations 2016. A licensing scheme applies to asbestos removal. You must notify WorkSafe New Zealand of licensed asbestos removal work at least 5 days before work commences. You can find more details here.
Even when the material is still intact, precautions must be taken and asbestos-cement must be disposed of appropriately. Precautions when removing these products include:
- extracting the fixings rather than trying to pry up brittle sheets which may break
- keeping the asbestos-cement wet during removal
- lowering the pieces of asbestos-cement to the ground instead of tossing down.
See Asbestos for information on handling and removing asbestos safely.
Maintenance, if required, primarily includes the removal of lichen and mosses and could include painting if the roofing is still sound, although BRANZ recommends that this only be done by a specialised contractor.
Prepare the roof for painting by removing dirt and contaminants, but under no circumstances use a waterblaster as this can blast asbestos fibres free of the roofing. Collect dirt and contaminants and dispose of correctly (see Asbestos).
Lichen and mosses should be removed by treating with a proprietary moss and mould treatment product.
Clay and concrete roof tiles are durable and long-lasting, but they require occasional inspection and maintenance. Tiles installed during the 1940s are likely to be at, or nearing, the end of their serviceable life, making them more brittle and crumbly.
Tile maintenance primarily involves routine checks for:
- loose, cracked or broken tiles
- an accumulation of debris on the roof
- loss of pointing from ridge or hip tiles
- surface erosion of early concrete tiles
- blocked valleys
- corroded metal valley flashings
- corroded wire ties (or no ties at all).
If a tile is broken, cracked or has an eroded surface it should be replaced. Both Marseille (clay) and concrete tiles are available new, but if a tile with the same degree of weathering as the roof is required, check building demolition yards.
If there are indications of roof damage such as internal leaking, but no damage is visible, the problem will be underneath the tiles and should be dealt with by a qualified roof tiler.
Note: Do not walk on a tiled roof as the tiles are brittle and likely to break.
The original corrugated metal roofing on 1940s houses may still be sound in areas of very low corrosion risk, but elsewhere it is likely to have been replaced because the original lapped end joints (particularly) and side laps are likely to have corroded out.
Roofing in very poor condition can be replaced with zinc/aluminium alloy coated steel longrun roofing, which can be used unpainted (depending on the environment) or factory painted, and installed without end laps. New zinc/aluminium alloy coated steel should not be used in contact with or installed above existing galvanised steel roofing.
Low slope tray section roofing available from the 1960s may be showing signs of deterioration if it has not been well painted or the low slope has meant that water has not been effectively drained from it. Such roofing also had a propensity to deteriorate from the back surface upwards at the gutter line.
Metal tile roofs also available from the 1960s were prone to loss of the stone chip coating which affected the aesthetics of the roof. In mild environments the base galvanised steel may still be sound although it is likely that many of these roofs have been recoated.
Building paper was generally installed behind wall cladding and metal roof cladding in houses built during the 1940s and 1950s except in a few areas, particularly in northern parts of New Zealand. If renovation work is being carried out, underlay should be installed. Options include:
- installing underlay when replacing roofing
- installing underlay to the underside of the rafters of an existing roof. While not an ideal solution, it will provide a measure of condensation absorption. A 25 mm gap must be maintained between flexible roof underlay and the insulation.
- installing underlay from the inside by folding into the framing cavities when replacing the interior lining.
Translucent roofing was available in the 1950s but much of the original material will have been replaced (possibly more than once). If still remaining it is likely to be brittle and have lost much of its translucency and be in need of replacement.