Common problems with roofing and cladding include corrosion of fixings, lack of underlay, and breaking of shingle cladding.
Asbestos-cement shingles or slates will typically last about 50-60 years, with the thicker ones lasting up to 70-85 years. An asbestos-cement shingle roof is likely to require replacement.
As asbestos-cement shingles get older, they become brittle and less impervious to water which can result in moss growth. When they absorb water, it can freeze during winter which can cause the tile to break.
If the shingles are extremely weathered to the point that the material can be crushed by hand, they should be professionally removed. Even when they are still intact, precautions must be taken, and they must be disposed of appropriately (see asbestos)
Precautions for the removal of existing asbestos cement shingles include:
- extracting the fasteners securing shingles, rather than prying up brittle shingles which are likely to break
- keeping the shingles wet while working with them
- lowering the shingles to the ground instead of tossing them down.
The Health and Safety at Work (Asbestos) Regulations (2016) regulate working with asbestos. They set out the new rules around the removal of asbestos, and the circumstances where WorkSafe must be notified.
It was not uncommon for walls and roofs of bungalows to be clad without building paper or underlay. Even if they have previously been re-roofed or reclad, they may still have no underlay.
Options for installing underlay:
- Install underlay when replacing roofing or exterior cladding
- Install underlay to the underside of the rafters for existing roofing. While this is not an ideal solution, it will restrict air flow into the roof and provide a measure of condensation absorption – remember too, that a 25 mm gap is required between a flexible roof underlay and the insulation.
- Install underlay by folding into the framing cavities for walls where the interior lining (but not the exterior cladding) is being replaced.
Corrosion of metal components such as roof cladding (including the flat roofs to bay windows), fixings, flashings or accessories may appear as the familiar red rust, or as a white discolouration on the surface of materials containing zinc, which is called white rust. It may be caused by:
- moisture combined with salt spray, dirt, or hydrogen sulphide from coal-burning fires or geothermal activity
- contact between dissimilar metals resulting in electro-galvanic action
- run-off from copper-based timber treatments.
Houses built close to the sea are likely to be affected by salt or chloride contaminants. The parts of the house most likely to be affected are the subfloor, roof flashings, window and door flashings, cladding nails, and external pressed metal panels.
Flashings, fixings and accessories that are significantly affected by corrosion (for example, pitted or pinholed surfaces) should be replaced.
Where corrosion on mild steel is on the surface only and is able to be removed by sanding, the steps that should be carried out are:
- Sand with grit paper to expose shiny metal.
- Prime with a zinc-rich primer.
- Apply a solvent-borne metal primer.
- Apply two finish coats.
The risk of future corrosion can be reduced by painting steel, specifying factory-coated steel and using hot-dip galvanised steel or stainless steel fixings and brackets.
Rust may chemically attack timber around corroded fasteners – referred to as ‘nail sickness’. An example is the dark staining around nails in old floorboards, and although it is unsightly and indicates a slightly weakened part of the timber, it does not generally pose a structural risk.