CB Bitumen and fibre cement

Bitumen-impregnated cellulose fibre

Impact damage
Replace damaged sheets.

Erosion of the coating
This is normally due to weathering. Check if the coating is still under warranty. If not, repaint to the manufacturer’s specifications, using a bitumen-compatible water-based paint.

Moss and mould growth
This is caused by moisture build-up.

Treat with water-based chemical treatments only. Consult suppliers of roof and wall cleaning materials for suitable products.

Fibre cement

It can be difficult to tell the difference between fibre cement and older asbestos cement. If you think the product is asbestos cement, you should get it professionally tested. You should not try to repair or replace asbestos cement yourself. See the article on asbestos cement.

Note: it is important to maintain a good waterproof or paint coating on all fibre-cement. Water entering fibre-cement can cause swelling, delamination and eventual failure of the material. Fibre-cement can also act as a moisture ‘reservoir’ and cause an increase in moisture loadings within the wall. This would lead to rapid deterioration of the structure if not remedied. Jointers, soakers (galvanised corners to weatherboard) and cover boards should also be maintained in good condition and be kept painted.

Degradation of sealant joints
Fibre-cement sheet should not be joined using sealant, but windows and other elements may be sealed to the fibre-cement using a sealant. Degradation of sealant could be due to sunlight or loss of adhesion. Scrape out the sealant and replace it with an appropriate filler to manufacturer’s specifications.

Splitting/cupping of timber battens/trims
This is caused by timber movement, the back of the board being unprimed, or double nailing.

Replace badly-damaged timber, nailing with a single line of nails. Ensure all surfaces are primed before installing.

Surface damage/damaged jointers
This could be caused by impact damage.

For holes up to 30mm, fill with an exterior grade filler. For larger holes, repair using additional pieces of fibre cement. For damaged jointers remove and replace the damaged jointer and repair the cladding as required. This is a big job and you may need professional help, particularly if there is a risk that the sheet contains asbestos.

Edges delaminating
This is due to moisture.

Ensure fibre-cement cladding is clear of the ground and if soil has built up against the cladding, remove it. If possible, address other causes of moisture such as shading or a leaking tap. Replace damaged cladding and wall framing materials.

Cracks showing through
This is due to a lack of movement control joints or the joints not being properly stopped.

Fibre-cement cladding can have either flush finished joints, or have the joints showing or ‘expressed’ using back flashings or battens over the joints. Manufacturer’s literature should be followed for the ongoing maintenance of the particular fibre-cement cladding system used.

Seek professional advice.