How long does an asbestos roof actually last, and do fibre-cement sheets crack over time?

Asbestos cement roof sheeting was sold on a service life of roughly thirty to fifty years. Manufacture in South Africa effectively ended in the 1990s and the material was prohibited in 2008. Do the arithmetic and it lands somewhere uncomfortable. Every asbestos roof in Cape Town is at least eighteen years old, most are forty to sixty years old, and a large number of them are still perfectly serviceable.

That is the strange thing about this product. It was very good at its job. It does not rust, it does not rot, it takes the sun without going brittle in the way early plastics did, and it is dimensionally stable through the temperature swing between a February afternoon in Durbanville and a July night. Roofs that were meant to last until 1995 are still keeping the rain out in 2026.

But they do not last forever, and the way they fail is specific and predictable. Once you know the three stages you can look at your own roof and place it fairly accurately.

The sheet is cement with asbestos fibre through it for tensile strength. Think of the fibre as reinforcing and the cement as the thing holding it. Over decades, three processes work on that matrix.

1. 1Carbonation. Carbon dioxide in the air slowly reacts with the cement, which changes its chemistry and reduces its strength. This is why a sixty year old sheet snaps where a new one would flex.
2. 2Surface erosion. Rain, wind-driven sand and ultraviolet light gradually wash the cement away from the surface, leaving fibre standing proud. That is what the chalky, slightly furry surface on an old sheet actually is.
3. 3Biological growth. Lichen and moss hold moisture against the surface and their root structures work into it mechanically. On the southern slopes in Cape Town, where the sheet stays damp through winter, this does the most damage.

In our experience across the Northern Suburbs, most of the asbestos roofs we look at are firmly in stage one or early stage two. Which is exactly the window where the cheap answer is also the correct answer, and where almost everybody is being told to do the expensive one.

• Go inside the roof space on a bright day with the lights off. Daylight through the sheet means a crack or a missing fixing. Mark where it is.
• Look at the underside of the sheets for dark tide marks fanning down from the fixings. That is a slow leak that has been happening for years.
• From the ground with a zoom lens, photograph each slope and look for cracks radiating from the screw crowns.
• Look in the gutters and at the base of the downpipes. Grit and fibre debris there is the roof telling you it is eroding.
• Check the ridge capping. On old roofs the ridge mortar and the capping fixings usually go first.
• Do not walk on it. Asbestos cement carries no load between purlins once it has carbonated, and going through it is both a fall risk and the single worst way to release fibre.

Once a liquid rubber membrane is over the sheet, the three ageing processes largely stop. Rain no longer touches the cement. Ultraviolet light no longer reaches it. Lichen has nothing to hold onto and nothing to feed on. The sheet stops eroding because the weather is now landing on a flexible waterproofing layer instead.

The honest framing is this. Encapsulation does not make a failing sheet strong again. What it does is take a sound but weathering sheet and stop the clock. On a stage one roof, coated properly and maintained, you are realistically buying another fifteen to twenty five years of service out of a roof that everybody told you was finished. Our system carries a written guarantee, and the practical life beyond that is a function of how much sun and rain the roof takes and whether anybody walks on it.

You are not restoring the roof. You are removing the weather from the equation, which is the thing that was aging it.

The visit is free and it takes about an hour. We get up there, photograph every slope, place your roof honestly in one of the three stages above, and put the recommendation in writing along with what it costs. If the answer is that it needs to come off, that is what the report will say.

If it is sound, the work is a controlled clean, crack and fixing treatment, and the full Rubber Sealant liquid rubber system in three coats with all details taken in. Asbestos encapsulation starts from around R310 per square metre. On a typical 120 square metre house roof that is well under a third of what a licensed strip and a new roof costs.