Asbestos roof sealing: how to fix a leaking asbestos roof without removing it

That is how most people describe it to us. It is fine all summer. Then the first proper Cape front comes through in May, the wind swings, and there is a damp patch on the passage ceiling. By August it is a stain the size of a dinner plate and there is a bucket in the spare room.

Somebody has usually already been out. They put a smear of bitumen on a screw, or a patch of aluminium tape over a crack, or that thick black tar-like compound around the flue. It held for one season. Now it is leaking again, in a slightly different place, and you are being told the roof needs to come off.

It very probably does not. But the patch approach was never going to work, and it is worth understanding why.

In roughly the order they show up.

1. 1The fixings. Every hook bolt and drive screw is a hole punched through the crown of a corrugation, sealed with a washer that perished twenty years ago. Water tracks down the thread. This is the single most common source and there are hundreds of them on a roof.
2. 2Hairline cracks radiating from those fixing holes. As the sheet carbonates and the fixing works with thermal movement, the hole becomes a star of fine cracks. Individually invisible from the ground, collectively a sieve.
3. 3The end laps. Where one sheet overlaps the next, capillary action pulls water uphill against the overlap. On a shallow pitch, and with the lap gap now full of forty years of lichen and grit, it beats the overlap regularly.
4. 4The ridge. Old ridge capping and its mortar bedding is often the worst detail on the whole roof.
5. 5Flashings and abutments. Where the roof meets a parapet, a chimney or a higher wall. The original flashing is usually cracked mortar and perished mastic.
6. 6Penetrations. Vent pipes, flues, aerial mounts, geyser pipework, and above all solar mountings that were drilled through the sheet without anybody thinking about what they were drilling.
7. 7Broken corners at the four-sheet junctions, where the sheets were mitred on installation and the corner has since chipped away.

The approach that actually works is to stop hunting individual holes and instead put one continuous flexible membrane across the entire roof, taken up every flashing and around every penetration, so there is no longer any joint, fixing or crack exposed to weather. That is encapsulation, and on an asbestos roof it does two jobs at once. It stops every leak, present and pending, and it locks the fibre into the sheet at the same time.

The systems used for this are liquid rubber membranes. They go on wet and cure into a seamless elastomeric skin that stays flexible, so it moves with the sheet through the daily temperature cycle instead of cracking away from it. On asbestos-cement roofs that application is regulated asbestos work: Williams & Co is an approved Rubber Sealant® Liquid Rubber applicator on non-asbestos substrates only.

You are not repairing forty holes. You are removing the weather from all forty at once, and from the ones that have not opened up yet.

1. 1Access and safety. Crawl boards and roof ladders spread the load, because nobody walks directly on the sheets. This alone rules out a lot of contractors.
2. 2Controlled clean. Low-pressure water, soft brushing and a biocide to kill the lichen and moss. No high-pressure washing at any point. This is the step where most jobs go wrong before they start.
3. 3Dry down and biocide dwell time. The growth needs to be dead, not just knocked off, or it comes back through the coating.
4. 4Fixing and crack treatment. Every fixing point and every hairline crack is treated individually, brushed in, with reinforcing fabric bedded into the wet coating where a crack is more than superficial.
5. 5Detailing. Ridge, end laps, flashings, abutments, penetrations, gutter lines. The details are where the leaks live, and they get done before the field of the roof.
6. 6First full coat over the whole roof, worked into the corrugation profile rather than bridged across it.
7. 7Second and third coats, each after proper cure, building to the specified film thickness. Three coats is not marketing. It is what gets you the thickness the guarantee is written against.
8. 8Final inspection with you, photographs of the finished details, and the written guarantee.

Costs vary widely from roof to roof. Only a registered contractor's measured inspection produces a real number.

The comparison that matters: licensed removal of the same roof plus disposal plus a new roof covering will very rarely come in under R150,000, and often considerably more once the timber gets exposed and somebody discovers what forty years of leaking did to the battens.

• Are you going to pressure wash it? The right answer is no, and the reason should be about fibre release.
• How will you walk on it without cracking the sheets?
• How many coats, and what wet film thickness is the guarantee written against?
• What happens to each fixing and each hairline crack, specifically?
• Are the ridge, laps, flashings and penetrations in the price, or extra?
• What is your position if you get up there and find the sheets are too far gone?
• Show me photographs of an asbestos roof you have done, before and after.

If somebody cannot answer the first two, they should not be on an asbestos roof at all.