Asbestos roof: repair it, seal it, or replace it? A straight decision guide

There are only three things you can do about an old asbestos cement roof. Patch the specific spots that leak. Seal the whole thing under a membrane and stop it weathering. Or take it off and put a new roof on. That is the entire list.

The trouble is that the loudest voices in the market only sell the third one, so that is the one you hear about. Nobody is lying to you exactly. They are just describing the only product they have. Here is how to work out which of the three roads your particular roof is on, before anyone quotes you a number.

1. 1How many places does it leak? One stain under one ridge is a detail problem. Stains in four rooms means the surface of the sheeting has given up, and patching will keep you busy for years.
2. 2What does the surface look like from close up? A sound sheet is firm and dense. A sheet that has weathered through is chalky, fibrous and soft, and you can see the fibre structure standing up out of it. Chalky is still sealable. Soft and delaminating is not.
3. 3Are any sheets actually broken? Look for cracks that run right across a corrugation, missing corners at the fixings, and previous foot damage. A handful can be reinforced. Widespread cracking across the roof is a different conversation.
4. 4How much moss and lichen is there, and how deep? Heavy growth means the surface has been holding water for a long time. It can be treated, but it tells you the sheets have been working hard.
5. 5What is holding it up? If the purlins or battens are rotten, sagging or rusted, the roof needs structural attention no matter which coating goes over it. Look along the ridge line from the ground: a wave in it is a warning.
6. 6What are you actually trying to achieve? Stopping leaks for the next decade, or preparing the property for a sale, or getting rid of the material entirely for peace of mind. All three are legitimate, and they point to different answers.

Look at the patching row honestly, because it is the one most people are already living in. Four hundred rand here, a tube of sealant there, a bucket in the passage when the northwester comes through. Ten years of that costs more than sealing the roof once, and you spend the whole decade with a roof that leaks.

We turn work down for these reasons, and you should be suspicious of anyone who does not.

• Sheets that are delaminating, meaning the layers are separating and coming away in flakes rather than just chalking
• Cracks through many sheets rather than a few, especially around fixings
• A roof structure that has failed: sagging ridge, rotten battens, rusted purlins
• A roof that has been pressure washed hard and repeatedly, leaving an eroded friable surface with nothing sound left to bond to
• Any building being demolished, re-roofed or substantially altered, where the sheets have to be cut anyway
• A written requirement from a bank, insurer or purchaser that the material be removed

1. 1Get someone competent onto the property to look at the sheets properly and give you a verdict in writing, with photographs.
2. 2Deal with the structure first if the structure is the problem. No coating fixes a sagging batten.
3. 3If the sheets are sound, seal the whole roof rather than chasing individual leaks. Partial work on a weathered roof is a subscription, not a repair.
4. 4If the sheets are past it, get a registered asbestos removal contractor and budget for the new roof at the same time, not afterwards.
5. 5Either way, get the area, the method, the products and the guarantee in writing before you pay anybody a deposit.

Most of the roofs we look at in Durbanville, Bellville, Brackenfell and Kraaifontein are on road two. They are old, they are tired, they leak at the laps and the ridges, and the sheets themselves are still perfectly sound. Those roofs do not need to come off. They need to be sealed properly, once.