Tiled roofs have ridge capping. These are the upside down Vee shaped elements at the top and sloping side edges of your tiled roof.
Firstly, the ridge capping is held down and supported by a “BED” of (sand and cement based) mortar. Unfortunately, the bedding will crack as it naturally shrinks and it does not bind the ridge capping very well. Also, the colour of the bedding does not blend in with the colour of the roof tiles or the ridges. And this is where the pointing comes in…
In the (not too distant) past, the pointing was just a thin layer of oxide coloured sand/cement. The ‘cement’ pointing was put together relying on the skill of the roofer and the thin layer had a good chance of sticking onto the bedding without cracking.
With age and movement on the roof, the bedding (and attached pointing) would start to crack – and the ridge capping will then start to dislodge itself and sometimes slide off the roof. This is when the roof has to be “repointed” – to maintain the ridge security. Usually, only the very loose bedding is removed and fresh bedding installed locally to support the ridge capping. Then the rest of the existing (sometimes cracked) bedding is prepared to receive the new pointing.