We have an a lean-to extension on the back of our house built be the previous owners around 20 years ago. We have been here for 12 years with no problems with the roof but recently it has started to leak in a couple of places when we have very heavy rain. The tiles are concrete aboy20mm thick and the roof has a shallow pitch. Is there a product that I can spray or paint or to seal the roof or do I need to strip the old tiles off and install a waterproof membrane? The property is for sale so a short term, cheap fix is preferable. We will, of course, inform the new buyers of the problem.
You need to find out where the water's really coming in. If it's a slipped or broken tile then you can fix that easily. If the tiles are okay and water is getting underneath then is there an existing felt which is failing in places where the water's coming in? If so, then lift the tiles above and paint a bitument roofing paint over the leaks. Wickes do a really good one: http://www.wickes.co.uk/Wickes-Brush-Applied-Bitumen-Roof-Waterproofer-5L/p/241213. I've just done this to fix a flat roof underneath a decked terrace we have with our house in 16, which was bodged by a dodgy british builder for the previous owner. It didn't cost much, except about 6 days!
If the water is coming in a different way then perhaps another approach is needed!
Like many old roofs here there is no covering under the tiles just open to the ceiling below. The tiles are all OK and fairly clean. Ineed something tp paint over the tiles as the leaks are not coming in through the gaps. Also, no Wickes near here!
You still need to find out why and where the water's getting through, when it didn't before. If the tiles are actually porous, then I'd suggest simply replacing the culprits.