Many years ago i had an extension fitted with Fortecrete (?) tiles fitted and to gain lenght the builder made it to shallow. I had always had water entry in bad rain. Had a roofer look at it and he was able to put a sealant on tiles to stop water getting under tiles. At the time he said this is not a correct repair but there was no other answer. Water coming in again does anybody know what sealant he used ? looked like a mastic gun type tube.