low pitch roof

Hi. we have a one story ground floor extension which leaks along two lines. Roofers have told us that the velux is fitted incorrectly and that the tiles (Redland 49) are wrong (we know that some Redland 49s are ok for low pitch and some aren’t -how can we tell? Our builder disagrees but...

Has anyone had problems with Cambrian (or similar) reconstituted slate tiles leaking at low pitch (15 degrees). Tiles are installed with correct overlap. If so, what did you do?

Have had an outhouse in the garden rebuilt. Roof was designed to be essentially a flat roof with 10 degree single pitch with reclaimed hand made pantiles as a covering to make it look prettier with a. Seems the builder didn't quite get it and installed it as follows: 20mm marine ply, tyvek...