House downpipe onto extension mono pitched roof

[^woody^]

Tile ratings ie. suitable for certain pitches, and the semi hip roof discharge is part of the same roof, not discharging one roof on to another roof.
They realised it was problematic years ago to discharge one roof on to another
without taking any measures to prevent flooding.
Old buildings or church roofs you see it often they will have a shute to take
the water across one roof to the gutter.

What you're saying then, is that its not a probelm with the tiles but one of gutter capacity of the roof.

So its not a probelm with any domestic gutter situation then, because 110 gutters have a large capacity, and its not a probelm for the OP.

And most of all, its nothing to do with the roof pitch, because when a tile is rated for a given pitch, with a given headlap for a given exposure location, then its rating stands even with a downpipe discharging on to it.