Just been on my mate's garage roof, removing ice from his condensate
drain. Hopeless positioning, pipe run is about 12 feet with the last
foot inaccessible from inside because of overhanging eaves. I think what
happens is that a plug of ice forms on the outside, sometimes it slides
out until it hits the inside of the gutter (no scope to re-route over
the gutter), at which point it all freezes solid. It would be difficult
to get trace heating in there without removing tiles and felt, the pipe
runs in a 2 inch gap between a floor joist and the wall.

Boiler is on gable end wall over the garage roof, I reckon the best fix
would be to go out through that wall and live with it draining onto the
felted roof.

My inclination is to drill the through-wall pipe so that it is draining
down at about 45 degrees (the internal pipe has the usual slope of 10
degrees or so), and extend it out far enough so that it is not draining
on to the felt to wall join. Might put in a bit of alloy plate or
plastic sheet at the drip site to help protect the felt.

Cavity wall with polystyrene bead insulation. I'd drill from inside
through a suitable vertical joint in the blockwork (outside is brick). I
have long SDS drill bits of various diameters, obviously I would start
small.

I'd wondered about using 32 mm waste pipe for this section rather than
the usual stuff.

Any thoughts, comments, suggestions?