Screwing flashing to corrugated roof - screws only to battens?

[smp50]

I am flashing a woodburner flue in a garden building. The roof is bitumen corrugated stuff, screwed down onto battens in the usual way.

The flashing needs to be siliconed and screwed, but surely there will be no purchase on the screws where they go into the roof alone, and not also into a batten below?

Should I use something like a plasterboard fixing that expands behind (under) the roof and pulls back on itself?

Was I supposed to frame under the flue opening so that there was in fact wood there to screw into?

Many thanks,

 

[Alastairreid]

What type is the flashing?

Assuming twinwall flue.

 

[smp50]

What type is the flashing?

Assuming twinwall flue.

I was thinking of trying this sort:

Yes, twinwall.

 

[Alastairreid]

The flu pipe will be braced inside, the weathering collar wont be taking any strain.
The sealant and self tappers should be adequate.