I'm planning to replace the polycarbonate roof on my uPVC conservatory
with a traditional "flat" timber roof, plus a skylight.

You do realise that this would probably be illegal under building
regulations? Basically, it would no longer be a conservatory and would need
to comply with the entire myriad of building regulations, including
insulation, which may not be possible with the fully glazed walls. The
foundations would be a particular worry, too.

Assuming that you just don't care about the regs, I would have suspected
that the uPVC frames would be a bit marginal to hold the roof up, but this
is only a guess. Why do you want to replace the polycarbonate? Perhaps there
is another solution?

Christian.