I would like to hear some discussion/advice on the feasibility of
putting
an antenna for, say, 30m in the attic versus on top of the roof...

Here is the situation :
The roof peak of my house is about 50 feet and is straight. Given
the
need for a dipole which is 44 feet wide, it can be mounted in either of
two
places:
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Andy , You didn't provide the shape of your roof space eg width at loft
bottom and whether there is any metal around (apart from nails/screws
etc). For an HF roof space antenna I would recommend a wire loop as
much as possible away from any metal and with total loop length
exceeding the the length of the largest wavelenght you wish to operate
on. So in your case a loop of 10m or say 35 ft would be adequate

A square would be the optimum ,but a 'wide rectangular' is acceptable.
Feed the loop with any balanced feeder ,twinlead electrical cable'(75
Ohms ?) or 300 -450 Ohms ribbon or 2 wires with insulator spacers.

It would be best to feed the loop somewhere near the middle of a side

You obviously need a balanced tuner/matching unit ,but might get away
with a simple current balun

Good luck

Frank KN6WH / GM0CSZ