gogmagog wrote

My mum n dad have a garage attached to their house. It's rather long,
since the guy who built it (previous owner did it himself, we're told)
also extended the kitchen sideways at the same time. So, it's the same
length as the house (25 feet or so). Neighbour says he used pretty much
any bits of chipboard he could find. There seems to be no incline, since
water tends to sit it pools on top. It has a few leaks, and has once
caused water to pour out of a light switch in the kitchen. I patched that
one up, but we're thinking it would be nice to do something so that we can
just forget about it in future. Having it properly replaced is out of the
question - the money will never be available in their lifetimes. I was
wondering if I could cover it all up with some corrugated sheets, making
sure they slope down to the back, where there is already a gutter.

You're better off with metal decking if there is a minimal slope.

Corrugated prefers a decent slope.

Is this a reasonable thing to do,

Yes, if you use metal decking.

or just plain bonkers?

Not if you do it right.