The pipe to the soakaway for one of our gutters is choked-up with leaf
mold and general crud. I've rodded-ed it but suspect that the soakaway
itself is gunged-up so am tempted to dig it out and replace it with a
pre-fabricated plastic one; especially as student labour is around at
the moment ;-)

Presumably the necessary volume can be calculated from the roof area and
type of soil - but how?

Dave