In message , Joan Riley
writes

You say the butyl is very heavy, June. We have an unlined pond here
which is a good 20ft by 30ft, full at the moment but we've been told
it dries out over the summer, so we'd like to put a Butyl liner in.

Don't forget that a butyl liner keeps the pond water away from any
pondside plants so if you've lots of lush growth you'll need to arrange
for its welfare.
Saddest thing I've seen in ages was a newly-lined pond with everything
around it dying in the first dry spell.

--
Sue ]