Background: In the summer we had the back bedroom roof of our
victorian terraced house reroofed. It has the original rafters but
eveything else is new. It has slate/impermeablemebrane/air gap/
celotex/thinsulex/plasterboard .

The problem: On clear nights, when the temperature drops fast, the
roof starts to "crackle and pop". The sound come from all over the
ceiling and is quite loud. It's as if someone is dropping small
stones onto the roof from a great height. If you go out in the back
garden you can hear it clearly and it seems to be coming from the
slates. It does this for about 20 minutes and then it stops; last
night it began at about 11 pm (in Cambridge).

Another thing: when I went in the garden to listen, I noticed that I
could hear a similar noise from another house that also had recently
had the roof redone (by a different contractor).

Is this a well-known problem? How do I cure it? We couldn't sleep in
that room. In fact, it's so loud it wakes us up in the front bedroom!

Robert