This has come up on this newsgroup a few times at least.

A friend who I've been helping out over the last year who bought a
house which needed lots of work doing has finally just had a new
roof (stripped back to rafters, new insulated sarking, battens,
and clay tiles), together with facias and gutters. Very nice job
all round, but done by a roofer, not us, although I helped with
the odd bit.

It's a terraced house, and where the roof runs on across the
neighbour's house, the roofer couldn't move any tiles to
straighten them up or even to replace broken ones near the
boundary, because it's been sprayed underneath with foam and
everything's stuck together rock solid. Neighbour's roof does
look very tatty now that there's a brand new one joining onto
it.

Apparently, neighbour has asked about having theirs redone too,
and the cost will be much higher because of the foam treatment,
and the difference is nearly twice what the foam treatment cost!
Talking with the roofer afterwards, he said in the time it took
to strip my friend's roof, he'd only be able to get about 4 rows
of tiles off the foamed roof. It has to be done by smashing the
tiles off, and then sawing along the rafter sides to separate
the foam/battens from them, and then cleaning it off the rafter
tops. The mess is unbelievable and needs lots more skips. If
it's been on there for more than 5 years, you always find some
of the rafters have rotted and need replacing too.

Anyway, thought that was a brilliant confirmation of what we've
always said here when the question has arisen.

He gave me another example too. Old lady had a roof leak which
she asked him to fix, as 3 other roofers had refused. Thought to
himself, can't be that difficult, although it was in a valley
where two parallel roofs met. Got up there, found that nothing
could be moved or replaced and the whole lot was glued into a
solid lump with foam. Had to go back and tell her he couldn't
do anything without replacing the roof, so that made 4 roofers.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]