Hello,

When building a cavity wall, how deep should the cavity go, i.e. how
far below ground level?

I ask because you may remember I was posting about removing asbestos
floor tiles. I wanted to remove them because the concrete floor
beneath them was not level and this had caused some of the tiles to
crack.

I have now safely removed the tiles thanks to all your help but I have
found that the concrete is cracked beneath them. In some places this
is because the central heating pipes were buried directly into the
concrete and over the years the heat must have caused the concrete and
tiles to crack. BTW I have read that some people worry that concrete
causes pipes to corrode. There doesn't seem to be any leakage but the
copper had turned green. Does that mean anything? I realise it is
green will be copper carbonate from a reaction with the concrete but
would it have affected the integrity of the pipes?

My plan is to replace these pipes with ones that are lagged.

But I have digressed. What about cavity walls? Well, there is a crack
in the concrete in front of my patio door. I have chiseled away the
concrete and there are no pipes underneath, so it's a bit of a mystery
why it has cracked. I have noticed that there is concrete up to a
course of bricks, then a gap full of rubble, sand, and concrete dust,
then another brick course. It seems to me that this would have been
the inner wall and the cavity, had there been a wall there, only there
isn't because there's a door instead. I hope that makes sense!

Should the cavity gap be full of rubble? Should I remove it and what
should I put there instead? Should I plug it with polystyrene and then
concrete over the top or can it remain full of rubble?

Thanks in advance.