In an earlier contribution to this discussion,
Mark wrote:

Hi,

I have just lifted some floorboards to replace them and have
discovered that someone has previously notched out the top of several
joists to accommodate some CH pipes in a first floor bedroom.

I know this is a difficult question to answer but can anyone tell me
how I find out if this is a serious problem and what can be done about
it. The house is a 1960's semi if that's any use.

I guess I'll need to measure the sizes of the joists and their
spacing, which I will do when the kids aren't in bed!

TIA, Mark

It's very common - and not a problem provided the rules have been followed.
I can't quote the rules precisely, but the notches must be fairly close to
the ends of the joists and mustn't be deeper than a certain fraction of the
joist height. I'm sure someone will come along and quote chapter and verse.
Joist sizes are based on limiting the deflection of floors, rather than on
strength requirements per se - and invariably have a lot of strength in
hand. If you are over zealous with your notches, you may end up with a floor
which is a bit springly - but it's unlikely to collapse.
--
Cheers,
Roger
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