Jim Ingram wrote:
Hi,

I've discovered a series of holes used to carry a cable through a number of
joists. These are nowhere near the centre of the joists, in fact I doubt if
any of them are actually 50mm from the surface. Should the cables be re-run
or could I just put steel plates over the top. I could do with running
another cable through these holes and it seems a bit ridiculous to further
weaken the joist adding even more holes.

In your situation I wouldn't worry about it. It's much worse to notch
joists, and even then you can usually get away with it. The worst place
to notch joists is the 25% of the joist at either end, and the 20% in
the middle. It's in the 16thEd wiring regs somewhere. And and holes
should not be more than 1/4 of the diameter of the depth of the joist.
So you can lose quite a bit before anything terrible will happen! Steel
plates are a good idea if only to stop you banging nails into
pipes/cables. But it was common practice to notch joists for years and
as long as they are small and in the centre of each floor board
shouldn't be a problem.

Regards,

Jon.