sm_jamieson wrote:
I know it sounds silly, but how much does it weaken a joist to screw
sideways right through it ? If I fitted noggins (extra support for
plasterboard) between joists but flush with the bottom of the joists,
by screwing right through the 3 inch joists and into the noggin ends,
how much would it weaken the joist ?

You will not weaken the joist - in fact, if you really stop and think -
you are in fact reinforcing it!

What about drilling small holes near the bottom of a joist - say for a
screw pilot hole.
Or drilling a countersink for the screw ?

When you thing plumbers, electricians, carpenters and all and sundry, drill
through joists for there relevant bits of pipe, cables etc to go through
(and usually in the centre or top third) then how are small pilots going to
affect the strength of the thing?

I want to make sure the BCO doesn't turn up and say I've wrecked my
roof supporting joists.

He's go more intelligence and experience to say that.

Should I devise another fixing method ?

Yes, *skyhooks*

Ah well.