Good Practice

I prefer to clip to joists, plus, it above a route classed as an escape, don't they need to be fire cliped?

The premature collapse in the event of fire requirement is now general - not it's specific to escape routes any more. However above ceilings the plasterboard is often sufficient to prevent the cables collapsing prematurely even if the clips melted, so probably easier all round just to continue to use plastic clips in that particular case.

Do we always use screws 50mm or less to fix to walls/ceilings ??? 

I think the main worry is the fixings for plasterboard below and floor boards above that are driven in by the thousand (usually using rapid power tools these days) - hence the 50mm rule applies in both directions. Most board fixings are 50mm or less in length, so given the thickness of the board will penetrate the joist by a fair bit less. When it comes to other things fixed to ceilings (e.g. light fittings etc) I think the assumption is that a a little more care will be exercised and most likely those doing it will be far more aware of the likely presence of cables.

  - Andy.

Hi Andy,

While both methods are accepted, from a workman ship point of few in a loft space, would your answer be leading to that clipped is best workman ship if possible, but if joists are sub thickness to achieve 50mm, then laying across joists and on top of the ceiling is next best thing? 

Hi Andy,

While both methods are accepted, from a workman ship point of few in a loft space, would your answer be leading to that clipped is best workman ship if possible, but if joists are sub thickness to achieve 50mm, then laying across joists and on top of the ceiling is next best thing?