Good Practice

1. Yes. I also like to leave a little slack at JBs.

Supplementary question is do you clip cables to joists, or just leave them loafing on the top of the ceiling? (I am thinking of older properties which don't have wadding in the ceiling/floor void.)

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

Another supplementry queston....... What about cables in old property lofts, we're the joists are barely 80mm think, and therefore can not meet the 50mm requirement from the bottom.

I read once that the 50mm only applies to through joist holes, but someone did point out the vulnerability of someone miss g the joists if screwing in to the ceiling and hitting a side clipped cable below 50mm requirement.

So are you then better to just float on ceiling or insulation 

 "What about cables in old property lofts, we're the joists are barely 80mm think, and therefore can not meet the 50mm requirement from the bottom."

Well if it is 50mm from the underside of the joist then it`s "OK" -  If the topside does not normally have traffic and is not hidden by boards. Is our 50mm minimum a satisfactory distance? cables rested on ceiling and not restrained do have a chance that they would often be nudged out of the way by any nail/screw/drilling although no guarantee on that one.

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

I saw an "door entry system engineer" drilling holes thru the ceilings and up thru the floorboards above with first removing a piece of floorboard to confirm no pipes/cables in the proposed route, said he`d never had such an accident even though he`d done loads of `em. Did make me cringe though! Having said that, against my advice he did wire a door entry system to a semi detached house with the bell push on the front door and the door lock mechanism on the back door and I was called to attend with him 12 months later to correct this - I am not kidding!

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? 

For lofts I'd never drill ceiling joists - they're way less substantial than floor joists and in the case of where they're part of a trussed rafter system definitely not meant to be cut into in any way by design. Loft thermal insulation complicates things - generally (especially for non-lighting circuits) you'd want to avoid running the cables within the insulation - traditionally they were laid above, but as the requirements move towards thicker layers, I'm tending to prefer to keep all the cables below the insulation - on top of the plasterboard between joists to allow heat from the cables to escape downwards and hopping over joists where the cables need to run at right-angles. Clipping varies depending on the situation - if the ceiling are yet to go up then some clips to keep things in the right place during construction certainly, on a retrofit, fewer are likely to be needed. Certainly clipped neatly where cables are run along woodwork above the insulation (along perlins and the like), but otherwise I'm happy to leave cables resting on plasterboard between joists.

    - Andy.

Hi Andy,

Sorry, wasn't referring to drilling or notching loft joists, instead clipping to the sides but below the 50mm requirement. As I had read that the sides of joist can be vulnerable, so the 50mm requirement should be applied there also?

Good point on sinking the heat down through the plasterboard when laying on top.

Sorry, wasn't referring to drilling or notching loft joists, instead clipping to the sides but below the 50mm requirement. As I had read that the sides of joist can be vulnerable, so the 50mm requirement should be applied there also?

The regs requirement is just for holes. Clipping on the side of the joist does pose a bit of a risk in terms of fixings 'just missing' the joists - if the ceiling plasterboard is already up then the risks are probably small since non-ones likely to be putting any more plasterboard fixings in - and there is a thermal advantage where there is insulation to having the cable close to the plasterboard rather than mid-way up the joist where it'll be surrounded with thermal insulation on three sides and not particularly thermally conductive timber on the fourth. So no one single answer really, just whatever's least worst for the particular circumstances.

   - Andy.