cables over joists not through them?

[officer dibble]

Regulations state cables through joist on a centre line at least 50mm from the top or bottom. In a modern warm flat roof there are firrings which are long wedge shaped soft wood fillits varying from 80mm to 0mm which sit on top of the joists and are used to tilt the roof for deck to assist drainage. Is it permissable to drill holes in these at the thick end in order to pass cable through or would this contravene current regulation. The firrings are sandwiched between the joists and the bottom boarding and there is 125mm of insulation and 18mm top deck above that.The firrings although screwed to the joists but are not part of the joists. I would rather drill through these than the middle of the joists as there is no structural issues here.

 

[Darkwood]

A pic paints a thousand words ....

 

[Engineer54]

So you would rather degrade these wedges, than run your cables where they should be run, between the structural joists?? There will be no structural issues drilling joists in the designated areas....

 

[MDJ]

If it is in a loft do you need to drill any joists?

 

[timo1]

Just checking the chin strap on my tin hat....I rarely clip and drill through joists in lofts, to my mind this has to be better than drilling through the joist. In most lofts this can be done easily and neatly, unless the loft is used for anything other than a storage area.

 

[Deleted member 26818]

I believe the requirement in BS7671 is 50mm from the surface of a floor or ceiling.
No idea about the requirements of building Regs.

 

[thekingiam]

i try to get as close to edge as possible if it means i can get away with not drilling through joists.<br>that usually involves on my belly, a mouth full of spiders webs and the ocasional concussion from banging my head.<br>maybe i need a tin hat?

 

[Engineer54]

If it is in a loft do you need to drill any joists?

I'm assuming this is a single storey flat roof extension type install, if he's talking about wedges being used for the drainage angle...

 

[Fin170]

Just checking the chin strap on my tin hat....I rarely clip and drill through joists in lofts, to my mind this has to be better than drilling through the joist. In most lofts this can be done easily and neatly, unless the loft is used for anything other than a storage area.

That and it will take an extra few hours (unpaid) of labour to move the itchy, pull the itchy out of your drill for using it near it then coughing up said itchy and blood.....

After all that, the cable is still better on top

 

[Stroppy]

As the firrings are not load-bearing (other than the roof covering) I can't see a problem with drilling them, as long as your holes are more than 50mm down from top edge where the roof ply will be screwed. This does however restrict you to the "thick end" of the firrings. As the underlying joists can be safely holed in the correct zones without weakening them I would do this and leave the flat roof structure alone. In the future the flat roof and its firrings may have to be replaced a lot sooner than the joists, so this would be simpler without cables going through them.

 

[Rockingit]

The point of the 50mm rule is to avoid penetration from flooring/ceiling fixings, and the 'distance ratio' is for structural. However, given the way that plumbers never ever seem to give a monkeys about the drilling zones (they have the same rules as us) and I've yet to see a house fall down after a heating install, I'm happy to be, er, 'flexible' about where things can go. And I've also never yet see a BCO even look at holes/notches in joists around my way.

 

[Engineer54]

It was never a problem where you drilled your holes in the older buildings, but then they knew how to build houses and joists were JOISTS!! lol!! Not like today's new builds where everything's paper thin...

 

[Rockingit]

It was never a problem where you drilled your holes in the older buildings, but then they knew how to build houses and joists were JOISTS!! lol!! Not like today's new builds where everything's paper thin...

Exactly my point! Rip up the floors in anything more than 50 years old and there's normally so much swiss cheese to choose from that you don't even need to get the drill out again! That having been said, I reused some existing holes in a joist the other day on a new build - well, if the plumber will drill them and walk away for the day..........

 

[Steve_P]

If you're drilling the thick end, how far can you get from the end of the joist?

 

[ayjay]

you can do it but you will use much more cable if youve only got one run of holes at one end
also if the roof is fitted the holes will have to be drilled from below at an angle as the drill wont be horizontal when drilling that far up thro the firlings
complete waste of time imo, just drill the joists in the centre