Drilling Through Joists

[Diesel9a1]

Running cables through the house loft now and before I go wild with the drill on the joists I thought I'd just check with you lot what I have read and summise is correct.

25% - 40% from either end of a joist is the zone that is mandatory to drill through. Is this correct?

Length from front to back of bungalow is 10.03m This gives a zone of:
2.50m to 4.01m from the joist ends.

This also means I cannot drill through the middle 2m of the joist - is this correct???

Joist height of 95mm = 23.75mm max hole diameter

How many cables can I fit through the holes? I.e. do I have to leave a % of air space? Obviously don't cram them in...

 

[plugwash]

generally its better not to drill loft joists especially in modern houses where the things are decidedly undersized in the first place.

 

[cozycats]

Agree never drill through loft joists, They are only designed to take the weight of the ceiling, never designed to be boarded over and several tons of books, out-of-fashion clothes dumped on them! All the F**8**g rules and regs about this and that and they can't even get the basics right!!!!!

 

[Albert]

Could you please detail the source of these figures?
According to BS7671, 522-06-05 (brown book), and OSG page 52:
The cable should be installed in a "position that it is not liable to damage by contact with the floor or the ceiling or their fixing. Unarmoured cables passing through a joist shall be at least 50mm from the top or bottom as appropriate or inclosed in earthed steel conduit"...etc.
1. Max dia of hole 0.25 x joist depth
2. Holes on centre zone between 0.25 and 0.4 x span (I think that this is your 25% to 40% figure).
3. Holes in the same joist should be at least 3 diameters apart.

 

[Diesel9a1]

That's what I've read Albert. It was just that I was going to drill through a joist or three to get across to a drop down point to a socket below, then thought I'd best re-measure and check. I'm not up for drilling loads of holes etc. Looks like I'll be ok - just need to drill in a different location to what I thought in the first place!

I'm fortunate with this bungalow in that there is a breeze wall straight down the middle of the walk way of the loft (underneath). So support there is fine. The bungalow is 1960's so not this modern cr@p that gets thrown up in 2 days. It is reasonably well built.

 

[masona]

This also means I cannot drill through the middle 2m of the joist - is this correct???

Why do you need to drill holes in the middle of the joist or any joists

 

[securespark]

Could you please detail the source of these figures?
According to BS7671, 522-06-05 (brown book), and OSG page 52:
The cable should be installed in a "position that it is not liable to damage by contact with the floor or the ceiling or their fixing. Unarmoured cables passing through a joist shall be at least 50mm from the top or bottom as appropriate or inclosed in earthed steel conduit"...etc.
1. Max dia of hole 0.25 x joist depth
2. Holes on centre zone between 0.25 and 0.4 x span (I think that this is your 25% to 40% figure).
3. Holes in the same joist should be at least 3 diameters apart.

Albert!! Long time no hear! Hope you are OK.

These figures are on P27 of IEE guide to building regs.

 

[Albert]

Albert!! Long time no hear! Hope you are OK.
These figures are on P27 of IEE guide to building regs.

Hi, Yep I am ok hope you too...

 

[Diesel9a1]

I need to get across to a drop down point near the middle of the loft space. The loft will eventually be boarded to improve storage etc, so unable to go direct to the drop down point.

Any alternatives?

 

[gingervit]

You can now buy plates from city electrical factors to put over notches in joists/rafters to comply with the regs on this matter. Prenailed too!

 

[Albert]

I need to get across to a drop down point near the middle of the loft space. The loft will eventually be boarded to improve storage etc, so unable to go direct to the drop down point.
Any alternatives?

Could you, may be, get the cable to a easier point and clip the cable on the joist (upper part of the joist side) up to the drop point? I had to do something like this, I used more cable and more work but if it is safe and complies it could be the solution, and you drill if you need to where you can.

 

[Diesel9a1]

Can do. I think I had ££££ in my head and now I think about it, I will only really be adding about 3m on to route in total. So the addition will cost an extra £4 ish compared to drilling wrong and costing me £xxxxx by Mr Part P bloke to put it right!

 

[Albert]

Can do. I think I had ££££ in my head and now I think about it, I will only really be adding about 3m on to route in total. So the addition will cost an extra £4 ish compared to drilling wrong and costing me £xxxxx by Mr Part P bloke to put it right!

Just to be a pain... make sure that the 3m extra will not effect the circuits volt drop value...

 

[Diesel9a1]

Thanks pain, I mean Albert, The cable is on the ring main and I don't think I'll worry about any volt drop there.