Running cables through joists.........

[JDLDIY]

Looking at the rules for running cables through joists to plan a run of cable, using this sourcehttp://www.tlc-direct.co.uk/Book/4.4.1.htm

I don't think any of the original wiring in this house complies with this. I've manages to get away with not cutting to many holes in joists, but where I am running some cables in some previously cut notches (where old CH pipes ran) I was hoping to cover with strike plates obtained from TLC. I take it that is still acceptable?

Next Q: When they say 25 to 40% of joist length from the ends, do they mean that the holes/notches must be cut/drilled BETWEEN a point 25% along the length of a joist and another point 40% along the joist? (10- 25% if notching). That is the way I read it, how many times is that not complied with I wonder?

So, they are indicating that the notching method is more acceptable nearer walls, but I would have thought that it is the most likely to cause structural problems in the joist. If drilling a hole is better for the joist, why not allow it nearer it's end-bearing?

JD

 

[TraineeSpark]

The regs state that: 522-06-05 "A cable passing through a timber joist ... (e.g. under floorboards), shall:
i) be at least 50mm measured vertically from the top, or bottom as appropriate, of the joist or batten, or
ii) ... be protected by enclosure in earthed steel conduit securely supported, or by equivalent mechanical protection sufficient to prevent penetration of the cable by nails, screws and the like"

Therefore, to cover with strike plates seems totally acceptable.

The rest of the question seems to be more concerned with the Building Regs, as opposed to the Wiring Regs.

Yes, "holes must only be drilled in a zone which extends 25% to 40% of the beam length from both ends." (Electricians Guide.)

Also hole centres must be at least three diameters apart.

Notches must be in a zone that is from 10% to 25 % of the beam length from both ends.
The depth of the notch must be no greater than one eigth of the joist depth.

Of course, wether it is a question of the Wiring Regs or the Building Regs, it all must be taken into account to comply with Part P.

Hope this helps.

 

[plugwash]

the real question seems to be

is it acceptable to run through existing holes or notches that were drilled or cut without regard to location issues

with most building regs you aren't required to bing existing work up to current standards so i would think the answer would be yes

 

[JDLDIY]

Thanks for your responses guys. I just don't quite get this regulation. I mean for me it would make sense to drill holes/cut notches near the ends of the joists. Weakening the middle of the joist just seems to be wrong, or am I missing something? If drilling for a circuit a joist could end up with two quarter-joist-size holes near the middle, at 40% and 60% of length, surely this does more structural damage than holes near the ends?

 

[dingbat]

Bending moments are highest at the centre of an evenly loaded beam, with shear forces always greatest at the ends.

In simple terms, to resist bending you need the greatest distance between the extremes of the joist section, so drilling holes does not significantly weaken a joist in terms of bending, as it takes material from the neutral axis, which is not actually subject to either compression or tension in the act of bending. Even so, you would not want to remove material from a joist at the point of maximum bending, i.ie. the middle.

Notches,on the other hand, reduce the overall depth of the joist and so are only allowed in an area where bending is at a minimum, that is, closer to the ends. To best resist shear forces you need to maintain the full depth of the joist, which is why neither method is allowed at the ends.

In modern construction material strenghs and loading are more accurately calculated with the intention of using the minimum amount of material for any given structure (£££). If you must notch/drill then follow the rules. In older properties it is likely you'll find joists riddled with holes and notches. If they are already there and you can make use of them, do that. If you must drill new ones, then all you can do is be sensible about it. It's only if a building inspector is involved, or you are issuing a certificate of conformity, that you really need to worry.

 

[JDLDIY]

Thanks for the reply Dingbat. What I don't understand is why holes have to be kept 25% away from the end, whereas notches can be 10% along the joist.
I totally agree with you in that as long as common sense is used, no harm should be done. Good to hear from someone else though!
JD

 

[JDLDIY]

Thanks for the reply Dingbat. What I don't understand is why holes have to be kept 25% away from the end, whereas notches can be 10% along the joist.
I totally agree with you in that as long as common sense is used, no harm should be done. Good to hear from someone else though!
JD

 

[dingbat]

Thanks for the reply Dingbat. What I don't understand is why holes have to be kept 25% away from the end, whereas notches can be 10% along the joist.
I totally agree with you in that as long as common sense is used, no harm should be done. Good to hear from someone else though!
JD

Study structural engineering for a bit... it'll all fall into place then.

 

[TraineeSpark]

I am a new poster here mate so don't really qualify to answer questions, but maybe you might get your answer from a "Joinery" or "Woodwork" Forum. Best of luck.

 

[steve190]

TraineeSpark stated:

The regs state that: 522-06-05 "A cable passing through a timber joist ... (e.g. under floorboards), shall:
i) be at least 50mm measured vertically from the top, or bottom as appropriate, of the joist or batten, or
ii) ... be protected by enclosure in earthed steel conduit securely supported, or by equivalent mechanical protection sufficient to prevent penetration of the cable by nails, screws and the like"

Now the phrase states: "or by equivalent mechanical protection sufficient to prevent penetration of the cable by nails, screws and the like" - so why can't we use safeplates http://www.tlc-direct.co.uk/Products/TLSP1.html if they are of a sufficient thickness?

The reason I ask is I have had to replace some cabling in an old flat in Glasgow and the previous sparky has run the cabling through slots in the joists (under the floorboards) and along side gas & heating pipes.
Now, can I, with a clear consience replace those floorboards without doing anything to protect those cables, as the owner is reluctant to spend on a complete rewire and knowing that I'm going to have to install a consumer unit with the susequent inspection & test.