Adding holes/notches to joists

[noggin14]

Hi - I've been using the forum for all sorts of research and have found it very helpful.

Whilst decorating a room I wanted to add a network point and a phone point and was going to notch one joist and drill through the others using this information:

http://niceic.com/Uploads/File3777.pdf

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Before proceeding I wondered if there were already any unused holes and lifted the other boards, and found that other notches and holes have already been made for electricity and plumbing - but some not within these recommended zones.

Is it a better idea to try and drop the cables from the loft and chase them into the wall rather than making more holes?

The joists are 2.5m long - and 4.5 inches deep x 3 inches wide (11cm x 7.5cm).

One joist has a notch and a hole 30mm apart, within 0.25 of the span.

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Several joists have two holes and one notch. The notch is within 0.07 of the span (virtually against the wall for radiator pipes), one hole in the correct 0.25 of the span and another hole in the middle of the span.

The final joist in the room I was hoping to use to get the cables into the room has four holes and one notch. One hole is in the middle, two are in the correct area at one side and another in a correct area on the other side, plus a notch between the wall and 0.07 of the span.

All of these holes carry twin and earth - the kitchen below has a concrete floor so the floor space carries the sockets for the room, plus lights, sockets and cooker for the kitchen. Trying to maintain 50mm separation between the power and data cables is proving pretty hard...

Many thanks,

Jimmy

 

[sparkwright]

The joists seem quite shallow to hold a floor.

THOUGH I SUPPOSE IT COULD BE THE ANGLE THE PICTURES HAVE BEEN TAKEN.

This isn't a converted loft space by any chance, is it?

Can understand your reluctance to make further holes.

On a positive note, I have seen many houses riddled with holes, which haven't fallen down yet.

May pay tp post to the building forum as well, for peace of mind.

 

[JohnW2]

A very common picture!

We've discussed this before. To my mind, if the only reason for the non-compliance is 'structural' (e.g. holes in wrong part of span) then it would be absolutely ludicrous to drill 'yet more' holes in compliant positions, rather than use existing holes. It is done, but when it is, I think the only reason is a perceived protection of the backside of the electrician - who fears that (s)he could be 'blamed' for the old hole if his/her cables went through it if (which never seems to happen, even with joists that looks like Swiss cheese!) the floor collapsed.

If non-compliance relates to hazard to cables (notches, or holes too close to surface) then some sort of metallic protection would need to be installed if the same notches/holes were going to be used for new cables.

That's how I see it, anyway!

Kind Regards, John

 

[PrenticeBoyofDerry]

We have discussed this before, if it were electrical cable, then as I have often said comply to regulation concerning span, hole/notch sizing and distance between multiply holes/notches. If introducing more holes/notches is structural concern then it should be avoided. In my mind if using existing non-compliant holes/notches I would suggest the use of suitable mechanical protection.

But we talking about data cable and the issue here would be that this cable, unless the insulation is rated to the same or greater than the highest voltage present on the electrical circuits, they should be separated from it.

I believe the requirements against mechanical damage are different between the two!

 

[JohnW2]

We have discussed this before, if it were electrical cable, then as I have often said comply to regulation concerning span, hole/notch sizing and distance between multiply holes/notches. If introducing more holes/notches is structural concern then it should be avoided. In my mind if using existing non-compliant holes/notches I would suggest the use of suitable mechanical protection.

Indeed, and I've expressed my views about this, now and in the past. As a matter of detail, as I just wrote, if existing holes were non-compliant only for 'structural' reasons, 'suitable mechanical protection' would obviously not be required - and, in any event, from what you have said in the past, I suspect that you would not want (feel able) to use existing holes in 'structurally-non-complaint' positions, anyway (for any type of cable).

But we talking about data cable and the issue here would be that this cable, unless the insulation is rated to the same or greater than the highest voltage present on the electrical circuits, they should be separated from it. ... I believe the requirements against mechanical damage are different between the two!

True. My comments were general, probably more relevant to 'electrical' (LV) cables. In fact, even if insulation conditions are satisfied, there are other, functional, reasons for separation of 'mains' and data cables being desirable. I'm not so sure about regulatory requirements as regards mechanical protection differing, though - are you sure about that?

Kind Regards, John

 

[noggin14]

Thank you for the replies.

The joists seem quite shallow to hold a floor.

THOUGH I SUPPOSE IT COULD BE THE ANGLE THE PICTURES HAVE BEEN TAKEN.

This isn't a converted loft space by any chance, is it?

These joists are also in the loft space, and there are two steps down into this room. I think the house originally had a single story extension - and this room has been added on top, so originally they could have been ceiling/loft joists?

I'll post to the building forum too - have found some other building bits which I'm not sure about in the room. With all the other things in the house it is nice to hear a positive note!

To my mind, if the only reason for the non-compliance is 'structural' (e.g. holes in wrong part of span) then it would be absolutely ludicrous to drill 'yet more' holes in compliant positions, rather than use existing holes. It is done, but when it is, I think the only reason is a perceived protection of the backside of the electrician - who fears that (s)he could be 'blamed' for the old hole if his/her cables went through it if (which never seems to happen, even with joists that looks like Swiss cheese!) the floor collapsed.

If non-compliance relates to hazard to cables (notches, or holes too close to surface) then some sort of metallic protection would need to be installed if the same notches/holes were going to be used for new cables.

Thanks - this makes sense. I've seen the suggestion to use metal plates over cables in notches and will try and get some where a notch is used. If the cable should be 50mm from the top and bottom of the joist, which itself is 110mm high it leaves 10mm for the hole? The notches are pretty tight carrying 2.5mm T&E and heating pipes - directly on top of each other at some points, so may need widening to give the electric cable some distance from the pipes?

We have discussed this before, if it were electrical cable, then as I have often said comply to regulation concerning span, hole/notch sizing and distance between multiply holes/notches. If introducing more holes/notches is structural concern then it should be avoided. In my mind if using existing non-compliant holes/notches I would suggest the use of suitable mechanical protection.

But we talking about data cable and the issue here would be that this cable, unless the insulation is rated to the same or greater than the highest voltage present on the electrical circuits, they should be separated from it.

I believe the requirements against mechanical damage are different between the two!

I think my main concern is about weakening the joists - the rest of the house uses 7" x 2" and I've only found one or two holes for a ring main and thermostat in those. The data cable is unshielded twisted pair and phone is CW1308, so no built in protection. There are some points where the cables will cross the electrical cables at 90 degree angles - but with some separation hopefully these will be OK. I'm using the 'safe zone' principle for data cables and avoiding diagonal runs, etc.

 

[JohnW2]

Thanks - this makes sense. I've seen the suggestion to use metal plates over cables in notches and will try and get some where a notch is used. If the cable should be 50mm from the top and bottom of the joist, which itself is 110mm high it leaves 10mm for the hole?

Indeed - but, as has been said/implied, 110mm is very little for a floor joist - what you have appear to be ceiling joists and, as has been asked, are you sure they are man enough to support a floor - or, more to the point (given the conservatism of the regs), are they big enough to be compliant with regulations for floor joists?

I think my main concern is about weakening the joists

I can understand that, given my comments above about the joist size. With 'proper' size floor joists, the regulations are probably ultra-conservative, but with only 110mm to play with, I would personally be hesitant to create any more holes or notches (or enlarge existing ones).

The data cable is unshielded twisted pair and phone is CW1308, so no built in protection. There are some points where the cables will cross the electrical cables at 90 degree angles - but with some separation hopefully these will be OK.

I don't think that there is a significant issue (in terms of 'coupling') with cables crossing at 90°, even if the data cables are unshielded - it's parallel runs of significant distance that should be avoided, even if the data cable is shielded (since I don't think that shielding does much {if anything} in relation to inductive coupling, anyway)

Kind Regards, John

 

[bernardgreen]

A couple of small holes for phone and data cables in the right place will have no significant effect on the strenght of the joist

Holes drilled midway between top and bottom of a joist have very little effect on the strength of the joist other than at the ends of the joist.

The top of the joist is in compression and the bottom is in tension as the wieght tries to bow the joist downwards.. Midway down the joist there is no horizontal stress. Provided there is enough material to keep the top and bottom layers apart small holes half way down have little effect.

At the ends of the joist it is necessary to prevent the layers delaminating and holes here can weaken the joist

With steel only a thin layer is needed at the top and bottom to take the stress.

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• bernardgreen
• 14 Apr 2014
• 1

With timber the top and bottom need to be much thicker due to the variations and imperfections in timber

 

[DetlefSchmitz]

Approved document Part A 1992 still contained the Trada tables for joist sizing. (It was later removed so that Trada could make money out of selling the tables.)

Anyway,the size quoted, together with the span doesn't actually look that bad. I leave it up to others to be specific, but I don't think it's anywhere near as bad as it looks.