Existing cable routes through joists etc

[rsmck]

I am rewiring a property built in 1953.

Where possible all new cables have been passed through holes drilled in the permissible areas of the floor joists however there are some areas where this is not practical.

Notably there are some very large notches cut out of some joists (way bigger than is permitted under current regulations) which appear to have been there since the house was constructed. I am slightly concerned about drilling these, weakening them further.

I know it is less than ideal to have T&E run in notched joists and would never do it by choice but what would you do in this situation ?

I am inclined to think for the few small areas where this is done following the existing route is probably safer than making MORE holes and I will mark the floorboards clearly that there's a cable beneath them (and the route of it) before replacing the carpet. This is in the middle of a hallway and not where anyone is conceivably going to nail or drill anything through the floor.

The circuits in question are 2.5mm² radial power circuits protected by a 30mA RCD and a 20A MCB,

 

[Paul.M]

In these circumstances and if you have no other option I would use the existing notches and place a metal strip to protect the cables from screws and nails.

 

[Deleted member 9648]

You want these...............Electrical wholesalers will sell small quantities.
Safe Plate | Expamet

 

[nickblake]

It is better to use existing holes and notches on a rewire as drilling more hole will make the joist weaker and as wire puller has said use some safe plates to protect the cables ,these will need notching in slightly so not to cause the board to go back down proud

 

[Engineer54]

If this is an ex council house, there will be 1 1/4'' holes along and close to the outside facing walls up-stairs. there will be bracing noggins between the length of joists, that will have about a 3/4'' gap between ceiling and noggins to allow cables to pass. There will also normally be access traps in the floorboards where existing downstairs lighting points are located. The notches were probably made by plumbers/central heating guys, they weren't there, when the houses were built, especially if it's a council house, ....They were generally better built, than the private houses in those days. The bricks were as bloody hard as iron too....loll!!!

As most of these joists are supported at various points by internal walls etc, i wouldn't be that conserned about drilling an extra 20mm or so hole in these joists, The older houses of that time period used thicker joists than they use today, and were not talking about these joists looking like Swiss cheese now are we ...lol!!!

 

[telectrix]

totally agree with engineer54. the old joists were well big for the job, seasoned timber, not weak green force grown kiln dried crap like they use nowadays.

 

[rsmck]

If this is an ex council house, .... The notches were probably made by plumbers/central heating guys, they weren't there, when the houses were built, especially if it's a council house, ....They were generally better built, than the private houses in those days. The bricks were as bloody hard as iron too....loll!!!

It's not an ex-council house - was built by John Lawrence of Glasgow in 1953, the joists are a bit odd in that they span the width of the house in a single span and in some parts of the hallway are not resting on an internal wall at all.

The house had one owner since new (until me), the heating was original (yes, the boiler had been replaced as had the water tank but all the heating pipes ran down the walls (single pipe system) none of them were below the floor.

The existing notches were made for the original VIR rubber wiring by the builder, there's been one or two sockets added by a DIYer (I assume, as there was no earth to them!) around the 1980s but everything else is original...

Interesting property and a fun project for the years ahead

 

[Engineer54]

As it's your own house, then you will obviously decide what's in your best interest. Personally, i would have no hesitation in drilling a line of 25mm holes in the joists for cables either side of the house upstairs. Even private houses from that period of building, used thicker joists than those used today, and as Tel quite rightly posted, were better seasoned too... I'm sure there aren't many of your joists that are unsupported over the whole of there length from the ground floor internal walls...


It's very rare to see a joists in such a condition as to make them ultimately structurally dangerous. As stated above, as this is your own house, only you will know what condition your joists are in and if you think drilling holes will be detrimental to the overall strength of your upper floor...

 

[JD6400]

As it's your own house, then you will obviously decide what's in your best interest. Personally, i would have no hesitation in drilling a line of 25mm holes in the joists for cables either side of the house upstairs. Even private houses from that period of building, used thicker joists than those used today, and as Tel quite rightly posted, were better seasoned too... I'm sure there aren't many of your joists that are unsupported over the whole of there length from the ground floor internal walls...


It's very rare to see a joists in such a condition as to make them ultimately structurally dangerous. As stated above, as this is your own house, only you will know what condition your joists are in and if you think drilling holes will be detrimental to the overall strength of your upper floor...

Hi Engineer54
You mentioned drilling 25mm holes but i am pretty sure building regs say no more than 20mm may be used and even then they have to be 3x the the size used apart and with in , i think the two in number safe zones .
I only say this because we got caught out on a favour job in a new build for a mate and he used his polish tractor driver to drill our holes ( in the wrong place) to keep his costs down and the nhbc inspector near on passed a kitten and wanted it rebuilt !
We eventually got it sorted with a wee bit of groveling!

 

[JD6400]

I do stand to be corrected , it was a few years ago now and the memory is by no way what it used to be !

 

[rsmck]

As stated above, as this is your own house, only you will know what condition your joists are in and if you think drilling holes will be detrimental to the overall strength of your upper floor...

We found one of them with a 'notch' about 3/4 of the dept of the joist - apparently to accommodate a bathroom pipe... - that one has been appropriately attended to

The plumbers who installed the central heating added a fair few of their own too unfortunately, thankfully much shallower.

In general I'm sure the holes wouldn't be detrimental to the strength of the floor but since the notches are already there then reusing them with appropriate protection and clear markings is probably a good solution. I'm not talking about running them beside pipes or anything like that, these are old notches for electrical cables which helpfully allow me to bring the new cables down the same route as the old to the consumer unit.

 

[rsmck]

I thought the rules were holes only within 0.25 and 0.4 x of the clear span of the joist with the holes a maximum of 0.25x the depth of the joist, so 25mm for a 100mm joist and 44mm for an 175mm joist with adjacent holes no closer than 3x the maximum permissible diameter apart.

Oh, and no holes within 100mm of a notch...

Notches can be closer to the edge, but I can't remember off the top of my head how much closer.

But yes, in practice for many years builders, plumbers and dare I say electricians have just put holes in them wherever convenient in many many buildings and they're all still standing

 

[JD6400]

Sorry i was corrected !
Just bothered to find my IEE guide to the building regs and as usual i have just had another Senior moment , which is not good at just 35 !

 

[Des 56]

If you do a rewire and was to try and comply with building regs, you would be lifting boards up here there and everywhere
Real situations are better overcome by using our common sense,as said, how many houses have collapsed because of a few holes in the non stipulated zones

 

[Engineer54]

If you do a rewire and was to try and comply with building regs, you would be lifting boards up here there and everywhere
Real situations are better overcome by using our common sense,as said, how many houses have collapsed because of a few holes in the non stipulated zones

Totally Agree.... commonsense Rules!! ...lol!!

I often wonder who the hell dreams up all these meaningless dimensions. Had nothing like this in bygone years, doesn't seem to have done any lasting damage over all those years either!!!!

All the times i've brought up different things about structural requirements of various mediums, of wood, steel, concrete etc, over the years on numerous projects, with ''Real'' working Structural Engineers. Only to be told nothing to worry about. Seems that most of these calculations are based on very worst case scenarios that the real world rarely see's. Lapped up by various organisations, so that even more rules are banded about, along with all the scaremongery that goes with them....

There are many buildings in the UK, that are hundreds of years old, built without a fraction of today's building Reg's. ...How many with ALL of today's building Reg's will be around in a hundred years time, ....not very many at all is my guess!!!

 

[rsmck]

There are many buildings in the UK, that are hundreds of years old, built without a fraction of today's building Reg's. ...How many with ALL of today's building Reg's will be around in a hundred years time, ....not very many at all is my guess!!!

Case in point - the changes on equipotential bonding from 16th to 17th

But yes, my first flat was in a building over 200 years old. Nothing wrong with it to this day and built without todays modern mass-produced components (every nail a different shape, every door made by hand and a slightly different size) - the plasterwork is a dying art now, and yet even the most basic rooms in that building had huge ornate cornices - I imagine the builders were proud of what they produced back then.

I can't imagine many of the buildings springing up left right and centre today will still be standing in 200 years time... yet somehow I believe, and hope, that my first flat might