wiring through joists.

[dtec dave]

Morning All...

Another quick question I'm sure one of you will have the answer to...

I am going to be running cables under my floorboards but as the house is over 100 years old this has been done many times before. The joists have already been notched out but not to the latest depth as per regs i.e 'in a zone between 0.07 and 0.25 x span'. Would you need to re-notch in the correct zone (weakening the joist further) or can you use the existing.

Also the regs state that a cable passing through a joist should 'be at least 50mm from top or bottom' on the same page it states that the max depth of a notch is '0.125 x joist depth'. That would mean a joist would have to be 400mm to achieve this and seeing as I don't live in a castle my joists aren't that depth.

Cheers in advance

 

[SPARTYKUS]

Common sense suggests not weakening the joist further, as long as you can get them through the middle

 

[brman]

I think you are getting confused with the regulations. The notches don't have to be at least 50mm deep. Have a look at P73 of the on-site guide.

As to whether you should re-use notches in a place not according to the regs. I would, but I'd also make sure the cables are protected if they are less than 50mm depth in a place where they should not be. Others may say differently.

 

[Richard Burns]

If there are notches already in place then I would tend to use them, though strictly speaking it would not comply with the regs, there are enough houses with older installations and if you have cables there already you are not making it any less safe.
The 50mm is for holes through joists not notches over them.

 

[brman]

btw guys, having looked at the on site guide and guide to the building regs again, neither of them appear to mention 50mm depth although I too thought it was a reg. Where does it come from?

 

[Dunc]

btw guys, having looked at the on site guide and guide to the building regs again, neither of them appear to mention 50mm depth although I too thought it was a reg. Where does it come from?

50mm is generally down to cables buried in walls. As for the notches I would reuse them but to satisfy regs you have to use safe plates or a similar item to protect the cables from nails and screws when floors are re fixed.

 

[malcolmsanford]

View attachment Notches and Holes in Timber Joists.pdf

Because you have a guide ​The actual dimensions are part of Buildings Regulation approved Document A

 

[Octopus]

I have to say this but the OSG, to me, appears to be written around new builds or new extensions.

If it were to cover the plethora of old builds and modifications it probably wouldn't fit in your tool bag!

 

[brman]

View attachment 11060

Because you have a guide ​The actual dimensions are part of Buildings Regulation approved Document A

I had a quick look in approved document A. tells you even less than the OSG (assuming I didn't miss the relevant bit)

 

[telectrix]

use existing notches if you are worried about weakening the joists, but fit safe plates over cables. have to say, though that with buildings that age, the joists are generally far stronger than the modern force grown kiln dried unddersized crap that is thrown in buildings today.

 

[SPARTYKUS]

use existing notches if you are worried about weakening the joists, but fit safe plates over cables. have to say, though that with buildings that age, the joists are generally far stronger than the modern force grown kiln dried unddersized crap that is thrown in buildings today.

yes! alot to be said for over engineering

 

[dtec dave]

Thanx..If you take the osg literally it does state on page 59 (still on red..just ordered my green) 'a cable passing THROUGH a joist be at least 50mm from the top' then on same page 'figure 7.1 cables THROUGH joists' figure 7.1 illustrates a notch in a joist, which to me suggests that a cable passing through a notch is the same as cable passing through a joist and has to be 50mm from the top!

 

[PC Electrics]

use existing notches if you are worried about weakening the joists, but fit safe plates over cables. have to say, though that with buildings that age, the joists are generally far stronger than the modern force grown kiln dried unddersized crap that is thrown in buildings today.

I've been working on a large ex manor house, the core building of which is 400 years old. The groundsman reckons the joists are solid oak. They are certainly solid! Even with new razor sharp Armeg Wood Beavers it's been a sweat to drill through some of them! And then there's the 2 foot thick stone walls!

 

[malcolmsanford]

Dave it clearly says that the maximum depth of a Notch is ....................0.125 x the joist depth. so if it is a 100mm joist say then it is 100 x 0.125 = 12.5mm a bit less than your 50mm

 

[telectrix]

now i would work taht out in english. 4" joist, 1/8 of the depth = 1/2". who said imperial measurements were complicated?

 

[malcolmsanford]

now i would work taht out in english. 4" joist, 1/8 of the depth = 1/2". who said imperial measurements were complicated?

Personally mate I would have gone 4" joist 4/32 = 8/16 but know what you mean

 

[telectrix]

ore even 8/64 ths

 

[dtec dave]

I'm not disputing any of this..I know it is totally impractical to cut a notch to 50mm deep. I was just trying to point out a confusion on page 59..fig 7.1 clearly states that a notch is a route for a cable to pass through it..above fig 59 it states that any cable passing through a joist should be 50mm from the top. So if we were to take this literally a notch should be 50mm deep, if they were to make this clearer there should be 2 diagrams. 1 showing cables passing through a joist i.e a hole and 1 showing dimensions of a notch. Anyone in agreement???

 

[SparkyDips]

stronger than the modern force grown kiln dried unddersized crap that is thrown in buildings today.

they didn't do too bad a job holding up boxes of boating magazines in a recently mentioned dwelling lol

 

[Andy78]

It tells you the maximum depth your notch can be in relation to your joist. If this is less than 50mm then it must satisfy the other stated requirements to comply. Simples.

 

[lauriehurman]

IMHO where you put the notches and holes is building regs,
Where you run cables is wiring regs.
Yesterday's cables are "selected and erected" to yesterday's wiring regs.
When you select and erect it has to be to BS7671:2011. In this case 522.6.100.
That means just because there's a convenient hole in the joist doesn't mean that you can use it for twin&earth, unless of course you fulfil one of the conditions for mechanical protection. I don't have sufficient wisdom to assert whether a notch plate is sufficient protection.
Interestingly the 50mm is from the top or bottom of the joist not the surface of the floor or ceiling, i.e. does not include the depth of the floorboard. Strange.

Laurie

 

[telectrix]

Interestingly the 50mm is from the top or bottom of the joist not the surface of the floor or ceiling, i.e. does not include the depth of the floorboard. Strange.


​that's to allow for 3" nails. bear in mind that the average wood butcher or wet-head will use whatever he has to hand when nailing boards.

 

[Richard Burns]

Interestingly the 50mm is from the top or bottom of the joist not the surface of the floor or ceiling, i.e. does not include the depth of the floorboard. Strange.
Laurie

Also means that it is not possible to put cables through joists in the mid level floor of a property with 4" joists!!

 

[Octopus]

Also means that it is not possible to put cables through joists in the mid level floor of a property with 4" joists!!

Excellent, that sounds like fun complying with.

How daft, old houses often have joists with more holes than Swiss cheese and they seem to stay up.

 

[telectrix]

How daft, old houses often have joists with more holes than Swiss cheese and they seem to stay up.

that's because they were built, not thrown up designs by pencil pushing bean counters.

 

[krishcanag]

I could not care less about BS7671, the first priority of any trade working on site is make sure that at the end of it he/she has not weakened the structure, stuff the regs, use what's there and inform the IET to issue a guideline of old houses with existing holes in joists (weakened joists). Holes in joists should never be allowed in the firstplace!! or should be thinker than required in the first place to conpensate for electricans and plumbers
KC