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?