Building regs- back box depth?

[Chri5]

If a vertical chase can be up to 1/3 of the depth of the inner leaf, and the horizontal chase 1/6.......

Does that mean a 45mm back box is illegal per building regs?

What is the back box depth allowed?

 

[regsmyth]

You wouldn't normally sink the back box in to the full depth, but set it to finish a little below finished plaster level. And you wouldn't be channelling the wall to the same depth as the box. Otherwise the wall would be seriously weakened.

 

[Taylortwocities]

Never had a house fall down when I've chased in a 47mm box - yet.

I expect some numpty will suggest putting a mini lintel over the top.!

This was suggested in another thread re meter enclosures. These need a 400mm hole in the brickwork - no structural support needed...

 

[Chri5]

Thanks guys for the details so far, but is there a definative?

Could I assume 1/3 rd wall depth?

So a 47mm would be 40mm odd into a wall (7mm for plaster) and at 2/5 ths of a 100mm wall would then fall foul of the building regs!

 

[regsmyth]

If you're just putting in cables, why make the channel so deep? Perhaps make it wider instead if you've got lots of cables in the run? As per regs, with a 100mm inner leaf, 33mm max deep for vertical chases, 25mm max deep for horizontal.

 

[Chri5]

I wouldn't chase that deep for a cable run.

This is purely a fictive post, based on curiosity and having read the EI Guide to the building regs can't find anything definitive regarding back boxes.

Seems strange that they have made rules for chases, but there does not appear to be anything of clarity regarding boxes and the accepted depth of wall to sink them in to.

 

[ban-all-sheds]

As per regs, with a 100mm inner leaf, 33mm max deep for vertical chases, 25mm max deep for horizontal.

Q1. 100 ÷ 6 =

• 25
• 600
• 16.67
• 0.06

 

[regsmyth]

doh!
serves me right trying to be too quick off the mark.
Anyway, OP, clearly it's ok to make holes, even big ones, right through a wall, so there's probably no limit to how deep you can have a back box. It's the structural weakening effect of a long channel in the wall they're trying to avoid.