# Wall Chase Maximum Depth

#### OuterSpaceHopper

Hiya Folks

I'm planning on chasing a channel in a wall for cables for a 'lounge plate' e.g. TV, FM, SAT, network, phone.
I'd like to put these in in conduit, it will probably have to be 25mm round.

What is the maximum depth I can chase the wall out vertically?

I've read that the maximum vertical chase depth is 33%.

My house has three layers of bricks in the walls. Normal outside red bricks, a cavity wall with breeze block, then an internal cinder or clinker type block at 50mm wide with plaster on top(12-15mm).

Is chasing out this internal wall(leaf?) limited by the building regulations Part A 33% limit also?
If so does the limit include the finishing plaster?
e.g. 0.33 x 65mm = 21.5mm?
or 0.33 x 50mm = 16.5mm?

Or does the regulation not apply to this inner leaf of blocks and I can merrily chase a channel deep enough for 25mm conduit?

It is hard to tell if these internal cinder blocks are taking any load from above. I suspect not as in places there is plaster board between the cinder block and the joists. I think the entire load of the joists is taken by the middle row of breeze block where the joists sit on metal plates on top of the breeze blocks, not by the cinder block internal wall.

Here is a photo of the joist end. The cinder blocks are under the narrow strip of plaster board, so probably not load baring.

http://www.diynot.com/diy/attachments/joistnotchresize-jpg.107292/

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#### EFLImpudence

The limit is 33% of the leaf, usually 33mm. of 100mm. blocks/bricks etc.

It is half that for horizontal chases.

#### securespark

Use 2 x 20mm conduit to reduce your chase depth.

#### OuterSpaceHopper

33% of 50mm doesn't give me much wiggle room to get the conduit flush with the brick to allow for boarding out in the future when the rest of the plaster fails.

I had hoped that the rule might be different for none load bearing walls that are supported from behind by another wall!

Either way it looks like it is going to have to be 2 x 20mm oval, but the other problem I have is that I need to drill up through the brick for 30cm or so to get past some nice looking coving, so I would then need two holes drilled, and they ain't easy to drill at the correct angle. It might have been luck that landed me right dead center of the top block on the first pilot hole!

#### EFLImpudence

Of course - you don't have to put the cables in conduit.

You can chase just deep enough for each cable.

#### seasickstevie

Use shallow, but wide trunking.

#### John D v2.0

I don't believe anyone noticed your question about plaster being included. The answer is no, you can chase plaster as far as you like no problem as it's not structural. However there's a limit to how thickly you can apply plaster without it pulling itself off.
With 100mm leaf and 12mm plaster you should be able to fit 45mm deep socket boxes even

#### JohnW2

I don't believe anyone noticed your question about plaster being included. The answer is no, you can chase plaster as far as you like no problem as it's not structural.
Indeed. I don't think that is explicitly stated, but it is certainly implied (and corresponds with common sense).
With 100mm leaf and 12mm plaster you should be able to fit 45mm deep socket boxes even
One interesting thing about Approved Document A (which creates the "1/3" and "1/6" rules) is that it does not define what it considers to be a "chase" - in particular in relation to how long it has to be before it counts as a chase. A double back box in the common 'landscape' orientation has a greater horizontal dimension than its vertical one. Hence, if it were considered to be 'a chase' it would presumably be considered to be a horizontal one (hence maximum depth of 1/6).

That might sound silly in relation to just one box (in which situation I would hope that common sense would prevail), but I have seen situations (in kitchens, utility rooms, workshops etc) in which a horizontal row of accessories has been sunk into what is essentially a fairly long horizontal 'chase'. With 35mm boxes and 12mm plaster that would involve 'chasing' a lot deeper than 1/6th of the thickness of a 100mm wall.

Has anyone ever thought about this, I wonder?!

Kind Regards, John

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#### John D v2.0

Well, I was being flippant really, but technically you can remove a brick completely without lintelling over, you could make your inner leaf into a honeycomb wall and put sockets all the way along and all the way up. Call it an art installation!

#### JohnW2

Well, I was being flippant really, but technically you can remove a brick completely without lintelling over ....
I realised you were being a bit flippant in talking about 45mm (47mm?) boxes, but I was being deadly serious in mentioning horizontal 'chases' (done that way because it was 'easier' than individual holes for boxes), more than a metre wide, accommodating a row of 35mm boxes (hence, say, 23mm into the wall proper, much more than 1/6 of a 100mm wall).

Whilst one can remove one brick "without lintelling over", one can't remove a row of half a dozen or more adjacent ones!!

Kind Regards, John

#### seasickstevie

A 150mm dia duct (or bigger even) takes a hell of a lot out of a wall, but not heard of any major concerns.

I wonder how big a duct/fan has to be before it's considered an issue (other than the common sense approach)?

#### JohnW2

A 150mm dia duct (or bigger even) takes a hell of a lot out of a wall, but not heard of any major concerns.
Not really. As has been said, one can remove a whole brick without any significant structural effects. One can also (within reason) remove the 'triangle' of bricks below that one removed brick. A 150mm diameter hole is therefore not really significant.

Kind Regards, John

#### OuterSpaceHopper

Thanks Guys, I had thought about the backbox depth(in relations to the chasing rule), especially where there is a row of double gangs, and then decided there was no way round it, other than surface mount(shrieks from her in doors)

The internal blocks are only 50mm deep cinder\clinker type block(420mm x 220mm), do you think that adding a deep backbox would count as breaking the building code?

Even a 25mm backbox seems to break the code as in places as the plaster is only 8mm think, so the backbox must be in the brick at least 17mm. It looks like the lounge plate needs 45mm deep backbox to accommodate the back of the TV and FM connectors.
It is virtually impossible to make a backbox this deep(or even 35mm) without the back of the block cracking and revealing the breeze block behind.

These cinder blocks don't seem like the strongest and as far as I can tell, are not even built on a sole plate! Upstairs I can find any sign of screws through the floorboards underneath them, so I presume they are just laid on the floor boards. They have been up 60 years though!

What do you think?
should I stop worrying about it, as the wall doesn't appear to be load bearing(still heavy if it fell on you while sitting on the sofa!
or
space the back boxes to avoid a row continuous row of several accessories?
or
ultra conservative and surface mount the new stuff?

#### OuterSpaceHopper

I just read JohnW's reply, so I think we are ok with even the deep back boxes!

I might have a closer look at the long row of face plates in the kitchen though!

As far as the cable chases are concerned, I may have to abandon the prospect of easy future replacement via plastic conduit and just chase deep enough to recess the cables in the brick, e.g. 12-15mm. I'd prefer not to fix them to the surface of the brick the plaster is very thin in places and I think that she-who-must-be-obeyed has her eye on reboarding the room and plastering in the future.
I'm guessing that recessing the cables in the brick now will make the dot and dab boarding that bit easier and certainly make it easier to get the old base plaster off without fiddling around cables?

#### JohnW2

The internal blocks are only 50mm deep cinder\clinker type block ....(420mm x 220mm), do you think that adding a deep backbox would count as breaking the building code? Even a 25mm backbox seems to break the code as in places as the plaster is only 8mm think ....
Oooooh - that's not very thick.
What do you think? .... should I stop worrying about it, as the wall doesn't appear to be load bearing(still heavy if it fell on you while sitting on the sofa! .... or ... space the back boxes to avoid a row continuous row of several accessories? .... or ... ultra conservative and surface mount the new stuff?
Hmmm. Difficult. Even though the wall is not load-bearing, as you say, it could be dangerous if it collapsed, although I'm far from sure that it would actually be subject to the Building Regs regarding chases if (as would be the case) the two load-bearing leafs remained intact. Given the non-weght-bearing nature of the wall, it would be tempting to 'stop worrying about it', but that would rely on your being able to dig out holes (of any depth) for boxes without running the risk of destroying the wall - and I think that would remain true even if the accessories were well-spaced.

Depending upon how much work/disruption you are prepared to contemplate, I suppose one approach would be to put some sort of lintel into that inner leaf, and then you could do more-or-less whatever you wanted below that lintel (maybe even replacing the blocks with some other material - maybe even wood).

Another, non-destructive and non-disruptive approach would be, rather than to have lots of separate surface-mounted accessories, to have a box/raised panel/whatever 'surface mounted' on the wall (thereby protruding a bit), and to 'flush mount' the accessories into that box/panel/whatever. Personally, I think I'd find that aesthetically preferable.

Kind Regards, John

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