Plastering chases and back boxes.

[GSDog]

I've had the electricians around and the back boxes are installed along with all the wiring and chasing have been done, they say it's ready to be plastered now.

I'm going to put PVA glue on 24 hours before using bonding plaster to fill the chases, leaving about 3 - 4mm for the finishing plaster. Is that right?

How long should I leave the bonding plaster for before using the finishing plaster?

How should I fill the holes and gaps around the back boxes, is finishing plaster OK to use for that?

Any help would be appreciated.

 

[Richard C]

That’s’ broadly OK; you can finish plaster as soon as the Bonding has set; around 1 1/2 hours. You will find it much easier if you use Easyfill rather than finishing plaster which can be difficult to blend in if you’re inexperienced. Another little trick to reduce the chance of cracking is to chip the original finish plaster away from the base coat adjacent to the chase, fill out the chase with Bonding to the same level & then tape the join before applying the finish plaster. Fill around the boxes with Bonding in the same way, then Easyfill.

I always fit steel capping over the cables.

 

[1john]

I prefer to use one coat to fill out as it hasnt got the lumpyness in it, and use easy fill over top, i never use capping, it doesn't do anything and imo steel capping can be more dangerous.

 

[Richard C]

I prefer to use one coat to fill out as it hasnt got the lumpyness in it, and use easy fill over top, i never use capping, it doesn't do anything and imo steel capping can be more dangerous.

I don’t agree with the usual reason trotted out; “it’s to protect cables from the plasterer” as no decent spread would damage cables or risk damaging their trowel by hacking away at cables but what a steel capping will give is a vital few seconds warning to “Mr DIY shelf fixer” with a drill not understanding safe zones; or even folks that should know better being careless (& Ive done it so me included ). Conventional masonry bits won’t cut through steel that easily & it may give just enough time for the penny to drop & figure out why the drill won’t move but the most determined will get through it eventually.

Why do you consider steel capping dangerous?

 

[GSDog]

I've been a bit delayed with this but I'm planning to do the plastering this weekend.

I'm not sure whether to install steel capping or not. The electricians told me to plaster straight over the wires but other people have told me to use the capping.

 

[Richard C]

You need to hold the cables in place somehow capping or cable clips, you can do either; I've said why I prefer to fit steel capping & I think it's easier to plaster over but your choice really.

 

[stevethespreader]

I've been a bit delayed with this but I'm planning to do the plastering this weekend.

I'm not sure whether to install steel capping or not. The electricians told me to plaster straight over the wires but other people have told me to use the capping.

ive done mine recently and ive gone straight over the wires

 

[paul78]

should be the sparky's job to cap them i think...... but most never do as everything is always left to the spread to sort out

 

[paul78]

should be the sparky's job to cap them i think...... but most never do as everything is always left to the spread to sort out

 

[PrenticeBoyofDerry]

Capping is not needed if the cables are deep enough that trowel damage to the cable will not occur.
If the cables are a bit loose and causing problems they can be clipped back or set back with the bonding and left until taken hold.
If I was capping I would use plastic rather than metal, the metal caps should really be connected to earth.

 

[joe-90]

I like steel. A battery drill on non hammer will soon tell you when you've hit the capping. It has a sort of 'feel' to it.

 

[Richard C]

Capping is not needed if the cables are deep enough that trowel damage to the cable will not occur.

PBD; I know you as one of the regulars who post on several of the trade forums; I also believe you “spread” so I can’t believe you posted that. I really don’t understand where on god’s earth this total myth that cable capping (metal or plastic) is required to protect cables from clumsy, ham fisted plasterers comes from; FFS, no decent plasterer will ever damage cables full stop. I can’t imagine any rendering/plastering scenario where a spread’s trowel would damage cables or he would risk damaging any of his trusty trowels (even a screeder) by hacking away at PVC cables; but how & why any spread would contemplate or even manage that is beyond my comprehension; you’ve got to be pretty intent to even damage the outer sheaving of a PVC shielded twin & earth.

metal caps should really be connected to earth.

No that’s not correct or necessary in accordance with Electrical Regs; conduit yes, capping no.

 

[PrenticeBoyofDerry]

Although they may be no requirement to earth metallic caps, the reason I stated this was if a nail/screw went through the capping and into the line conductor without making contact with the cpc then the fault is earthed by the capping.
RCDs should take care of this if the existing installation is compliant to BS7671:2008, but there are no assurance that it is and often is seen, where cables are routed outside the prescribed safe zones and capping is then placed over this as a means of mechanical protection, which it is not.
Metal capping is used to hold the cables in place and to prevent trowel damage,(whether trowel damage would happen in the real world, by any competent spreader is a different issue) it's as simple as that. It is not a means of mechanical protection or a method used as containment for cables to be pulled!
In the days of the 16th edition of wiring regs, pre the requirements of RCD protection as now in the 17th. The mind set was to earth anything that was metal; baths, sinks, windows, caps etc....