Securing backboxes to service void

[jayc89]

I have a 25mm service void, which will be finished with 12.5mm plasterboard so I plan on using 35mm backboxes, which will be secured through the sides and bottom to studs/nogs. Can't secure through the back as that's my airtight layer. 

 

I plan on getting these - https://www.screwfix.com/p/lap-2-gang-galvanised-steel-installation-boxes-35mm-10-pack/88287 - but the top/bottom/side holes look quite close to the edge of the box so I'm worried they'll be proud of the stud work around them (where the PB will sit).

 

Am I missing a trick? That's generally the case!

[Post and beam]

Appleby 2-Gang Dry Lining Knockout Box 35mm | Back Boxes | Screwfix.com  ?

[jayc89]

1 minute ago, Post and beam said:

Appleby 2-Gang Dry Lining Knockout Box 35mm | Back Boxes | Screwfix.com  ?

 

I really don't like the feel on PB backboxes, was hoping to stick with the metal ones. 

[MikeSharp01]

Try These then: They include the fire proofing as well: https://www.tlc-direct.co.uk/Products/FB4235.html?source=adwords&ad_position=&ad_id=&placement=&kw=&network=x&matchtype=&ad_type=pla&product_id=FB4235&product_partition_id=&campaign=shopping&version=finalurl_v3

 

[Kelvin]

You can screw them through the airtight layer as the membrane self seals (depending on the membrane). You can also put butyl tape on the membrane where the box is going to go for extra air tight security. The issue is if you remove a screw and don’t cover the hole. 

[jayc89]

27 minutes ago, Kelvin said:

You can screw them through the airtight layer as the membrane self seals (depending on the membrane). You can also put butyl tape on the membrane where the box is going to go for extra air tight security. The issue is if you remove a screw and don’t cover the hole. 

I have some double-sided butyl tape so that would work! It'd be screwed into PIR though, so unlikely to grip for long? Directly behind is brick so I could use pretty long concrete screws, but they'd act as cold bridges. 

Edited January 30, 2023 by jayc89

[Kelvin]

Ah right. In my case I’m screwing into OSB. 
 

@Nickfromwalesshowed a detail he does where he builds a frame around the services metal box to give you something to screw it to. 

[jayc89]

1 minute ago, Kelvin said:

Ah right. In my case I’m screwing into OSB. 
 

@Nickfromwalesshowed a detail he does where he builds a frame around the services metal box to give you something to screw it to. 

 

That's what I'm thinking, but I probably screwed up by only allowing myself a 25mm service void. I don't have the space to put OSB behind the backboxes and them not protrude beyond the PB. 

 

Perhaps I just need to bite the bullet and go for drywall boxes in this room and chalk it off as experience...

[Mattg4321]

Drill extra holes where needed on the metal knockout boxes. 

[jayc89]

5 minutes ago, Mattg4321 said:

Drill extra holes where needed on the metal knockout boxes. 

 

It did cross my mind, is that allowed? I'll be getting our sparky back in to connect everything up, I'm just doing the legwork. 

[Kelvin]

50 minutes ago, jayc89 said:

 

That's what I'm thinking, but I probably screwed up by only allowing myself a 25mm service void. I don't have the space to put OSB behind the backboxes and them not protrude beyond the PB. 

 

Perhaps I just need to bite the bullet and go for drywall boxes in this room and chalk it off as experience...

Yes my timber wall is a closed panel system so makes all this a bit easier as you have something to screw into 

Edited January 30, 2023 by Kelvin

[Mattg4321]

27 minutes ago, jayc89 said:

 

It did cross my mind, is that allowed? I'll be getting our sparky back in to connect everything up, I'm just doing the legwork. 

Yes, no problem. Suggest drilling where there isn’t a knockout if possible. That’s probably obvious though.