Backboxes in plasterboard on top of OSB

[Dartlec]

Recently sorting out first fix for a loft extension for a friend and ran into a situation I don't often run into (I avoid new builds usually)

In the internal walls, they had the normal wooden studs, then one side was boarded 15mm thick OSB - and plasterboard screwed directly on top.

I assume it's to add structure, but it threw up several issues.

Since the OSB was covering one side of the studs, when the builders plasterboarded that side they just screwed to the board with standard plasterboard screws without checking where the studs were - so had to remove several screws protruding through very close to wiring in the studs.



Then the issue with back boxes - in the end I multi tooled the board out and clipped the back boxes to that, and left the plasterer to worry about the recess depth.



Is this a standard method these days? If so what's the 'correct' or easiest way to fit back boxes for the future?

Only other option I see would be to batten in advance for metal back boxes, or find some plasterboard boxes that can cope with the double depth.

I guess cutting a large hole in advance in the OSB would have been the better option, but it was boarded before I realised that.

 

[Baddegg]

Don’t usually get osb on the inside on loft conversions, depends on the architect really, but when I’ve come up against it I cut through both materials and then use a multi tool to remove a section of osb where the back box wings meet the plasterboard....think that makes sense?.....works for me anyways ?

ps I’d be concerned about the builder as well using that ---- insulation!

 

[buzzlightyear]

Stick the box in and see what the build say when it sticks out fertheir then it is.
Till him you need to but a noggin in and if he's where, open your mouth so I can put between your ears.. Lol

 

[loz2754]

You could try a 47mm plasterboard box as the fixing lugs go further into the cavity.

 

[Pretty Mouth]

You could try a 47mm plasterboard box as the fixing lugs go further into the cavity.

Good shout. The lower quality plasterboard boxes, eg the blue tab ones from toolstation, are good for this too as they can be fitted to thicker boards than the better quality appleby ones.

 

[richy3333]

We multitool the box hole in the OSB slightly larger/longer than required, before the plasterboard goes on. Then once plasterboard its easier to find the 'hole' and cut out for the back box. Just use fast fix boxes.

One builder we work for uses it for noise suppression and says it makes it easy screwing anything to the wall as no plasterboard fixings are required (this includes his 2 bathroom wall lights which were £500 each)!

 

[FatAlan]

I had this with a reinforced stud wall in my place. I used standard dry lining boxes and multi-tooled through the plasterboard and OSB or ply in my case. I then had to multi-tool out sections of the underlying ply each side of the box so the back box lugs would fit. A pain in the arse but it worked ok.

 

[Midwest]

I think the use of ply or osb behind plasterboard, is to help comply with acoustics regs and fixing bathroom furniture, specifically with bathrooms or en suites. They normally put in some insulation in there as well. Its aimed at stopping you hearing people going for a tiddle. Not sure if that's the case in your new build.

 

[Dave OCD]

You could also cut the plasterboard and OSB accurately and fix a metal box using the edge fixing holes with small screws into the edges of the boarding, the box would be a bit deep but gives you more wiring space, just need 2 slightly longer platescrews.

 

[Dartlec]

Good shout. The lower quality plasterboard boxes, eg the blue tab ones from toolstation, are good for this too as they can be fitted to thicker boards than the better quality appleby ones.

Typical - and I made sure the customer bought Appleby because I wanted decent ones ? Good to know though for future reference!

 

[Dartlec]

Don’t usually get osb on the inside on loft conversions, depends on the architect really, but when I’ve come up against it I cut through both materials and then use a multi tool to remove a section of osb where the back box wings meet the plasterboard....think that makes sense?.....works for me anyways ?

ps I’d be concerned about the builder as well using that ---- insulation!

I did consider doing that - but was told the plasterer was in on Monday so didn't have a lot of time. Of course typical there has been no sign of the plasterer this week....

 

[Dartlec]

We multitool the box hole in the OSB slightly larger/longer than required, before the plasterboard goes on. Then once plasterboard its easier to find the 'hole' and cut out for the back box. Just use fast fix boxes.

One builder we work for uses it for noise suppression and says it makes it easy screwing anything to the wall as no plasterboard fixings are required (this includes his 2 bathroom wall lights which were £500 each)!

Sounds like the best option when you are prepared in advance...

Though any length of screw and you have sharp pointy bits through right where the cable is dangling in the cavity....

 

[FatAlan]

I think the use of ply or osb behind plasterboard, is to help comply with acoustics regs and fixing bathroom furniture, specifically with bathrooms or en suites. They normally put in some insulation in there as well. Its aimed at stopping you hearing people going for a tiddle. Not sure if that's the case in your new build.

In my case the use of ply on the stud wall was a structural requirement. Basically had to rebuild the house apart from the walls. ?

 

[lozarus]

Good shout. The lower quality plasterboard boxes, eg the blue tab ones from toolstation, are good for this too as they can be fitted to thicker boards than the better quality appleby ones.

You say lower quality - personally I'm never going back to Appleby since discovering the blue winged ones (manufactured by CED).

The wings are on more of a 'ratchet' style, and you can lock them quite firmly in place to the plasterboard. A misplaced m3.5 won't push the wing back. Plus they go very far back - 20mm or more on a 35mm box. The spring loaded ones only seem to go back about 6mm. The spring winged ones have, occasionally in scenario is such as this one, caused issue by being a bit 'unsteady' or not going back far enough as in this case.

And on top of that they're cheaper! Used recently in a very similar situation.

 

[Resu]

For new builds here it's predominantly timber kit houses with service voids on outer walls. Internal walls can often be found with one side sheeted in OSB along load-bearing walls. What you've run into is common for us to find now. If we haven't been able to cut a section out ahead of the walls being plasterboarded, we will carefully cut through both layers with a multitool then fit metal boxes or appleby fastfix's with the wings cut off and wood screws in either side.

Personally wouldn't chose to use any other brand of fastfix other than Appleby having had more issues in the past with any other brand we've tried. Might pick up a couple of CED and give them a try though.