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Metal Back Boxes and Dry Lining Boxes — Flush Mounting Sockets, Switches, and Wiring Accessories for UK Electricians
Metal Back Boxes and Dry Lining Boxes — Flush Mounting Sockets, Switches, and Wiring Accessories for UK Electricians
Every flush-mounted socket, switch, and wiring accessory in a UK property sits in a mounting box recessed into the wall. Getting the box type and depth right the first time prevents rework, cracked plaster, and accessories that sit proud or rack when fitted. This guide covers galvanised steel back boxes for solid masonry walls and dry lining boxes for plasterboard and stud partition walls — selection, depth choice, preparation, and installation technique for each wall type.
Back Box Types
Galvanised Steel Back Box (Solid Wall)
Pressed galvanised steel mounting boxes, also called "metal back boxes" or "steel pattresses," are the standard fitting for sockets, switches, and wiring accessories in solid brick, block, and concrete walls. They are chased into the masonry and secured with plaster or wall plugs and screws. The face frame of the accessory clamps over the box with machine screws into the box's internal lugs.
Key dimensions:
- Gang size: 1-gang (1G) fits single-width accessories; 2-gang (2G) fits double-width accessories. 3G and 4G boxes are available for socket clusters.
- Depth: 16mm is the minimum depth for most modern slimline accessories. 25mm suits standard accessories including back boxes with earth tails or cable dressing. 35mm and 47mm are used where larger accessories (USB outlets, smart controls, dimmers with deep components) need space for rear wiring and cable loops.
- Standard face size: 86 × 86mm for 1G, 86 × 146mm for 2G (matches UK accessory face plate dimensions).
Dry Lining Box (Plasterboard / Stud Wall)
Dry lining boxes — also called "plasterboard boxes" or "plastic pattresses" — are designed for use in plasterboard partition walls and dry-lined solid walls. They are fitted from the front of the plasterboard by cutting a square hole and pushing the box through, then tightening integral gripping wings or lugs that clamp the plasterboard from behind. No plaster is required. Most dry lining boxes are PVC.
Key dimensions:
- Gang size: 1G and 2G are most common. Dual 2G versions allow a double socket and a single socket in one combined aperture.
- Depth: 25mm suits most standard accessories. 35mm and 47mm allow deeper accessories and better cable management behind the box. Dual-depth boxes accommodate both shallow and deep accessories.
- Nose depth: Verify the box depth matches the plasterboard thickness. Standard plasterboard is 12.5mm; a 25mm box sits correctly. For 15mm fire board or double-layer board, check the nose length allows the face of the box to sit flush with the board surface.
When to Use Which Box
| Wall Type | Box Type | Fixing Method |
|---|---|---|
| Solid brick / block / concrete | Galvanised steel back box | Chased recess, plastered in or plugged and screwed |
| Plasterboard on stud (new build / partition) | Dry lining box | Square hole cut, box pushed through, wings tightened |
| Plasterboard over masonry (dot and dab) | Dry lining box (preferred) or extended steel box | Hole through plasterboard; may reach masonry for deeper fixing |
| Timber stud with plasterboard both sides | Dry lining box with stud plate if near stud | Avoid cutting studs — position between studs |
| Exposed brick / unplastered surfaces | Surface-mounted metal trunking or surface back box | Screwed to surface (surface installation, not flush) |
Depth Selection Guide
Choosing the wrong depth is the most common back box mistake. Too shallow and the wiring cannot be dressed safely behind the accessory, the accessory is loose in the box, or connections foul the back of the accessory. Too deep and the box protrudes beyond the plaster surface or the accessory face plate does not sit flush.
| Depth | Typical Use | Notes |
|---|---|---|
| 16mm | Slimline accessories; refurbishment where recess depth is limited | Not suitable for accessories with deep rear components (dimmer modules, USB chargers) |
| 25mm | Standard 1G and 2G sockets, switches, FCUs — most common depth | Suitable for most standard accessories; allows one cable fold behind |
| 35mm | USB outlets, smart switches, shaver sockets, data outlets | Required where accessories have internal transformers or circuit boards |
| 47mm | Consumer unit connections, large accessories, high-cable installations | Allows two cable loops; useful for twin-and-earth + earth tail dressing |
Installation — Galvanised Steel Back Box in Solid Wall
Tools Required
- SDS drill with chisel bit
- Bolster chisel and club hammer
- Steel back box (correct gang and depth)
- Spirit level
- Retractable knife (to cut conduit knockout)
- Conduit and grommets (where cables enter via knockout)
- Wall plugs and screws (for direct fixing to masonry)
- Patching plaster for the perimeter
Procedure
- Mark the position — use the box as a template; mark the perimeter with a pencil. Use a spirit level. Check for hidden cables with a cable/pipe detector before cutting.
- Chase the recess — use the SDS chisel or angle grinder with masonry disc to cut the recess to the box depth. Remove material with bolster and club hammer.
- Cut knockouts — remove the required cable entry knockouts using a screwdriver or retractable knife. Fit rubber grommets to all knockouts through which cables will pass.
- Plug and screw or plaster in — either drill two fixing holes through the back of the box and plug the masonry, or offer the box into the recess and plaster around it. For plastered fixing, ensure the box face is flush with the plaster surface when set.
- Level the box — ensure the box fixing lugs are horizontal before the plaster sets. Accessories with a horizontal orientation will rack if the box is skewed.
- Thread cables and terminate — pull cables through grommets, leaving sufficient tail length for dressing inside the box. Connect to the wiring accessory and secure the accessory face plate to the box lugs using the machine screws provided with the accessory.
Installation — Dry Lining Box in Plasterboard
Tools Required
- Dry lining box (correct gang and depth)
- Padsaw or oscillating multi-tool with plasterboard blade
- Dry lining box template or the box itself for marking
- Flathead screwdriver (to tighten gripping wings)
- Cable detected before cutting
Procedure
- Mark and cut the aperture — most dry lining boxes include a paper template. Mark the cut line on the plasterboard and cut with a padsaw or oscillating multi-tool. Cut inside the line for a tight fit; the face flange of the box covers a small gap.
- Fish cables into position — ensure cables are fished to the cut-out position before fitting the box. Pull cable tails through the appropriate knockout on the box body before offering the box into the aperture.
- Fit the box — fold gripping wings flat, push the box into the aperture, then tighten the gripping screws. As the screws are tightened, the wings rotate and grip the back face of the plasterboard. The box should grip firmly when correct.
- Check depth and flush — the face of the box should sit level with or very slightly proud of the plasterboard surface. Recess depth is important where the fitting is to be over-plastered — in this case a metal box and skim is more appropriate.
- Connect and fit accessory — dress cable tails neatly within the box, connect to the wiring accessory, and fix the face plate with the machine screws to the box's integral lugs.
Common Faults and Fixes
| Fault | Cause | Fix |
|---|---|---|
| Accessory rocks when fitted | Box too shallow; box not set flush; box lugs misaligned | Deepen recess; adjust box position; check accessory fixing holes align with lugs |
| Dry lining box spins when gripping | Aperture too large; wing not gripping properly | Pack with offcut plasterboard; use deeper box; check box is correct size for aperture |
| Metal box rusting in bathroom/damp area | Galvanised box used in high-humidity zone | Use stainless steel or non-metallic boxes in IP-rated zones; check IP rating of accessories |
| Cables too short to terminate | Cable not fished far enough before box fitted; box too deep | Use cable jointing in a suitable junction box; avoid proprietary connectors inside back boxes in plastered walls (not accessible) |
| Accessory face plate sits proud | Box set too deep; plaster built up over box face | Use box extension ring (frame ring) to bring box face forward; or use shallower replacement box |
BS 7671 and Part P Requirements
The installation of socket outlets and switches is notifiable work under Part P of the Building Regulations in England and Wales (except in rooms other than kitchens and bathrooms for like-for-like replacements). New socket circuits and wiring in kitchens, bathrooms, and outdoors always require notification. All metal back boxes and accessories must be earthed where accessible metal parts exist — the earth terminal in the box must be connected to the circuit protective conductor.
Under BS 7671:2018+A2:2022, all new cable routes in walls should either be at specified zones or protected by RCD. Ensure the circuit feeding the new socket or switch is on an RCD-protected way in the consumer unit. Document circuits on the installation schedule and update the domestic electrical installation certificate (DEIC) or EIC as appropriate.
Recommended Products
Thrion 1G 25mm Galvanised Metal Back Box
Single-gang 25mm deep pressed galvanised steel back box for flush mounting 1-gang wiring accessories in solid masonry walls. Standard 86 × 86mm face plate footprint. Two cable entry knockouts on the back and sides. Internal earth lug for CPC connection. Machine screw fixing lugs at standard centres for UK wiring accessories.
Why buy: The go-to 1G back box for single sockets and switches in solid walls. 25mm depth accommodates standard cable dressing without overcrowding. Galvanised finish provides corrosion resistance in normal dry internal environments.
Thrion 2G 47mm Galvanised Metal Back Box
Double-gang 47mm deep pressed galvanised steel back box for flush mounting 2-gang accessories in solid masonry. Standard 86 × 146mm face plate footprint. 47mm depth provides ample cable dressing space for twin-and-earth plus earth tail, and accommodates deeper 2G accessories including USB outlets and smart socket modules.
Why buy: Deep 2G box suited to modern accessories with rear-mounted components. Eliminates rework when USB socket chargers or dimmer modules prove too deep for a 25mm box after the plaster has set.
Appleby Dry Lining Box 1G 35mm
Single-gang 35mm deep PVC dry lining box with gripping wings for plasterboard and stud partition walls. Pre-formed cable entry knockouts. Tighten the integral gripping screws to secure the box — no plastering required. Standard 86 × 86mm face plate footprint. Works with plasterboard from 9.5mm to 15mm thickness.
Why buy: Fast installation in plasterboard — cut aperture, push through, tighten wings. 35mm depth suits modern accessories including USB outlets and data faceplates. The workhorse 1G dry lining box for new build and partition work.
Appleby Dry Lining Box 2G 47mm
Double-gang 47mm deep PVC dry lining box for plasterboard walls. Gripping wing design for tool-less wall fixing once aperture is cut. Standard 86 × 146mm face plate footprint. 47mm depth accommodates back-to-back wiring in multi-cable circuits or deep 2G accessories. Compatible with all standard UK 2G wiring accessories.
Why buy: Deep 2G dry lining box for double sockets and 2G switch plates in plasterboard walls. The 47mm depth prevents cable congestion in high-cable circuits and ensures modern USB double sockets fit without fouling rear connections.
Shop Back Boxes at APM Electricals
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APM Electricals
24 Western Avenue, Acton, London W3 7TZ
Phone: 020 8702 8080
Web: www.apmi.uk
Same-day collection available for West London trades.
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