Back Boxes Explained: 16/25/35/47mm Depths & Types

The wrong back box is the number one reason a new socket or screwless switch won't sit flush. This guide covers the three box types, which depth each accessory needs, and how to check before you order.

The three types of back box

  • Metal (flush) boxes — for brick/block walls, chased in and plastered. The default for solid walls; knockouts for cable entry, adjustable lug for levelling, earth terminal inside.
  • Dry-lining boxes — for plasterboard. Fitted into a cut-out and clamped with wing lugs. Fast for stud walls and refurbs.
  • Surface pattress boxes — mount on the wall face. For garages, plant rooms, retrofits where chasing isn't practical, and hotel back-of-house.

Depth diagram: which box fits what

Wall face (plate sits here)16mmPlate lightswitches only25mmStandard sockets,most switches35mmUSB sockets, dimmers,cooker switches, grids47mmShaver sockets,smart devicesDepth measured from wall face to box base. When in doubt, fit 35mm.
Side-view comparison (not to scale). Screwless plates also need the box flush and square with the wall face.

Depth: the measurement that matters

  • 16mm — basic 1-gang plate light switches only
  • 25mm — standard switches and most sockets; minimum for many screwless plates
  • 35mm — the safe choice: USB sockets, dimmers, cooker switches, sockets on ring finals with multiple cables
  • 47mm — shaver sockets, some smart devices and heavily-loaded grid plates

Rule of thumb: for anything with electronics inside — USB sockets, dimmers, WiFi extender sockets — specify 35mm and you won't be caught out. Deeper grid assemblies (3+ modules) also benefit from 35mm+; see the grid switches guide.

Matching plate to box

  • Screwless ranges (e.g. BG Nexus Screwless, Evolve): flat plates sit directly on the wall — the box must be flush and square or the clip-on front won't seat evenly.
  • Raised-profile ranges (e.g. Nexus Metal): more forgiving of slightly proud or uneven boxes — the practical choice for older walls.
  • Metal plates need earthing at both the box and the plate terminal.

Checking an existing box

  1. Isolate the circuit and prove dead.
  2. Remove the faceplate and measure internal depth from wall face to box base.
  3. Count the cables — two or more cables plus USB internals usually means 35mm.
  4. Check the fixing lugs: stripped 3.5mm threads can be rescued with repair lugs rather than replacing the box.

FAQs

What depth back box do I need for a USB socket?

25mm minimum, 35mm recommended — the charging electronics take the space of an extra cable.

Can I use a metal back box in plasterboard?

You can where the box can be screwed to a stud or noggin, but a dry-lining box is quicker and designed for board fixing.

Do screwless switches need deeper boxes?

Not always deeper, but they need the box flush and level. Where the existing box is shallow (16mm) many screwless and USB plates won't fit — check the product's stated minimum.