Guide to Back Boxes

Back boxes are the workhorses of the electrified building. They offer safety, versatility and compatibility for a whole range of switches, sockets and fused spurs. In this article, we’ll be covering the back box basics – what they are, why we have them, and the various types available.

What is an electrical back box?

A back box is an enclosure for holding the ends of electrical wires for sockets and switches, and is also a means for attaching the visible part (the switch or plug faceplate) to the wall. They provide a safe, controlled place for wiring to emerge from behind or within a wall, ready to be connected or blanked off.

What do we need back boxes?

If you consider the alternative – a rough, dusty hole chiselled into a masonry, stone or concrete wall, with wiring emerging from the back or side – their purpose becomes obvious. The box can be inserted into the rough hollowed-out space and the gaps around it filled with mortar or filler. That way, there’s a solid, neat finish ready for painting or wallpapering. 

A strong metal box can also provide some structural support too, although the amount of load-bearing capacity should not be overestimated. As a means of stopping loose masonry or mortar from filling the hole, they’re certainly useful.

Installing a back box

To install a back box in a mortar, concrete or stone wall, the space needs to be physically cleared, using a chisel, bolster, drill or other power tools, depending on the circumstances. Similarly, you’ll need to chase out a channel in the plaster, and perhaps into the masonry beneath, for the wiring to enter the box through one of the holes on the top, bottom, sides or back. 

Everyone has their preferred method of chasing the space for the box, but a good way is to drill the four corners to the depth of the box, then fill the rectangle and edge lines with more drill holes. This makes it easier to remove the masonry with a bolster or multi tool.

The box can be stuck to the wall with specialist adhesive and/or screws, then the edges can be cleaned up and filled to produce a smooth surface.

Types of back box

There are two types of back box – ones that sit inside the wall, as described above, and surface-mounted ones, often called pattress boxes. Traditional back boxes are much neater as only the faceplate will be visible. Also, as they don’t protrude, furniture and doors can be placed near to them more easily, and they are harder to damage.

Surface-mounted pattress boxes

Pattress boxes are, however, easier to install, as they simply sit on the wall, so there’s no hacking away at masonry. You might not even have to chase a channel for wiring, as that too can be surface-mounted inside conduit.

They’re useful where aesthetics aren’t so important, such as in a garage or workshop, or if they’re going to be hidden away, such as behind a TV cabinet. Sometimes, a surface-mounted back box might be the only option, too. An example would be if the wall is too thin to sink a standard back box into, or if its material is too hard to work. A pattress might be required where you don’t want to excessively damage the fabric of the building, such as if it is a listed building.

Plastic or metal?

Metal (usually galvanised steel) is a much stronger material than plastic, and comes with the benefit of being fireproof, which is an important consideration for electrical applications. It should also be used when you have metal-sheathed wiring, as the metal body is important for grounding the current. 

Plastic boxes are cheaper and are electrical insulators, so can be safer to work with and use. However, they should only be used with non-metallic cables. Dry lining back boxes (for plasterboard) are usually made of plastic.

Weatherproof back boxes

If the switch or socket is going outside, it will need to be a weatherproof model, as will all the components connected to it, such as the faceplate and the wiring. If it’s a socket, the whole unit needs to be encased in a weatherproof lid that lifts to access the sockets. For switches, a special outdoor sealed switch should be fine.

Depth of back box

The depth of the box is important, as some switches and sockets are deeper than others. For example dimmer switches and USB-enabled sockets can sometimes be deeper than their regular equivalents. This is an important consideration if you’re doing a like-for-like replacement, but also if you’re starting afresh.

Width and height

Just as important (but hopefully more obvious!) is the size of the box. A double socket will require a two-gang back box or pattress box. Just make sure you pick the right size when ordering back boxes.

Our range of back boxes

Now you know the basics, you can shop with confidence for your back boxes and pattress boxes. If you need any help with finding the right type, please get in touch so we can guide you in the right direction.