Pattress, Drylining & Metal Back Boxes

A pattress box, also known as a surface-mounted back box, is a plastic or metal enclosure fixed directly onto the face of a wall rather than being recessed into it. It is the right choice when chasing into the wall is not practical or possible, such as on solid stone walls, tiled surfaces, or where access to the wall cavity is restricted. Pattress boxes are a common choice in garages, utility rooms, and older properties where surface wiring run in conduit or trunking is the preferred installation method. They are also used to add sockets or switches to an existing installation without significant making good or replastering. Surface-mounted pattress boxes are available in single-gang and two-gang configurations and in varying depths to suit the wiring requirements of the faceplate being fitted. They pair directly with standard switches and sockets, so no special faceplate is required.

The minimum recommended depth for a standard single or double socket back box is 25mm, though 35mm is more commonly specified to give adequate room for wiring and terminations. Shallower 16mm boxes are available but are generally only suitable for switches controlling lighting circuits, where the cable volume is lower. For sockets with USB modules, RCD protection, or dual voltage shaving sockets, a deeper 47mm box is often required to accommodate the additional electronics behind the faceplate. Always check the faceplate manufacturer's minimum back box depth before ordering, as fitting a faceplate into a box that is too shallow can damage the wiring and prevent the cover plate from sitting flush. On dry-lining walls, the box depth also needs to account for the thickness of the plasterboard plus any skim, so a 35mm or 47mm box is typically specified. Choosing the correct depth at first fix avoids the need to swap boxes once second fix wiring begins.

A standard back box is designed for installation in solid masonry or timber stud walls where it is fixed into the wall with screws and held rigidly in place by the surrounding structure. A drylining back box, also called a plasterboard back box, is specifically engineered for walls built from timber or metal studs with plasterboard sheeting, where there is a hollow void behind the board. Drylining boxes use lugs, wings, or friction clips that expand behind the plasterboard once inserted through a cut-out, gripping the rear face of the board to hold the box securely without needing a solid fixing point. This means they can be installed at any point on a plasterboard wall, even where there is no stud directly behind the box position. Drylining boxes are the correct and compliant choice for partition walls and are faster to install than solid-wall boxes as no drilling or plugging is required. Using a standard masonry-type box in a plasterboard wall is a common first-fix error, as it will not hold the box securely under repeated use of the socket or switch.

Metal back boxes are made from galvanised or zinc-coated steel and are required whenever steel conduit is used to carry cables to the outlet, as earth continuity must be maintained throughout a metallic wiring system. They are also the standard choice on commercial, industrial, and high-specification projects where mechanical robustness, fire resistance, or electromagnetic interference shielding is a priority. Screwless flat plate sockets and switches with a brushed steel or chrome finish are often designed to pair specifically with metal back boxes, as the box provides a more rigid and stable mounting than plastic. In some local authority and social housing specifications, metal back boxes are mandated in all installations regardless of conduit type. Metal boxes are also preferred in areas subject to impact or accidental damage, such as communal corridors, plant rooms, and commercial kitchens, where a plastic box may crack or deform over time. Where steel conduit is used, the metal back box must be bonded to the conduit system to ensure a continuous earth path in compliance with BS 7671.

Drylining back boxes are secured to plasterboard using one of three main fixing mechanisms: expanding lugs, friction wings, or toggle clips. The box is inserted through a rectangular cut-out in the plasterboard, then the fixing mechanism is tightened from the front using a screwdriver, causing the lugs or wings to open up and press against the rear face of the board. This clamping action locks the box firmly in position without requiring access to the back of the wall. Some designs use a patented grip system that bites into the plasterboard for additional holding strength, which is useful in higher-load applications or on thicker boards. The box remains flush with the front face of the plasterboard, providing a stable mounting surface for the faceplate. Always ensure the plasterboard is at least 12.5mm thick before fitting a drylining back box, as thinner boards may not provide sufficient grip for the fixing mechanism.

An architrave back box is a narrow, slim-profile single-gang box designed for installation in locations where a standard-width box will not fit. They are most commonly used on or beside door frames, in narrow wall sections, alongside window reveals, and in corridor walls where the available wall width is too limited for a conventional 86mm-wide socket or switch. The reduced width of an architrave box allows a switch or socket to be positioned in these tight spaces without cutting into the door frame or adjacent structure. Architrave boxes are available in both surface-mounted and flush-fitting versions, and they accept standard architrave-profile faceplates rather than the square-format faceplates used with standard boxes. They are a practical solution for adding a local lighting switch beside a door without rerouting cables to a more accessible wall position. Because the gang size is single only, they are typically used for switches rather than sockets where a double outlet would usually be preferred.

Yes, a drylining back box is the correct product for any plasterboard or stud partition wall, and using a standard masonry back box in these walls is generally not appropriate. Standard boxes rely on fixings into solid material to stay in place, and plasterboard alone does not provide sufficient resistance to the repeated pulling forces applied when plugging and unplugging appliances. A drylining box's expanding clip or lug system is engineered to grip the rear face of the plasterboard, distributing the load across a wider area and keeping the box secure under everyday use. Some installers attempt to fix a standard box to the timber stud behind the plasterboard where one is present, but this limits placement options and is impractical across most of the wall area. Using the correct drylining box from the outset ensures a compliant, stable installation that will not loosen or pull away from the wall over time. The full range at MP Moran includes options for all plasterboard thicknesses and gang sizes, and sits within our wider switches, sockets and boxes category.

The standard cut-out size for a single-gang flush back box in a plasterboard or masonry wall is approximately 60mm x 60mm, though this can vary slightly between manufacturers so always check the product datasheet before cutting. A two-gang flush back box typically requires a cut-out of around 60mm x 123mm to 125mm. For surface-mounted pattress boxes, no wall cut-out is required as the box is fixed directly onto the wall face, with only a cable entry hole drilled through the wall. When cutting into plasterboard, use a pad saw or oscillating multi-tool rather than a drill, as this gives a cleaner edge and reduces the risk of cracking the board. Always check for cables and pipe runs behind the wall using a detector before cutting. Measuring and marking the cut-out carefully before you start will ensure the box sits flush and square, giving a professional finish once the faceplate is fitted.

Standard pattress and drylining back boxes are not designed for outdoor or exposed locations and should not be used externally. For outdoor sockets, switches, and electrical outlets, you need weatherproof back boxes and faceplates rated to at least IP44, or IP66 where high levels of water ingress protection are required. Weatherproof back boxes are typically made from UV-stabilised plastic or galvanised steel and are sealed at cable entry points to prevent moisture penetration. Using an indoor back box outdoors will allow water ingress, which creates a risk of electric shock and is non-compliant with BS 7671 wiring regulations. Any outdoor electrical installation should also be connected via an RCD-protected circuit. For external electrical accessories, the weatherproof switches and sockets range at MP Moran includes fused connection units, socket outlets, and isolator switches designed specifically for outdoor and damp location use.

MP Moran stocks back boxes from leading UK electrical brands including MK Electric, Knightsbridge, Lap, and Norslo, covering the full range of pattress, drylining, and metal back box types. MK Logic Plus is a consistently popular choice among electricians for its reliability, well-engineered fixing mechanisms, and compatibility with a wide range of faceplates. Knightsbridge back boxes are known for their build quality and value across both surface-mounted and flush-fitting variants. The range covers single and double gang sizes in standard and deep configurations, with options for both plasterboard and masonry walls. Stock is available for same-day collection from our six branches across London and Hertfordshire, making it straightforward to pick up back boxes alongside your switches and sockets in a single visit. If you need advice on the right back box for a specific wall type or installation depth, our branch teams are experienced in electrical first fix and can help you select the correct product.