Dry Lining Around Services: Boxing-In Pipes, Pattress Plates, Access Panels and Fire Stopping at Service Penetrations

Summary

Services — plumbing, electrical, and ventilation — must be coordinated with drylining on every project. Poor coordination results in back boxes without backing, isolation valves buried behind board, pipes that creak and cannot be repaired, and soil stacks that create fire risks inside boxings. Getting this right requires the drywall installer to communicate with the plumber and electrician before boarding starts.

The general rule is: all services should be run, clipped, and signed off before the first board goes up. Mechanical connections that will need servicing (isolation valves, gas cocks, drain points, boiler connections) must be identified and access panels positioned over them. Electrical tails and conduits should be positioned so back boxes will fall on a stud, or so a pattress plate can be fitted before boarding.

On fire-rated partitions, the additional requirement is that every penetration is fire-stopped to maintain the partition's fire rating. The dryliner is responsible for the boarding — but typically not for the services themselves. Clear sub-contract responsibility is essential, and checking for fire-stopping completion before signing off the work is the dryliner's professional obligation.

Key Facts

• Soil pipe boxing — 110mm soil pipe in a boxing creates a flanking path for both noise and fire; acoustic pipe wrap (Rockwool Rainscreen Duct Slab or equivalent) around the pipe inside the boxing reduces noise; intumescent collar required through any fire-rated element
• Copper and steel pipes — do not require intumescent collar; metal maintains integrity in fire; annular gap around pipe at fire-rated element must be filled with mineral wool or fire mastic
• CPVC, PB, PEX, polyethylene — all plastic; require intumescent collar at any fire-rated penetration
• Intumescent collar — fits around plastic pipe at the wall face; expands in fire to crush the pipe and seal the opening; must match pipe OD; install per manufacturer instructions
• Pattress plate — timber or proprietary metal plate fixed to stud to provide backing for switch/socket boxes where the box does not fall on a stud; plywood (12mm minimum) or proprietary metal pattress
• Back box depth — standard UK back box: 25mm (shallow), 35mm (standard), 47mm (deep); deep boxes required where conduit bends or multiple cables enter
• Service void partitions — a 25mm service void created by a secondary layer of 25×25mm battens behind the board provides space for first-fix cables without chasing; standard in new-build wiring
• Chase in insulation — on insulated wall linings (dot-and-dab systems), cables can be chased into the insulation up to 50% of the insulation depth; deeper chasing risks breaking the thermal continuity
• Access panel sizes — standard 150×150mm, 200×200mm, 300×300mm, 300×450mm; choose to allow full access to valve handle plus tool access; always larger than the valve itself
• Access panel fire rating — must match the partition or ceiling fire rating; many off-the-shelf panels are not fire-rated; specify EI 30 or EI 60 panels on fire-rated elements
• Boxing deflection — boxing around hot water pipes expands and contracts; leave 3mm minimum gap at boxing corners; use compressible backing before sealing with flexible mastic
• Gas pipes — must be accessible; cannot be permanently concealed; proprietary ventilated boxing or surface-mounted plastic trunking; gas supply must be identified and not buried

Quick Reference Table

Detailed Guidance

Soil Pipe Boxings

Soil pipe (110mm) is the most challenging service to box in. Issues:

1. Noise: water flow in a soil stack creates significant noise. Boxing in the stack without acoustic treatment results in audible flushing throughout the building.
2. Fire: plastic soil pipe through a fire-rated element is a fire path. The pipe will burn through and the opening will allow fire to pass.
3. Access: rodding eyes, inspection points, and branch junctions may need to be accessed for maintenance.

Acoustic treatment of soil pipe boxing:

• Wrap the 110mm pipe with minimum 30mm of mineral wool acoustic pipe wrap (Rockwool Rainscreen Duct Slab or similar) before boarding
• The wrap reduces noise transmitted from pipe vibration through the boxing structure
• Inside the boxing void, fill the remaining space with loosely packed mineral wool
• Use resilient mounting for any pipe clips that touch the boxing structure

Fire stopping at soil pipe boxing:

• If the boxing passes through a fire-rated floor or ceiling: fit an intumescent collar (Hilti CP606, Rockwool Firepro or similar) around the pipe at the structural element — not at the boxing face
• If the boxing is itself fire-rated (e.g. forming part of a separating wall): fit intumescent collar at both faces of the partition

Access to soil stack:

• Rodding eyes must remain accessible — fit an access panel over any branch junction or rodding point
• Cleanout plugs at the base of the stack need tool access — position the boxing with sufficient clearance

Pattress Plates and Back Box Backing

Electrical back boxes in drylining partitions or linings require backing material — either a stud behind the board, or a proprietary pattress.

Back boxes on a stud:

• Ideal solution: position back boxes so the box centred on a stud
• Mark stud positions on the floor or ceiling before boarding so the electrician can position conduit tails correctly
• Box is fixed through the board into the stud using appropriate screws

Back boxes between studs:

• Where the box falls between studs, a pattress plate (also called a nogging pad or back-pad) provides the fixing
• Timber option: 12mm plywood cut to the stud bay width and fixed to both studs at the correct height; box then fixed through board into plywood
• Proprietary metal pattress (e.g. Drylining Pattress Plate): a two-part plate that grips the back of the board; rated for lighter loads than timber
• Fire-rated elements: timber pattress is preferred; specify 12mm fire-retardant plywood if in a fire-rated partition

Back box depth on insulated linings:

• On dot-and-dab insulated board (e.g. 50mm PIR + 12.5mm board = 63mm total), a standard 35mm back box protrudes entirely through the insulation into the void behind
• The box must be fixed to the structural wall via a long extension screw or proprietary accessory — back boxes cannot be fixed only to the insulation
• Deeper boxes (47mm) are required where multiple cables enter, or where the conduit is rigid

Back boxes in acoustic partitions:

• Boxes on opposite sides of an acoustic partition must be offset horizontally by minimum 150mm
• Seal around the back box with acoustic putty pads before boarding
• Proprietary acoustic back box enclosures are the most effective solution

Back boxes in fire-rated partitions:

• Standard back boxes create an opening in the fire partition
• Intumescent pads inside the back box allow the box to remain, provided the pad is in contact with the board and swells to seal the box on fire exposure
• Box enclosures (proprietary fire-rated enclosures) provide full protection for up to 60-minute fire rating

Access Panels

Every isolation valve, service connection, or inspection point concealed behind drylining must have a correctly sized and rated access panel.

Sizing:

• The panel must be large enough to operate the valve handle with a tool in hand — allow minimum 150mm clearance around the valve in each direction
• For a standard 22mm compression isolation valve: 200×200mm panel minimum
• For a branch manifold or multiple valves: 300×450mm or 400×400mm
• For an electric motor or actuator: manufacturer's recommended service access clearance

Panel installation:

• Proprietary access panel frames fix to the stud framing around the opening
• Frame must be plumb and square; opening cut in board with jab saw after boarding
• Non-fire-rated panels: aluminium or uPVC frame with screwed or magnetic catch
• Fire-rated panels (EI 30 or EI 60): proprietary rated panels (e.g. Acudor, Karp, Nystrom) with intumescent sealing; these are significantly more expensive than standard panels
• Do not use a non-rated panel in a fire-rated partition — it destroys the fire rating at that point

Hidden services:

• Gas: cannot be permanently concealed. Ventilated boxing (open to room air at top and bottom) is acceptable; sealed boxing is not
• Water supply pipes (not isolatable): generally can be concealed, but any compression or pushfit joint in the concealed run should be avoided — capillary soldered joints or press fittings only
• Hot water: expansion must be accommodated; use flexible sealant at boxing joints, not rigid filler

Service Coordination Checklist

Use this before boarding starts on any project:

• All first-fix plumbing clipped, tested with water, and signed off
• All first-fix electrical conduit and tails in position, tested with continuity check
• All isolation valves, drain cocks, and service points identified and access panel positions marked
• Any fire-rated penetrations identified and intumescent products ordered or on site
• Gas pipes identified — surface-mounted trunking or ventilated boxing arranged
• Back box positions coordinated with stud layout — pattress plates fitted where needed
• Acoustic partition back box offsets checked against layout plan

Frequently Asked Questions

Do I need building control approval to box in a soil stack?

Not usually, unless the boxing forms part of a fire-rated element (e.g. a protected stairway enclosure, a compartment wall, or a ceiling forming a separating floor). If the boxing is simply concealing the stack inside a single dwelling, no formal approval is required. If the soil stack crosses a fire compartment boundary (e.g. from one flat to another), the penetration must be fire-stopped and this may require building control sign-off as part of the conversion works.

What happens to the fire rating if an electrician adds a socket to a fire-rated partition after I've finished?

The fire rating is compromised unless the new penetration is correctly fire-stopped. This is a common problem on building sites. As the party responsible for the fire-rated partition, you should ensure the main contractor or client understands that penetrations by follow-on trades must be fire-stopped. In practice, once the partition is handed over, you are not liable for subsequent trade damage — but good practice is to document the partition's specification and highlight the fire-stopping requirement at handover.

Can I use plasterboard filler to seal around a pipe at a fire-rated partition?

No. Standard plasterboard jointing compound and fillers are not fire-rated. Use only tested fire-stopping products — intumescent collars, intumescent putty (e.g. Hilti CP 620, Rockwool Firepro Putty), or mineral wool + intumescent sealant as appropriate to the pipe material and partition rating. Check the product's ETA (European Technical Assessment) for the specific rating.

Regulations & Standards

• Building Regulations Approved Document B — fire stopping at service penetrations through fire-rated elements

• BS EN 1366-3:2021 — fire resistance testing of penetration seals

• BS 7671:2018+A2:2022 (IET Wiring Regulations) — requirements for electrical back boxes, conduit, and earthing in drylining

• The Gas Safety (Installation and Use) Regulations 1998 — requirements for accessible gas connections

• CIBSE Guide Y — multi-service coordination and building services interface with structure

• Hilti Firestop Solutions — intumescent collar and cable transit product data

• Rockwool Fire Stopping — mineral wool fire stopping solutions and ETA data

• British Gypsum White Book — back box treatment in fire-rated partitions

• building regs part b fire lining — fire-stopping requirements for drylining systems

• drylining in wet areas — drylining adjacent to pipework in wet room conditions

• metal stud partition installation — stud layout for coordinating service penetrations

• fire stopping — comprehensive fire-stopping guide