Pipes through walls, in floors and in voids

Hidden pipework is where the most serious mistakes happen. A leak inside a wall cavity, floor void or duct can collect where nobody can smell it, and a nail or screw through a concealed pipe may not be found until gas escapes. The rules in this lesson aim to stop gas collecting, protect the pipe from damage and movement, and keep the building's fire protection intact.

Through walls and floors: sleeves

Any pipe passing through a wall or floor, cavity or not, must go through a sleeve, and take the shortest practicable route (GSIUR reg. 19(2)).

• The sleeve must resist corrosion, be impermeable to gas (for example copper, steel, PE or PVC), and protect the pipe if the building moves. It must have no joints along its length.
• It goes through the full width of the wall or the full depth of the floor, and its outside is sealed to the building at each end, for example with mortar or flexible sealant.
• The gap between pipe and sleeve is sealed at one end with a flexible fire-resistant compound. Through an outside wall, that seal is on the inside.
• There must be no screwed, mechanical or press-fit joints inside a sleeve.
• A factory coating on the pipe, including the cover on CSST, is not a sleeve.

Cavity walls

Never run pipework within a wall cavity. It may only cross the cavity from one side to the other, by the shortest route, sleeved (GSIUR reg. 19(3)). The one exception in law is the short, sleeved supply to a living flame effect gas fire with a fanned flue, set in the inner leaf of a cavity wall (reg. 19(4)).

Joisted floors and roof spaces

• Run pipework along or at 90° to the joists, never diagonally, and support it at the Table 4 intervals.
• Solid timber joists: use purpose-made notches or holes within the limits in BS 6891 Figure 13. For example, keep at least 100 mm between a notch and a hole, and keep holes at least three diameters apart. Don't notch joists less than 100 mm or more than 250 mm deep without a structural engineer's agreement.
• Engineered joists: never notch or drill the flanges. Go through the web as the joist manufacturer says, and keep pipe off metal webs.
• Roof timbers (rafters, purlins, trusses, bracing) may only be notched, drilled or cut with a structural engineer's approval. Don't lay pipe beneath or inside loft insulation.
• Roof spaces with vapour-permeable underlay and no specific ventilation count as unventilated. Pipework there needs at least two ventilators, each at least 50 cm², at high level for natural gas.

Intermediate floors (between rooms in the same home): research for the gas industry showed that natural gas leaks in these floors disperse through natural gaps, so no extra ventilation is needed. That finding doesn't apply to compartment floors between separate dwellings, where pipework is only allowed in the arrangements shown in BS 6891 Figures 14 and 15, such as within vented ducting. Elsewhere, a void under a floor must be ventilated (unless it is already well ventilated naturally) or the pipe run through a vented duct.

Solid floors

• Run parallel or at 90° to the walls, never in the structural slab or a power-floated structural floor, and never rigid stainless steel in screed.
• Protect against corrosion and against movement (for example in a duct, or with a soft covering at least 5 mm thick).
• Keep joints to a minimum, with no mechanical or press-fit joints buried in the floor.
• Give at least 25 mm of screed cover, and sleeve pipes passing vertically through the floor.
• Inspect the pipe for damage just before it's covered.

Walls

• Solid walls: run vertically where possible, in ducts or chases, by the shortest route, never diagonally. No mechanical or press-fit joints in walls. Keep chases within the depth limits in BS 6891 Figure 17, which allows a vertical chase to be deeper than a horizontal one.
• Dry lining: encase the pipe in building material so gas can't track behind the plasterboard. Thin-wall metal pipe (copper or CSST) within 50 mm of the plasterboard face needs protecting, for example with a 1 mm steel plate.
• Timber and steel frame: run in purpose-made channels or ducts, more than 50 mm from the plasterboard face or protected by a steel plate. Never notch studs, rails or noggings; drill holes as small as possible, near the centre line and within the limits in BS 6891 Figure 19. Remove no more than a third of a bottom or sole plate's width, and treat cut timber with preservative. Allow for the timber frame shrinking or moving. On a party wall, the pipe rises in front of the plasterboard.

Ducts, flats and protected areas

• Ducts containing pipework must be ventilated so minor leaks don't build up, to a safe place, preferably outside, and sealed from any cavity or void they pass through. GSIUR forbids pipework in any unventilated shaft, duct or void unless the pipe is in its own ventilated duct (reg. 19(6)).
• Flats: pipework passing through another dwelling must be in a purpose-made duct. In buildings with flats or maisonettes, pipework must be fire-stopped as it passes from floor to floor, unless it runs in its own protected shaft ventilated top and bottom to outside air. Where it comes out of such a shaft into a flat, it is fire-stopped at that point.
• Protected areas (escape stairs, protected corridors and lobbies, service shafts) may only contain steel with screwed or welded joints, a continuous length of copper, or continuous CSST that has passed the required fire test. For CSST made to BS EN 15266, check with the manufacturer.

Sources: GSIUR 1998 reg. 19enclosed pipes BS 6891:2015 8.7sleeves BS 6891:2015 8.9–8.10, Annex Cfloors, roof spaces BS 6891:2015 8.11, 8.20walls BS 6891:2015 8.16–8.19ducts, flats, protected areas Gas Safe Register TB 120 (2021)CSST CCN1 Issue 16 §5assessed

▸Tutor notes

• Show learners a section through a cavity wall and ask them to describe the sleeve and seal positions before revealing the artefact.
• Ask why the intermediate-floor research doesn't apply to the floor between two flats.
• CCN1 §5 items 13, 17–20 and 22 (ducts, fire stopping in flats, protected areas, multi-occupancy, under walls) are all covered here.