First Fix Plumbing: Pipe Runs, Notching Regulations, and New Build Inspection Requirements

First fix plumbing refers to all the pipe runs, drain connections, and rough-in work that must be completed before plastering, boarding, or finishing begins on a new build or major renovation. Getting first fix right is critical — mistakes are expensive to correct once walls are closed in, and failures can go undetected until they cause significant water damage. This guide covers water supply, drainage, heating, and gas first fix for UK plumbers working under Building Regulations.

What Is First Fix Plumbing?

In UK construction, trades divide their work into two phases:

• First fix: All work that is concealed in the structure — pipe runs through walls, floors, and ceilings; drainage stacks and branch connections; heating flow and return pipework; gas supply pipes; manifold positions for underfloor heating. First fix happens before plastering, floor screeding, or floor finishing.
• Second fix: All the visible, finished elements — tap fittings, radiators, boiler connections, sanitary ware, shower valves. Second fix happens after the structure is plastered and painted.

First fix plumbing must comply with Building Regulations Part G (sanitation, hot water, and water efficiency), Part H (drainage), and — for gas — Gas Safe Register requirements under the Gas Safety (Installation and Use) Regulations 1998. Heating pipework within a property is not typically notifiable under Part P (which covers electrical work), but may require commissioning certificates.

Water Supply Pipe First Fix

Pipework Materials for First Fix

The choice of material for concealed water supply pipe depends on location and local authority requirements:

• Copper (half-hard or soft): The traditional choice for in-wall and underfloor pipe runs. Durable, well-understood by all tradespeople, and resistant to UV and vermin. End-feed and solder ring capillary fittings are most common for concealed work — push-fit fittings are generally not recommended behind walls as O-ring joints can be disturbed during boarding/plastering and are not easily accessible for remediation. Compression joints behind walls are permitted but must be accessible via a suitable access panel.
• Plastic push-fit (Hep2O, JG Speedfit): Widely used in new builds where pipe runs are within floor screeds or under suspended floors with access. The Hep2O pipe is UV-resistant and approved for concealment in screed. Do not use standard flexible push-fit pipe within walls without metal liner protection against compression or drilling damage. Check manufacturer guidance — most push-fit systems specify that joints within screeds or walls must use the barrier pipe with clip inserts.
• MDPE (blue polyethylene): Used for underground supply between the meter or stopcock and the property, and for sub-slab cold water supply runs under concrete ground floors. Not used for hot water. MDPE requires electrofusion or compression-style fittings (Philmac UTC, JG Speedfit MDPE). See also: MDPE Blue Water Pipe — Article #107.

Cold Water Distribution

In a new build, the cold water distribution scheme depends on whether the property is direct-fed (cold water taken direct from mains to all outlets) or uses a cold water storage tank (CWST):

• Direct cold water (most common in modern domestic builds): A single rising main from the incoming stopcock feeds a distribution manifold or branch pipework to all cold outlets. Every cold outlet — including the WC cistern, kitchen sink, bath, basin, and shower — is fed at mains pressure direct. This eliminates storage tanks and simplifies first fix significantly.
• Indirect system (older properties, social housing, some commercial): The rising main feeds only the kitchen sink direct (for drinking water) and a CWST in the loft. All other cold outlets are fed by gravity from the tank. Pressure is low and depends on the head of water above each outlet. Not permitted in new dwellings under Part G (G1 — water supply) unless there is an engineering reason.

Cold Water Pipe Sizing

For domestic new builds under direct supply, 22mm copper or 22mm barrier pipe is standard for the main distribution run, with 15mm branches to individual outlets. The incoming main is typically 25mm MDPE from the meter.

Where simultaneous draw-off demand is high (large houses, multiple en-suites), the distribution pipe may need upsizing to 28mm. Consult BS EN 806-3 for demand units and pipe sizing calculations.

Water Regulations Compliance (WRAS)

All pipework and fittings used in the UK water supply system must comply with the Water Supply (Water Fittings) Regulations 1999. Key requirements:

• All fittings must be WRAS-approved or meet the standard
• No cross-connections between wholesome water supply and any non-wholesome source (rainwater, greywater, non-potable water)
• Backflow prevention required at every point of use — check valves, RPZ valves, and air gaps depending on the fluid category risk
• Notify the local water authority before making a connection to the supply (new build, extensions with additional connections, or material change)

Hot Water Pipe First Fix

Hot Water Distribution Options

First fix hot water distribution depends on the system type:

• Combination boiler (no cylinder): Hot water is generated on demand. A 22mm flow and return runs from the boiler position to each hot water outlet, with 15mm branches. Pipe runs should be kept as short as possible to minimise wait time — long runs from a combi mean customers wait for hot water while the cold water in the pipe clears.
• Vented cylinder: 28mm primary flow and return between boiler and cylinder. 22mm secondary distribution from cylinder top to all outlets. 15mm branches. A secondary circulation loop (22mm return from the far end of the hot distribution back to the cylinder) may be installed on large properties to provide hot water quickly at all points.
• Unvented cylinder (G3): Cold mains supply to the cylinder inlet (typically 22mm). Hot distribution similar to vented. The installation of an unvented system requires a competent person qualification (G3 Unvented) and must be notified to building control. All safety devices (pressure relief valve, temperature relief valve, expansion vessel, tundish, discharge pipe) must be installed correctly and discharged safely to a visible location. See: Unvented Hot Water Cylinders: G3 Qualification and Installation Guide (#87).

Pipe Insulation in First Fix

Building Regulations Part L (conservation of fuel and power) requires all hot water pipes in unconditioned spaces (loft, garage, basement) to be insulated. Best practice is to insulate all hot water distribution pipework. Use 19mm or 25mm foam tube (to BS EN ISO 10211 or equivalent). Pipe clips must be fitted outside the insulation — do not crush or compress the foam with clips applied over the top.

Cold water pipes in unheated spaces (loft, roof voids, external walls) must also be insulated against frost. Use closed-cell foam insulation — at least 25mm wall thickness in exposed loft areas.

Drainage First Fix

Above-Ground Drainage (Soil and Waste)

First fix drainage includes the soil and vent pipe (SVP) and all branch connections to WCs, basins, baths, and showers:

• SVP (soil vent pipe): 110mm diameter, runs vertically through the building and terminates above eaves level with a vent cowl or air admittance valve (AAV). The SVP must extend at least 900mm above any opening window within 3m, or 200mm above a non-opening window within 3m, to prevent sewer gases entering the building. In multi-storey properties, the SVP may run internally and exit through the roof.
• WC connections: WC pans connect via a 110mm pan connector to the SVP. Branch connections to the SVP use 110mm swept tee fittings. WC branch lengths should not exceed 6m without additional ventilation, or siphonage of trap seals in other connected appliances can occur.
• Basin, bath, and shower: Branch into the SVP via 40mm or 50mm waste pipes. Use air admittance valves at the appliance trap where direct ventilation to outside is not possible. Branch lengths: basin 32mm max 1.7m, 40mm max 3m; bath 40mm max 3m; shower 40mm max 3m. See: 50mm Waste Pipe (#21) and 32mm and 40mm Waste Pipe (#108).
• Gradient: All waste pipes must fall at the correct gradient — 18mm per metre (1:56) for 32mm, 44mm per metre (1:22) for 50mm. Too steep and water outruns solids; too shallow and solids deposit.

Underground Drainage Connection

The underground drain connects the SVP to the sewer (public or private). First fix at this stage includes:

• Installing the drain at the correct gradient (1:40 minimum for 100mm pipe under BS EN 12056-2)
• Rodding eyes and inspection chambers at changes of direction and at 22m maximum intervals
• Connection to the public sewer via a new lateral — requires approval from the water company and may require a Section 50 licence from the highway authority if crossing a public footway

Notching and Drilling Regulations

Concealing pipes in floor joists and stud walls requires notching or drilling through structural timber. Building Regulations Part A (structural) sets out zones where notching and drilling are permitted:

Floor Joists (Solid Timber)

For solid timber floor joists of span L:

• Notches: Permitted in the top of the joist only (never the bottom — this is the tension zone). Notch depth must not exceed 0.125 × joist depth. Notches must be within 0.07L to 0.25L from the joist bearing (i.e. near the ends, not mid-span).
• Holes: Drilled holes must be within 0.25L to 0.4L from the joist bearing. Hole diameter must not exceed 0.25 × joist depth. Holes must be in the neutral zone (between 0.25 and 0.4 of the depth from the top). Minimum spacing between holes is 3 × the hole diameter.
• I-joists (engineered joists): Always follow the manufacturer's specific notching and drilling guidance. These are not interchangeable with solid timber rules. Incorrect drilling of I-joist webs can cause catastrophic failure.

Stud Walls

Drilling through stud walls for pipe runs is generally straightforward — holes up to 25mm diameter through the centre of 75mm or 100mm studs are acceptable for domestic plumbing pipework. Keep holes away from noggings and structural studs (load-bearing walls). Install pipe clips or grommets to prevent the pipe abrading against the hole edge.

Sleeves and Fire Stopping

Pipes passing through fire-rated walls and floors must be fire-stopped. For pipes:

• Use intumescent pipe collars where plastic pipes pass through fire compartment walls or floors
• For copper pipes, close-fitting collars with fire-rated sealant are acceptable
• Do not leave gaps around pipes at structural fire stops — even a 5mm gap can compromise a 60-minute fire rating

See: Passive Fire Protection (#113).

Heating Pipework First Fix

Central Heating Pipe Runs

For first fix on a new build with gas boiler central heating:

• Boiler position: Usually fixed in first fix — decide if it is wall-mounted (ideal) or floor-standing. Mark out for boiler bracket, flue route, and gas connection position. The flue aperture through the wall must be formed during first fix.
• Flow and return routes: 22mm (up to 12kW single circuit) or 28mm (larger properties) flow and return from boiler position to each radiator position. Normally run through floor ducts, under floors, or along ceiling voids rather than through walls where possible — easier to access and less damage risk.
• Radiator positions: Mark out and rough-in 15mm flow and return drops to each radiator position. Leave capped stubs projecting from the wall at the correct height for the specified radiator.
• Underfloor heating: If wet UFH is specified in ground floor, install the screed-embedded pipework in first fix. The UFH manifold position must be determined before screeding. MDPE or PEX barrier pipe (typically 16mm) is laid in pattern across the floor at calculated spacings and connected at the manifold. After screed is poured and cured, the manifold caps off for second fix connection. See: Underfloor Heating: Wet UFH, Pipe Layout, and Manifolds (#25).

Cylinder and Indirect Pipework

Where a hot water cylinder is specified (indirect system, ASHP, solar thermal), the primary circuit (flow and return between boiler/heat source and cylinder) must be roughed in during first fix. Standard is 22mm copper for domestic cylinders up to 200 litres. Allow adequate height above the cylinder for the temperature relief valve and tundish discharge arrangement.

Gas Pipe First Fix

Gas installation work in the UK must be carried out by a Gas Safe-registered engineer. First fix gas work includes:

• Meter to boiler: The supply pipe from the meter position to the boiler route. Typically 22mm copper (up to 60 kW input) or 28mm for larger properties. Run in protected zones — do not run gas pipe through bedroom floors unless enclosed in a gas-tight duct to outside.
• Concealed gas pipes: Gas pipes within walls must be in a continuous sleeve or conduit without joints, and the sleeve must vent to outside or to a ventilated space. Alternatively, install only in accessible ducts. This is a key requirement under IGEM/UP/2 — no gas pipe joints should be concealed unless specifically permitted by design.
• Test before closing: All gas pipework must be pressure-tested before it is concealed. Test to 1.5× working pressure using nitrogen or air (not gas). Document the test — gas tightness test certificate required.
• Purge and retest: At second fix, after boiler connection, the gas supply is purged and a further let-by and tightness test performed before commissioning.

Pressure Testing and Pre-Plastering Inspection

Before plastering or boarding, all first fix pipework should be tested and signed off:

Water Pressure Test

Test all cold and hot water distribution pipework to 1.5× maximum working pressure (typically 1.5 × 10 bar = 15 bar) for one hour. Acceptable fall in pressure is less than 0.6 bar in the first 30 minutes. Document the test. See: Pressure Testing Plumbing and Heating Pipework (#59).

Drainage Test

Above-ground drainage: fill each trap and check for siphonage by running an adjacent appliance (if fitted). Underground drains: air or water test to BS EN 1610 — for new drains, a water test to 1.5m head for 30 minutes with no visible leakage or drop exceeding 0.5 litres.

Heating Pressure Test

Pressurise sealed central heating system to 1.5 bar using a filling loop and check all joints are sound before closing in. Flush the system before final fill — remove all flux, swarf, and debris. See: Sealed Heating System Pressure (#45).

Building Control Inspection

For new builds, the building inspector should be notified for a first fix inspection before boarding and plastering. Some local authorities require notification at first fix stage under Part G. Competent persons schemes (CIPHE, SNIPEF, APHC) allow self-certification for most domestic plumbing work — but the completion certificate must be issued.

Common First Fix Plumbing Mistakes

• Joints inside walls without access: Compression fittings on hot water pipes inside stud walls without an access panel will fail eventually. Use soldered end-feed fittings or plan access hatches for any compression or push-fit joints that are concealed.
• Incorrect gradient on waste: Too shallow and blockages occur repeatedly. Too steep and the water runs away leaving solids behind. Set gradient properly before securing pipe clips.
• No pressure test before closing in: The most common cause of expensive first fix failures is plastering over untested joints. Always test and document before walls close in.
• Over-notching joists: Removing too much timber from joists for 28mm pipes is a common structural problem. Where notching limits are tight, consider alternative routes or consult a structural engineer.
• Mixing pipe materials without appropriate fittings: Copper to plastic transitions require the correct jointing method — do not use copper compression fittings on plastic pipe without the correct insert liner. See: Push-Fit Plumbing: JG Speedfit and Hep2O (#55).
• Leaving UFH manifold in wrong position: The manifold must be accessible and at the right height in the finished room — an error here means cutting into screeded floors.
• No fire stopping at compartment walls: Pipe penetrations through fire compartment floors or walls must be fire-stopped on completion of first fix, not left for later.

Summary: First Fix Plumbing Checklist

• Cold water distribution: MDPE underground entry, 22mm distribution, 15mm branches — tested to 1.5× working pressure
• Hot water distribution: sized and routed per system type (combi/vented/unvented) — all joints accessible or soldered
• Above-ground drainage: 110mm SVP, 40mm/50mm waste branches at correct gradients — drained and tested
• Underground drainage: correct gradient, inspection chambers, approved connection — water or air tested
• Central heating flow and return: correct pipe size, radiator stubs roughed in, UFH laid and manifold positioned
• Gas supply: Gas Safe engineer, continuous sleeve in walls, no concealed joints, pressure-tested and documented
• Notching: within Part A limits, I-joist manufacturer guidelines followed, no over-notching
• Fire stopping: all compartment penetrations sealed with intumescent products
• Pressure tests: all completed and documented before boarding or plastering
• Building control notification: first fix inspection completed where required

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