Push-Fit Soil Stack Installation
Complete guide to installing a 110mm push-fit soil stack including branch connections, fire stopping at floor penetrations, and vent termination to BS EN 12056.
Safety Warning — Working at Height & Fire Safety
Soil stack installation involves working at height and penetrating fire compartment floors. Use scaffold or a suitable working platform — not a ladder for extended overhead work. Fire stopping at every floor penetration is a Building Regulations requirement (Approved Document B) and must not be omitted. When working with existing drainage, be aware of biological hazards — wear appropriate gloves and sanitise tools.
What You'll Need
Tools
- Hacksaw or reciprocating saw
- File and deburring tool
- Spirit level
- Drill with masonry and wood bits
- Core drill or hole saw (for floor penetrations)
- Tape measure
- Scaffold tower or working platform
- Mastic gun (for fire stopping)
Materials
- 110mm push-fit soil pipe (3m lengths)
- Push-fit branch connectors (92.5° for WC, 135° for waste)
- Push-fit couplers and bends
- Pipe clips (bracket and backplate type)
- Weathering slate or roof terminal
- Intumescent fire stopping collar or wrap
- Fire-rated sealant (intumescent mastic)
- Silicone lubricant
Technical Specifications
Step-by-Step Installation
1 Plan the Stack Route
The soil stack should run vertically from ground level to above roof line, as close as possible to the WC on each floor. Map out all branch connection positions — WCs, basins, baths, and kitchen waste. Ideally, the stack should be on an external wall for ease of installation and vent termination. If internal, plan the fire stopping at every floor carefully. Note: WC connections must enter below basin/bath waste connections on the same stack.
2 Prepare Floor Penetrations
Core-drill or cut holes through each floor slab or joist platform where the stack passes. The hole should be large enough for the pipe plus the fire collar (typically pipe OD + 40mm clearance). For timber floors, trim joists should frame the opening. For concrete floors, core drilling is cleanest. The annular space around the pipe is where the fire stopping collar will sit, so don't make the hole oversized.
3 Connect to Underground Drainage
At the base, the stack connects to the underground drainage via a rest bend (a long-radius 87.5° bend that transitions from vertical to horizontal). The rest bend should sit on a concrete support pad. Use a coupler to connect to the underground pipe run. The base of the stack is a critical stress point — it must be properly supported and not rely on the push-fit joint alone to carry the stack's weight.
4 Build Up the Stack
Working from the base upwards, assemble the stack using push-fit joints. Apply silicone lubricant to the spigot end and push into the socket until it meets the insertion mark. The stack must be plumb vertical — check with a spirit level on two faces at every pipe length. Fix pipe clips at 1.2m maximum centres and at every floor level. Each clip should have a backplate securely fixed to the masonry or structural frame.
5 Install Branch Connections
Fit branch connectors at the correct heights for each appliance. WC connections use a 92.5° boss connector or access branch at the level of the WC outlet — this is the lowest branch on each floor. Basin and bath waste connect via 32mm or 40mm boss adaptors at 135° (swept entry), positioned above the WC branch. Maintain a minimum 200mm vertical distance between the WC branch centre and the lowest waste branch connection above it (BS EN 12056-2).
6 Fire Stopping at Floor Penetrations
This is a Building Regulations requirement and must not be skipped. At every point where the stack passes through a fire compartment floor, fit an intumescent fire stopping collar rated to the required fire resistance (typically 30 or 60 minutes to match the floor). The collar wraps around the pipe and, in a fire, expands to crush and seal the PVC pipe, preventing fire and smoke from passing between floors. Seal any remaining gaps with intumescent mastic. Document the fire stopping with photographs for Building Control.
7 Terminate at Roof Level
The stack must vent to open atmosphere above the roof. The vent terminal should be at least 900mm above the top of any opening window within 3 metres horizontally. Where the stack passes through the roof, fit a weathering slate sized to match the roof covering, sealed with lead or proprietary flashing. Fit a wire cage or balloon guard to the vent terminal to prevent bird entry whilst maintaining full ventilation. Do not cap or restrict the vent opening.
8 Test the System
Flush each connected WC and run water from each basin and bath simultaneously. Check every joint for leaks — even a small drip from a soil joint is unacceptable and will create significant damage over time. Verify that trap seals on all connected appliances remain intact after flushing (no gurgling or siphonage). If trap seals are lost, check for blocked or insufficient venting. Perform an air test if required by Building Control.
Common Mistakes to Avoid
- Omitting fire stopping — This is a legal requirement under Building Regulations Approved Document B. Failure to fire-stop floor penetrations can invalidate insurance and endanger life.
- Waste branch below WC branch — Waste connections (basins, baths) must always enter the stack above the WC branch on the same floor. Connecting below causes back-pressure and siphonage.
- Inadequate vent height — A vent that terminates too low near windows will allow foul odours to enter the building. Check the 900mm / 3m rule carefully.
- Stack not plumb — A leaning stack creates turbulent flow, noise, and can cause siphonage of trap seals. Keep it vertically plumb throughout.
- No access points — Fit a rodding access at the base of the stack (above the rest bend) to allow future maintenance and blockage clearance.
- Unsupported base — The rest bend at the base takes the full weight of the stack. It must be supported on a concrete pad, not left resting on soil or gravel.
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