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
Plan work at height using a risk assessment and provide a suitable working platform where required. Before cutting a floor or wall, confirm the structural design and whether it is a fire-separating element. Any required penetration seal must be a tested system suitable for the pipe material and diameter, supporting construction, opening, orientation and required fire resistance. Follow its installation detail exactly. Wear suitable PPE when opening existing drainage and clean contaminated tools safely.
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 Kalsi push-fit soil pipe (3m or 4m)
- Branch fittings and waste adaptors selected for the approved layout
- Push-fit couplers and bends
- Pipe clips (bracket and backplate type)
- Weathering slate or roof terminal
- Tested penetration-sealing system where the fire strategy requires it
- Kalsi soluble lubricant
Technical Specifications
Technical basis: the current Kalsi 110mm Push-Fit Soil data sheet, Approved Document H and Approved Document B for England. Elsewhere in the UK, apply the relevant national building regulations and the building's fire strategy.
Step-by-Step Installation
1 Plan the Stack Route
Set out the stack, branches, rodding access and ventilation from the drainage design. Branches must not connect in a way that can cause crossflow into another branch or loss of trap seals. In a single dwelling up to three storeys, Approved Document H keeps branch discharge connections at least 450mm above the invert at the tail of the bend at the foot of the stack; larger buildings need the additional clearances and design rules in Part H. Identify every fire-separating wall or floor before forming openings.
2 Prepare Floor Penetrations
Do not cut joists, reinforcement or other structural elements without approval. Form each opening to the dimensions in the structural detail and, where applicable, the tested penetration-sealing system. The permitted annular gap and supporting construction vary between fire-stop products, so do not use a generic allowance.
3 Connect to Underground Drainage
Connect the stack to the underground drainage with the specified long-radius rest bend and compatible adaptor or coupler. Provide permanent support to the bend and stack using the project detail so the joints do not carry the stack weight. Include rodding access above the bend where the run cannot be cleared from another suitable access point.
4 Build Up the Stack
Working upwards, cut pipe square, deburr and chamfer it, and clean the spigot, socket and ring seal. Apply Kalsi soluble lubricant and assemble to the manufacturer's insertion depth without damaging or displacing the seal. Keep the stack plumb. Fix Kalsi clips to sound structure at no more than 2.0m centres vertically and 1.0m horizontally, with a clip within 150mm of fittings. Do not over-tighten clips; the ring-seal system must be able to accommodate thermal movement.
5 Install Branch Connections
Use a compatible pan connector and soil branch for a WC, and the correct Kalsi boss or waste adaptor for basin, bath, shower and sink branches. Kalsi supplies 92.5° and 135° branch fittings; select the fitting from the designed flow direction and access arrangement rather than applying one angle to every appliance. A single-appliance branch should be at least the diameter of its trap, and combined branches must be sized to Approved Document H. Check the completed branch geometry for crossflow, ventilation and maintenance access.
6 Fire Stopping at Floor Penetrations
Where the stack penetrates a fire-separating element, install the penetration seal specified by the fire strategy. Verify that its evidence covers 110mm PVC-U, the actual wall or floor construction, opening and annular gap, orientation, services present and required fire resistance. Install every collar, wrap, sealant, backing material and fixing exactly as shown in that tested detail; components from different systems are not automatically interchangeable. Label and record the completed seal before it is concealed.
7 Terminate at Roof Level
An open ventilating stack should finish at least 900mm above any opening into the building within 3 metres. Weather the roof penetration with a compatible proprietary flashing and finish the vent with a perforated cover that does not restrict airflow. An air admittance valve may be used only where the drainage design permits: it must comply with BS EN 12380, remain accessible and adequately ventilated, and must not be installed outside or in a dust-laden atmosphere. The below-ground system still needs provision to relieve positive pressure.
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
- Using a generic fire-stop recipe — The complete penetration seal must match tested evidence for the actual pipe, substrate, gap, orientation and fire rating.
- Poor branch geometry — Position and size branches from the drainage design so one discharge cannot crossflow into another or siphon a trap.
- 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.
- Wrong clip spacing — Follow Kalsi's 2.0m vertical, 1.0m horizontal and 150mm-at-fittings limits, using fixings suited to the supporting structure.
- Inadequate access — Provide access above the rest bend where the stack and downstream run cannot be cleared from another suitable access point.
- Unsupported base — Support the rest bend and vertical pipe from sound structure so push-fit joints do not carry the stack weight.
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