Soil and Vent Stack Replacement in Multi-Storey Buildings

Soil and vent stack failure in mid-rise residential and commercial buildings causes leaks, odours, and tenant disruption. Replacement in an occupied building is a coordination challenge — but standard now thanks to modern materials.

When stacks fail

• Cast iron from before 1970 — corrosion at joints and at base
• Asbestos cement — degradation, asbestos exposure risk
• Early plastic — UV degradation, joint failure
• Inadequate ventilation causing pressure imbalance and trap siphoning

Replacement materials

Cast iron (modern)

Used for high-end residential, hotels, and where acoustic performance matters. Heavy but quiet — masses absorb flow noise.

HDPE

High density polyethylene with electrofused joints. Lighter than cast iron, durable, increasingly standard in commercial.

PVC-U

Traditional plastic — cheap and easy but noisier and more brittle than HDPE.

Hybrid systems

HDPE for risers (durability), cast iron behind WC pans (acoustic) — common in modern hotel/residential.

Replacement process in occupied buildings

1. Survey existing system (CCTV, pressure tests)
2. Design new stack — match or improve diversity capacity
3. Plan phasing — typically one stack at a time
4. Tenant communications and decant if necessary
5. Temporary diversions (often days, not weeks)
6. Cut out old stack section by section
7. Install new stack with fittings
8. Test (water test or smoke test)
9. Reinstate finishes

Programme

• Single stack in 6-storey block: 2–4 weeks
• Full building (3 stacks): 8–16 weeks

Cost

• Per-stack replacement in occupied building: R300,000–R900,000
• Cast iron premium: +20–35% over HDPE
• Asbestos cement removal: +R40,000–R160,000 per stack

Vent and ventilation

SANS 10252-2 requires adequate ventilation to prevent siphoning of trap seals. AAVs (air admittance valves) can substitute for vent stacks in some configurations but not all — engineer's calculation determines.

Disruption mitigation

• Pre-fabricated stack sections speed installation
• Out-of-hours noisy work where tenants allow
• Temporary toilet provision during downtime
• Clear written schedule shared with tenants

MCFAR designs stack replacement for residential and commercial buildings.

Frequently Asked Questions