Foul Drainage Design for Domestic Extensions

Foul drainage from an extension must connect to the existing system without overloading it, with falls steep enough to flow but not so steep that solids leave water behind. Common-sense engineering with strict regulatory backing.

Pipe sizes

• 100mm (4"): main household drains, single dwelling
• 150mm (6"): larger drainage runs, multiple dwellings
• 225mm (9"): typically commercial

Falls (gradient)

• 100mm pipe: minimum 1:40 to 1:80 (between half-full and full flow)
• 150mm pipe: minimum 1:80 to 1:150
• Steeper than 1:40 — solids deposit as water races ahead
• Shallower than 1:80 — inadequate self-cleansing velocity

Manholes and access

Required at:

• Every junction
• Every change of direction over 45°
• Every change of gradient
• Every change of pipe diameter
• Maximum 22m intervals on straight runs (100mm), 45m (150mm)

Build-over agreements

If your extension overlaps a public sewer (typically anything over 150mm in the road or a shared adopted drain), you need a build-over agreement from the water company.

• Application fee: R3,000–R10,000
• Requirements: protective sleeve, granular bedding, manhole access
• Cost to construct compliantly: R30,000–R120,000

Soakaway vs sewer

Foul drainage usually connects to existing sewer. Soakaways are not permitted for foul water — only for surface water in appropriate ground conditions.

Common mistakes

• Connecting WC to a 50mm waste (must be 100mm)
• Inadequate falls causing repeated blockage
• No rodding access at junctions
• Manhole covers covered by patio (find them, mark them)
• Building over sewer without agreement

MCFAR designs drainage for residential and commercial extensions.

Frequently Asked Questions