Plumbing Contract Works · Question

Should rainwater drainage and sewage lines be kept separate?

Short answer

Yes, in most cases they should be kept separate. Rainwater is relatively clean and arrives in sudden large volumes, while sewage needs treatment and flows steadily. Mixing them can overload sewers or treatment units during rain and spread contamination. Local rules often set how each network must be built and where each is allowed to discharge, so confirm the requirement for your site.

In more detail

The two networks do different jobs. Sewage, from toilets and washing fittings, must reach a sewer or a treatment arrangement because it carries contaminants. Rainwater, from roofs, terraces and paved areas, only needs to be drained away safely or, increasingly, collected and reused or recharged.

When the two are combined, a heavy shower can push far more water into the sewage path than it was designed to carry. The result can be backing up, overflow at low points or a treatment unit receiving diluted, unmanageable flow. Keeping them apart avoids this and also keeps the choice open to use rainwater for harvesting.

Separation also helps in practice:

Maintenance is simpler because blockages in one network do not affect the other.
Treatment capacity is sized for wastewater alone, not for storm peaks.
Rainwater can be directed to storage, recharge or the approved stormwater outlet.

Local conditions vary. Some areas have sewers that were built to carry both, while others have separate stormwater drains, and some plots have neither. The competent local authority decides what is permitted, including where each network may connect, so the designer should check this at the planning stage and show the answer on the drawings.

Do not connect lines informally

Cross-connections made on site, to save pipe or time, can lead to contamination and to refusal at inspection. Any connection between the networks should appear only if the design and the authority allow it.

Ask the designer to mark both networks distinctly on the layout, along with their outlets and inspection points, and to confirm the discharge arrangement in writing before work begins.

Key points

Rainwater and sewage behave differently, so separate networks are generally preferred.
Mixing can overload sewers or treatment units during heavy rain.
Separate rainwater lines keep harvesting and recharge options open.
The competent local authority decides where each network may discharge.
Avoid informal cross-connections; the design should show each network distinctly.

Who this applies to

Owners of new commercial plots planning drainage from scratch.
Project teams coordinating roof and site drainage with plumbing.
Buyers and facility managers dealing with flooding or odour complaints.

Last reviewed 3 October 2026

This answer is general information, not advice on your particular case. Terms, eligibility, and requirements change, so check the current position with the relevant institution or authority.

Should rainwater drainage and sewage lines be kept separate?

Short answer

In more detail

Separation also helps in practice:

Maintenance is simpler because blockages in one network do not affect the other.
Treatment capacity is sized for wastewater alone, not for storm peaks.
Rainwater can be directed to storage, recharge or the approved stormwater outlet.

Do not connect lines informally

Ask the designer to mark both networks distinctly on the layout, along with their outlets and inspection points, and to confirm the discharge arrangement in writing before work begins.

Key points

Rainwater and sewage behave differently, so separate networks are generally preferred.
Mixing can overload sewers or treatment units during heavy rain.
Separate rainwater lines keep harvesting and recharge options open.
The competent local authority decides where each network may discharge.
Avoid informal cross-connections; the design should show each network distinctly.

Who this applies to

Owners of new commercial plots planning drainage from scratch.
Project teams coordinating roof and site drainage with plumbing.
Buyers and facility managers dealing with flooding or odour complaints.