Foul Drainage Design: Above-Ground Stack Sizing, Branch Discharge Units, Ventilation and Approved Document H

Quick Answer: Above-ground foul drainage must be designed in accordance with Approved Document H (2015) and BS EN 12056-2. Stack sizes are determined by discharge unit (DU) loadings: a single-stack 100mm soil pipe serves most domestic properties, while 150mm stacks are required when total DU exceeds 200 on any one stack. All stacks must be ventilated to atmosphere either by open-top vent or an air admittance valve (AAV) meeting BS EN 12380.

Summary

Foul drainage design is one of the less glamorous but most consequential aspects of building work. Get it wrong and you get blocked pipes, foul smells, and potential public health issues. The above-ground drainage system — everything from the sanitary fittings to the point where the drain enters the ground — must be sized and configured to handle peak discharge flows without losing trap seals or creating pressure surges.

Approved Document H (ADH) is the primary regulatory reference for England and Wales, setting out the requirements of Building Regulation H1. ADH draws heavily on BS EN 12056-2 (Gravity drainage systems inside buildings — sanitary pipework, layout and calculation) for its sizing methodology. In Scotland, Section 3.7 of the Technical Handbooks applies; in Northern Ireland, Technical Booklet N.

The discharge unit method is the standard UK sizing approach. Each sanitary fitting is assigned a DU value representing its peak flow contribution. The DUs are summed at each point in the system to determine pipe sizes. This is more accurate than simple fixture counting because it accounts for probability — not all fittings discharge simultaneously in normal use.

Key Facts

• Approved Document H (2015) — primary Building Regulation document for drainage in England and Wales
• BS EN 12056-2 — gravity drainage systems inside buildings; sanitary pipework calculation
• BS EN 12380 — performance requirements for air admittance valves
• Minimum stack diameter: 75mm for stacks serving up to 10 WCs with no urinals; 100mm for most domestic properties
• 100mm stack capacity: up to approximately 200 DU in frequency of use (system II, ADH Table 1)
• 150mm stack: required where DU exceeds 200 or where WC branch connections require it
• Offset limitation: no offsets in the 'wet' portion of a stack (zone from 450mm below lowest branch to 750mm above highest)
• Branch gradient: 18mm/m to 45mm/m (1:55 to 1:22) for 32mm–50mm waste branches
• Max branch length without secondary ventilation: 1.7m for 32mm pipe; 3m for 40mm pipe; 4m for 50mm pipe (single stack system)
• WC branch: minimum 100mm diameter, maximum length 6m without branch ventilation, 40mm fall minimum
• Trap depths: 75mm water seal for WCs; 25mm for baths/basins (32–40mm pipe); 50mm for shower traps
• AAV application: BS EN 12380 Type A or B; must be in accessible position; cannot replace open stack vent in all cases
• Stack vent termination: minimum 900mm above any opening window within 3m horizontally

Quick Reference Table

Detailed Guidance

Single-Stack System Design

The single-stack system, where all branches connect to one soil and vent pipe (SVP) without individual branch ventilation pipes, is the standard approach in UK domestic construction. It relies on careful pipe sizing and branch arrangements to prevent trap seal loss from self-siphonage or induced siphonage.

Critical rules for single-stack:

• WC connections must be 'boss' type or swept entry — no right-angle connections to stack
• Bath and basin branches must not connect within 200mm of a WC connection (below)
• Stack should be straight or with maximum 45° offsets in the dry section only
• Branch pipes must not combine before entering the stack (no shared branch traps)

For a standard 3-bedroom house the DU calculation typically looks like:

• 2 WCs: 2 × 14 = 28 DU
• 1 bath: 7 DU
• 2 basins: 2 × 1 = 2 DU
• Kitchen sink: 6 DU
• Washing machine: 6 DU
• Total: 49 DU — well within 100mm stack capacity

Branch Sizing and Gradients

Branch pipes carry effluent from individual fittings to the stack. The gradient must be steep enough to maintain self-cleansing velocity (0.7 m/s minimum) but not so steep that liquid runs ahead of solids.

Washing machines and dishwashers should discharge over a standpipe (300mm high minimum, 40mm diameter) with an air gap to prevent back-siphonage of grey water into the appliance.

Ventilation Options

Open stack vent (primary ventilation): The stack terminates above roof level with a wire balloon guard. This is the simplest and most reliable system. The vent pipe must be 100mm minimum and terminate at least 900mm above any openable window within 3m horizontally.

Air admittance valve (AAV): BS EN 12380 Type A valves (opens on negative pressure only) can replace individual branch vents and, in some configurations, the main stack vent — subject to local authority and building control acceptance. Key requirements:

• Must be accessible for inspection
• Minimum 200mm above highest connecting branch
• At least one open stack vent per drainage system connecting to the sewer (AAVs cannot replace all venting in a building)
• Not suitable in areas of positive pressure (risk of foul gases being emitted)

Secondary ventilation (relief vents): Required where branch lengths exceed the single-stack limits above, or in commercial buildings with high DU counts. The vent pipe connects back to the main SVP at least 750mm above the spillover level of the highest connected fitting, or to atmosphere.

Offset in Stacks

Offsets in the 'wet' portion of a stack (from 750mm above the highest branch to 450mm below the lowest branch) are prohibited in the single-stack system. An offset here creates a hydraulic jump that can pressurise the stack and blow trap seals. Where an offset is unavoidable:

• Use a ventilated anti-syphon trap on every branch within the offset zone
• Or install a 75mm vent pipe on the upstream side of the offset connecting to atmosphere
• Or use a secondary ventilated stack system

Multi-Storey and Commercial Applications

For buildings over four storeys or with DU totals exceeding 200, a secondary ventilated system is required. This adds individual vent pipes (32mm minimum) to each branch, connected to a common vent stack running parallel to the SVP. The vent stack connects to the SVP at the top and at the base, above the flood level of the lowest fitting.

In commercial premises (restaurants, schools, hospitals), discharge unit totals can be very high. A 150mm stack handles up to approximately 750 DU. Above this, multiple stacks or specialist hydraulic modelling is required.

Frequently Asked Questions

Can I use an AAV instead of running a vent pipe through the roof?

Yes, in most domestic situations. BS EN 12380 Type A AAVs are widely accepted by building control. However, every drainage system in a building must have at least one open vent to atmosphere — you cannot use AAVs exclusively. The AAV must be accessible (not buried in a cupboard with no access panel) and positioned at least 200mm above the highest branch connection.

What size soil pipe for a loft conversion with an en-suite?

For a single bathroom in a loft conversion, a 100mm SVP is standard. If the existing stack is 100mm and the loft en-suite adds one WC (14 DU), one basin (1 DU), and one shower (1 DU), the total addition is 16 DU — well within the stack's remaining capacity. Check the existing total DU load on the stack before connecting. The branch from the loft WC must not exceed 6m and must have a minimum 1:55 fall (18mm/m).

Do I need building regulations approval for a new bathroom stack?

Yes. New drainage installations require Building Regulations approval under Regulation H1. For most domestic work, you can use a competent person scheme (e.g., CIPHE, WaterSafe, NAPIT) to self-certify, or submit a Building Notice to the local authority. Replacement of like-for-like (same size stack, same connections) is generally exempt, but check with your local authority.

Why does the bath drain slowly but the WC is fine?

Slow bath drainage with a functioning WC usually points to a partial blockage in the bath branch (40mm pipe) rather than the stack. Common causes: accumulated hair and soap scum at the trap, a collapsed or kinked section of flexible waste pipe, or a branch gradient that's too shallow (less than 18mm/m). Rod from the inspection boss on the stack back toward the bath.

What is the 200mm rule for WC connections?

No other branch connection should enter the stack within 200mm below a WC connection. This is because WC discharge creates a slug of water that temporarily fills the stack bore; a branch entering immediately below would be submerged, potentially causing back-pressure into that fitting's trap. Space connections carefully when adding new fittings to an existing stack.

Regulations & Standards

• Building Regulations Approved Document H (2015 edition) — H1 covers foul water drainage for England and Wales; sets out DU method, stack sizing, gradient requirements

• BS EN 12056-2:2000 — Gravity drainage systems inside buildings. Sanitary pipework, layout and calculation. The technical backbone of ADH methodology

• BS EN 12380:2002 — Air admittance valves for drainage systems. Performance requirements and test methods

• Scottish Building Standards Section 3.7 — equivalent drainage requirements for Scotland

• Northern Ireland Technical Booklet N — equivalent requirements for Northern Ireland

• Water Supply (Water Fittings) Regulations 1999 — applies to standpipe connections for appliances

• BS 8000-13 — Workmanship on building sites — drainage of roofs and paved areas (referenced for general drainage workmanship principles)

• Approved Document H: Drainage and Waste Disposal — MHCLG official guidance

• BS EN 12056-2 — BSI standards portal

• CIPHE Technical Guidance — Chartered Institute of Plumbing and Heating Engineering

• NHBC Technical Standards Chapter 8.1 — Drainage below ground

• underground drainage — below-ground drainage sizing, inspection chambers and connection to public sewer

• soakaways — surface water disposal where sewer connection is unavailable

• pump stations — when gravity drainage is not achievable

• part h drainage — Approved Document H overview and compliance checklist