Approved Document H Guide: Drainage Falls & Gradients

Approved Document H is the official guidance on Part H of the Building Regulations in England: drainage and waste disposal. The document itself is free on GOV.UK; this guide starts with its most-used figures, the Table 6 minimum falls for foul drains, then covers all six requirements, H1 to H6, in plain English.

Approved Document H minimum falls (Table 6)

Table 6 gives the flattest gradients at which foul drains should be laid. They depend on the peak flow, the pipe size and the number of WCs connected (paragraph 2.34):

Source: Approved Document H (2015), Table 6 “Recommended minimum gradients for foul drains” and its notes. Sizes are nominal; capacity is the table’s maximum at that gradient. Fall per metre = 1000 ÷ X.

What the table means in practice:

• A 100 mm (110 mm OD plastic) drain carrying a WC from a house can go down to 1:80. Table 5 puts the peak flow from one dwelling at about 2.5 l/s, so it is above 1 l/s, and the 1:80 capacity of 6.3 l/s covers it.
• 1:150 is only for 150 mm drains with at least five WCs connected. A 150 mm drain from a single house does not qualify, and Table 6 gives no other 150 mm gradient.
• A drain with no WC is not covered by the 100 mm 1:80 row. Below 1 l/s it needs 1:40. A 75 mm drain should not carry WC waste: paragraph 2.33 asks for at least 100 mm.
• Table 6 sets minimums only. Approved Document H gives no maximum gradient for below-ground drains (see Is there a maximum fall?).

Rainwater drain gradients (H3)

Rainwater (surface water) drains have their own minimums in paragraph 3.15:

For larger pipes, paragraph 3.15 refers to BS EN 752. Approved Document H pipe sizes are nominal (paragraph 0.3): plastic drain pipe sold as 110 mm outside diameter is the 100 mm size, and 160 mm is the 150 mm size. To turn any of these ratios into a total fall over your run, use the Drainage Fall Calculator.

What Approved Document H covers

Part H has six requirements. Each has its own section of guidance:

H1 lists where foul water should go, in order of priority: a public sewer, then a private sewer that connects to one, then a septic tank with secondary treatment or another wastewater treatment system, and only then a cesspool. H3 does the same for rainwater: a soakaway or other infiltration system, then a watercourse, then a sewer.

H1 above ground: sanitary pipework

Section 1 of H1 covers the pipework inside domestic and small non-domestic buildings. It is written for WCs with a major flush of 5 litres or more (paragraph 1.2). Complex systems in larger buildings follow BS EN 12056.

• Traps (Table 1): 32 mm traps with a 75 mm seal for washbasins and bidets; 40 mm with a 50 mm seal for baths and showers; 40 mm with a 75 mm seal for sinks, washing machines and dishwashers. Every trap should keep at least a 25 mm water seal under working and test conditions (paragraph 1.3).
• WC branches (Table 2): an unventilated 100 mm branch can serve up to 8 WCs over up to 15 m, at a gradient between 18 and 90 mm per metre (about 1:56 to 1:11). A note to the table lets the 18 mm/m minimum drop to 9 mm/m on long runs where space is restricted, but only if more than one WC is connected. Basin branches have tighter limits that depend on their length and diameter. Outside the Table 2 limits, the branch should be ventilated by a vent pipe, a ventilating stack or an air admittance valve (paragraph 1.20).
• Direct connection: a ground-floor WC may discharge straight to a drain only if the drop from floor to drain is 1.3 m or less (paragraph 1.9).
• Stacks (Table 3): at least 100 mm for WCs with outlets over 80 mm (7.2 l/s capacity). The bend at the foot needs a centreline radius of at least 200 mm, and in houses up to three storeys no branch should join within 450 mm above the invert of the bend’s tail (paragraphs 1.11 and 1.26).
• Ventilation: open vents should finish at least 900 mm above any opening into the building within 3 m, with a cage or perforated cover (paragraph 1.31). The dry part of the stack may be reduced in one- and two-storey houses, but to no less than 75 mm (paragraph 1.32). Air admittance valves should comply with BS EN 12380 and should not be used outside buildings (paragraph 1.33).
• Boiler condensate: pipework of at least 22 mm through a 75 mm condensate trap (paragraph 1.14).
• Air test: the pipework should hold 38 mm water gauge for at least 3 minutes (paragraph 1.38).

H1 below ground: foul drains

Section 2 of H1 covers underground drains and sewers from the building to the public sewer or treatment system.

Outlets and connecting to the public sewer

Connect to a public foul or combined sewer wherever reasonably practicable. For small developments, connect where the sewer is within 30 m, provided you have the right to lay the drain across any land in between (paragraph 2.3). Notify the sewerage undertaker (the water company) at least three weeks before connecting (paragraph 2.5); section 106 of the Water Industry Act 1991 requires 21 days’ notice. Where the public sewer can surcharge, basements with sanitary appliances need a pumped system or an anti-flooding valve (paragraphs 2.8 to 2.12).

Pipe sizes and minimum falls

• A foul drain should have an internal diameter of at least 75 mm. A drain carrying WC waste or trade effluent needs at least 100 mm (paragraph 2.33).
• A sewer (a drain serving more than one property) should normally be at least 100 mm for up to 10 dwellings and 150 mm for more than 10 (paragraph 2.30).
• Table 5 gives approximate peak flows for design: 2.5 l/s for 1 dwelling, 3.5 l/s for 5, 4.1 l/s for 10, 5.1 l/s for 20 and 5.8 l/s for 30.
• The minimum gradients are the Table 6 figures at the top of this guide.

Is there a maximum fall for drains?

No. Paragraph 2.34 describes Table 6 as “the flattest gradients at which drains should be laid”, and Approved Document H sets no maximum gradient for below-ground drains. The 1:40 figure is itself the Table 6 minimum for 75 mm and 100 mm drains carrying less than 1 l/s. What the document does say:

• Lay pipes to even gradients, and combine any change of gradient with an access point (paragraph 2.19).
• Where a steeply laid drain up to 150 mm enters a septic tank, lay the last 12 m at 1 in 50 or flatter to limit turbulence in the tank (H2 paragraph 1.22).
• Above ground, the Table 2 gradient limits for unventilated branch pipes decide whether a branch needs ventilating. They are not limits on below-ground drains.

Some installers keep 100 mm below-ground runs within roughly 1:40 to 1:110 out of habit. That band is trade practice, not a Part H limit. The claim that solids are left stranded in drains steeper than about 1:40 does not come from Approved Document H either.

Access points and maximum spacing

Provide access for rodding (paragraph 2.49):

• on or near the head of each drain run
• at a bend and at a change of gradient
• at a change of pipe size
• at a junction, unless each run can be cleared from an access point

Access can be a rodding eye, an access fitting, an inspection chamber (working space at ground level) or a manhole (working space at drain level) (paragraph 2.48). Table 13 sets the maximum distances between them, for drains up to 300 mm:

Source: Approved Document H (2015), Table 13. Manhole-to-manhole spacing may be up to 200 m for man-entry size drains and sewers.

Bedding, cover and pipes near buildings

• Cover: Table 10 gives 0.6 m to 7 m of cover for SN4 plastic pipes of 100 to 300 mm laid in fields, and 0.9 m to 7 m under roads. Where the pipe has less cover than the tables allow, where necessary protect it with a reinforced concrete slab over flexible filler, with at least 75 mm of granular material between the pipe and the filler (paragraph 2.44).
• Under a building: surround the pipe with at least 100 mm of granular or other flexible fill. If the crown is within 300 mm of the underside of the slab, give it special protection (paragraph 2.23).
• Through walls: build in a short length of pipe with its joints within 150 mm of the wall faces, connected on each side by rocker pipes no more than 600 mm long with flexible joints. Or leave an opening with at least 50 mm clearance all round, masked with rigid sheet and sealed (paragraph 2.24).
• Trenches near foundations: a drain trench dug lower than a nearby foundation should be filled with concrete. Within 1 m of the foundation, fill up to its lowest level; further away, fill to a level below the foundation’s lowest level equal to the distance from the building less 150 mm (paragraph 2.25).

Testing

Gravity drains and private sewers should be tested for watertightness after backfilling (paragraph 2.59). In the air test for pipes up to 300 mm, the pipe is brought up to 110 mm water gauge and held for about 5 minutes. It should then hold an initial 100 mm with no more than 25 mm loss over 7 minutes (paragraph 2.60). The water test alternative is in paragraph 2.61. Where separate foul and surface water systems are provided, check that every connection goes to the correct system (paragraph 2.62).

Self-cleansing velocity

Approved Document H does not state a velocity. The Table 6 gradients and the Diagram 9 capacities, drawn for pipes running at 0.75 proportional depth, are its deemed-to-satisfy route. Where a design departs from Table 6, for example a flatter run justified by calculation, UK designers commonly check that the design flow reaches roughly 0.7 to 1.0 m/s. The check uses the Manning or Colebrook-White equation, and BS EN 752 is the alternative approach named in paragraph 2.63.

Invert levels and setting falls on site

A drainage design is built on site from invert levels. The invert level is the level of the inside bottom of the pipe, the surface the water actually runs along. Falls are set by fixing the invert at each end of a run so the drop matches the design gradient.

The basic relationship is:

Total fall (m) = pipe length (m) ÷ gradient ratio

So a 12 m run at 1:80 falls 12 ÷ 80 = 0.15 m = 150 mm. If the start invert is 24.500 m AOD, the end invert is 24.500 − 0.150 = 24.350 m.

To transfer these levels into the trench accurately, use one of:

• Laser level set to the design gradient, with a receiver on a staff at the pipe invert.
• Dumpy or automatic level and staff, reading invert levels off temporary benchmarks.
• Boning rods and sight rails (travellers), the traditional method: a sight rail is set at each manhole and the boning rod confirms the invert as the trench progresses.

Always work from a fixed, recorded benchmark so levels are consistent across the whole site, and double-check the connection invert at the receiving manhole or sewer before backfilling.

Worked example

A 15 m run of 100 mm (110 mm OD) foul drain carries the waste from one house: WC, bath, basins, sink and washing machine. Table 5 gives about 2.5 l/s peak flow for one dwelling, which is above 1 l/s, and the drain carries a WC. So the Table 6 row for a 100 mm drain with at least one WC applies: 1:80, with a capacity of 6.3 l/s.

• Total fall = length ÷ ratio = 15 ÷ 80 = 0.1875 m = 187.5 mm
• Fall per metre = 1000 ÷ 80 = 12.5 mm/m
• Percentage grade = 100 ÷ 80 = 1.25%
• If the start invert is 25.000 m AOD, the finish invert is 25.000 − 0.1875 = 24.8125 m AOD

If the same run carried only a sink and a basin, with no WC, the 100 mm 1:80 row would not apply. Lay it at 1:40 (15 ÷ 40 = 0.375 m of fall), unless you can show its peak flow is above 1 l/s, in which case Table 6 also allows a 75 mm drain at 1:80. You can run any of these conversions with the Drainage Fall Calculator.

H2: septic tanks, treatment plants and cesspools

Non-mains foul drainage should only be considered where connecting to a public sewer is not practicable (H2 paragraph 1.1). The main siting and sizing figures in the guidance are:

A notice inside the building must describe the maintenance the system needs. The example wording for a septic tank calls for monthly checks of the outlet and emptying at least once every 12 months. Discharges from these systems are regulated separately by the Environment Agency in England and Natural Resources Wales in Wales, so check whether a permit is needed before applying for building regulations approval (H2 paragraph 0.3).

H3: rainwater drainage, gutters and soakaways

Gutters and rainwater pipes

Table 2 of H3 sizes level, half-round eaves gutters with a sharp-edged outlet at one end:

The effective roof area allows for pitch (Table 1): plan area × 1.29 for a 30° pitch, × 1.50 for 45° and × 1.87 for 60°. For a roof pitched over 70°, or a wall, use half the elevational area. A rainwater pipe should be at least the size of the gutter outlet (paragraph 1.10). Where more than 25 m² of roof drains through one downpipe onto a lower roof, fit a distributor pipe to the shoe (paragraph 1.9). For valley, parapet, siphonic and flat-roof systems where overtopping would be serious, design to BS EN 12056 (paragraph 1.2). To check a gutter section by Manning’s equation, use the Gutter Flow Calculator.

Paved areas

Paved areas should fall away from the building, at 1 in 60 or steeper, with a cross-fall on paths of no more than 1 in 40 (paragraph 2.3). Where levels would send water towards a wall, form a reverse gradient for at least 500 mm from the wall (paragraph 2.2). A design intensity of 0.014 l/s/m² (about 50 mm/h) may be assumed for normal situations (paragraph 2.4). Gully gratings sit about 5 mm below the surrounding paving (paragraph 2.16). These provisions cover paved areas and small car parks up to 4,000 m² (paragraph 2.1).

Surface water drains

Rainwater drains should be at least 75 mm, and surface water sewers serving more than one building at least 100 mm (paragraph 3.14). The minimum gradients are 1:100, 1:150 and 1:225. Rainwater should never go to a cesspool or septic tank (paragraph 0.6). Surface water drains connecting to a combined sewer need traps on all inlets (paragraph 3.7). Car parks, petrol filling stations and other areas likely to have oil spills need an oil separator (paragraph 3.22): a by-pass separator with a nominal size of 0.0018 × the contributing area in m², or a full-retention separator of 0.018 × the area in fuel storage and other high-risk areas (Appendix H3-A).

Soakaways and attenuation

Soakaways and other infiltration devices should not be built (paragraph 3.25):

• within 5 m of a building or road, or in unstable land
• where the water table reaches the bottom of the device at any time of year
• so close to other soakaways or drainage fields that the ground’s soakage capacity is exceeded
• where contaminated run-off could pollute groundwater

Design soakaways for a 1-in-10-year storm, testing different storm durations to find the largest storage volume. For 25 m² or less of drained area, a rainfall of 10 mm in 5 minutes may be assumed as the worst case (paragraph 3.27). Test the soil with the H2 percolation test (paragraph 3.28). Larger soakaways follow BRE Digest 365 or BS EN 752 (paragraph 3.30). For a first estimate of pit volume and drawdown, the dry well (soakaway) calculator uses a US method, not BRE 365. Where infiltration is not possible, the discharge to a watercourse may be rate-limited (paragraph 3.3), and attenuation storage such as a detention pond holds back the peak flow to the sewer or watercourse (paragraph 3.35). For a buried attenuation tank, see the underground detention calculator.

H4: building over or near drains and sewers

H4 applies when you build, extend or underpin over or within 3 m of the centreline of a drain, sewer or disposal main shown on the sewerage undertaker’s sewer records (H4 paragraph 0.1). Consult the owner first (paragraph 0.3). For a public sewer that is the water company, which usually handles this through a build-over agreement. In 2011 most private sewers and lateral drains in England and Wales that connected to the public network were transferred to the water companies, so a drain shared with a neighbour is often a public sewer now.

The guidance says:

• Do not build over a manhole or inspection chamber on a sewer serving more than one property (paragraph 1.3).
• No more than 6 m of drain or sewer under a building without the owner’s permission (paragraph 1.5).
• No building over or within 3 m of a drain or sewer more than 3 m deep or larger than 225 mm without the owner’s permission (paragraph 1.6).
• A diversion route that avoids the building by at least 3 m should be available so the sewer could be rebuilt (paragraph 1.4).
• Where the drain runs under the building less than 2 m deep, extend the foundation locally so the pipe passes through the wall. Where it is deeper than 2 m to invert and passes beneath the foundations, design the foundation as a lintel spanning at least 1.5 m either side of the pipe (paragraphs 1.10 and 1.12).
• Rising mains, brick or masonry sewers, and sewers in poor condition (for example deformed by more than 5%) carry a high risk. Do not build over or within 3 m of them without special measures (paragraph 1.2).
• Keep piles at least twice the pile diameter from the sewer, measured outside to outside (paragraph 1.8).

H5: separate foul and surface water systems

Where the public sewers in an area are separate, or a separate surface water sewer is under construction, a new building’s rainwater system should be kept separate from its foul system (H5 performance and paragraphs 1.1 to 1.3). The aim is to keep rainwater out of the foul sewer, where it can overload capacity and cause flooding. Where only a combined sewer exists now but a separate system is being built, both systems connect to the combined sewer at first and are reconnected when the new sewers are complete.

Cross-connecting the two, with foul waste into a surface water sewer or rainwater into the foul system, is a common and serious defect. It pollutes watercourses or overloads treatment works. Run-off contaminated by stored materials needs the sewerage undertaker’s consent before it goes to the foul sewer (paragraph 1.4).

H6: solid waste storage

For houses and low-rise flats (up to the 4th floor), the guidance asks for:

• space for bins with a combined capacity of 0.25 m³ per dwelling, more if collections are less often than weekly, or whatever the waste collection authority agrees (paragraph 1.1)
• a place each dwelling can use for at least two movable bins, individual or communal; a 1.2 m × 1.2 m area is usually enough where each dwelling has its own store (paragraphs 1.2 and 1.3)
• a carry distance for householders of no more than 30 m, and bins within 25 m of the collection point (paragraph 1.8)
• a route to the collection point that does not pass through the building, unless through a porch, garage or car port (paragraph 1.9)
• for bins up to 250 litres, no more than 3 steps on the route and slopes no steeper than 1:12 (paragraph 1.10)

Communal enclosures should be at least 2 m high, with 150 mm clear around each bin, and refuse chutes at least 450 mm in diameter (paragraphs 1.14 and 1.18). For non-domestic buildings, agree capacity and access with the waste collection authority (paragraph 1.19). BS 5906 is the alternative approach.

England, Wales, Scotland and Northern Ireland

Common mistakes

• Laying a 150 mm foul drain at 1:150 when fewer than five WCs are connected. Table 6 only allows 1:150 with at least five WCs.
• Using 1:80 on a 100 mm drain with no WC. The 1:80 row needs at least one WC. Below 1 l/s, Table 6 gives 1:40.
• Changing gradient without an access point. Any change of gradient should be combined with an access point (paragraph 2.19).
• Setting falls off the trench bottom instead of the invert. Always level to the inside bottom of the pipe, allowing for bedding.
• Cross-connecting foul and surface water systems, overloading treatment works or polluting watercourses.
• Accumulated levelling errors over a long run. Re-check against a fixed benchmark at each manhole.
• Placing a soakaway within 5 m of the house or where the water table rises into it.
• Building an extension over a shared sewer or its manhole without the water company’s agreement.
• Forgetting capacity for surface water. The design storm, not just the minimum gradient, governs the pipe size.

When to involve building control

New drainage and material alterations to existing drains are covered by the Building Regulations (Appendix H1-B, paragraph B.7). In practice:

• Apply for building regulations approval before you start: a full plans application or building notice to the local authority, or use a private building control body (in England, a registered building control approver).
• Building control will want drainage layouts, pipe sizes, gradients and connection details, and will normally witness an air or water test before backfilling is signed off.
• Repairs and minor alterations are not normally covered by the Building Regulations. Section 61 of the Building Act 1984 still requires 24 hours’ notice to the local authority before you repair, reconstruct or alter a drain, except in an emergency (paragraph B.8).
• Connecting to a public sewer or building over or near one also needs the agreement of the sewerage undertaker (the water company), separately from building control.

If you depart from the Approved Document H figures, for example with an unusually flat gradient justified by a hydraulic calculation, be ready to show that the functional requirement is still met. A Manning-equation capacity and velocity check, or a design to BS EN 752 (paragraph 2.63), is the usual evidence.

Frequently asked questions

What is the minimum fall for a foul drain under Approved Document H?

Table 6 of Approved Document H gives 1:40 for 75 mm and 100 mm foul drains when the peak flow is below 1 l/s. Above 1 l/s it gives 1:80 for a 75 mm drain, 1:80 for a 100 mm drain with at least one WC connected, and 1:150 for a 150 mm drain with at least five WCs connected. Table 5 puts the peak flow from one dwelling at about 2.5 l/s, so a 100 mm drain carrying a house’s WC can be laid at 1:80.

Can a 150 mm foul drain be laid at 1:150?

Only when the peak flow is above 1 l/s and at least five WCs are connected. That is the note on the 150 mm row of Table 6. A 150 mm drain serving a single house does not meet it, and Table 6 gives no other 150 mm gradient, so either lay a 100 mm drain at 1:80 (with at least one WC connected; its 6.3 l/s capacity exceeds the 2.5 l/s from one house) or justify the flatter 150 mm run by calculation under BS EN 752, which paragraph 2.63 names as the alternative approach.

Is there a maximum gradient for drains in Part H?

No. Table 6 lists the flattest gradients at which foul drains should be laid, and Approved Document H sets no maximum for below-ground drains. Above ground, Table 2 gives gradient ranges for unventilated branch pipes, for example 18 to 90 mm per metre for a WC branch; outside them the branch should be ventilated. The 1:40 to 1:110 band and the idea that solids are stranded in drains steeper than 1:40 are trade lore, not Part H.

How close can I build to a public sewer?

Requirement H4 applies to building, extending or underpinning over or within 3 m of the centreline of a drain, sewer or disposal main shown on the sewerage undertaker’s sewer records. Consult the owner, usually the water company, which handles this through a build-over agreement. Its permission is needed to build over or within 3 m of a sewer more than 3 m deep or larger than 225 mm, or to have more than 6 m of sewer under the building, and you should not build over a manhole on a sewer serving more than one property.

How far must a soakaway be from a house?

Paragraph 3.25 of Approved Document H says infiltration devices such as soakaways should not be built within 5 m of a building or road, in unstable land, or where the water table reaches the bottom of the device at any time of year. Soakaways are designed for a 1-in-10-year storm; for 25 m² or less of drained area a rainfall of 10 mm in 5 minutes may be assumed, and larger soakaways should follow BRE Digest 365 or BS EN 752.

How far must a septic tank be from a house?

Paragraphs 1.16 and 1.17 of Approved Document H2 place septic tanks at least 7 m from any habitable part of a building, preferably downslope, and within 30 m of a vehicle access for emptying. The drainage field or mound that treats the tank’s effluent should be at least 15 m from any building, 10 m from any watercourse or permeable drain, and 50 m from any groundwater abstraction point.

Does Approved Document H apply in Wales, Scotland and Northern Ireland?

The 2015 edition published on GOV.UK is for use in England. Wales publishes its own Approved Document H (the 2002 edition incorporating 2010 amendments), with the same Table 6 foul-drain gradients, and since 7 January 2019 new developments of more than one dwelling, or with 100 m² or more of construction area, also need SuDS approval from the local SuDS Approving Body. Scotland uses Section 3 of the Building Standards Technical Handbooks and Northern Ireland uses Technical Booklet N.

Related tools and reading

• Drainage Fall Calculator: convert between gradient ratio (1:X), total fall in mm and fall per metre, with a Part H Table 6 check.
• Manning’s Pipe Calculator: full-bore capacity and velocity for a pipe at a chosen gradient.
• Dry well (soakaway) calculator and underground detention calculator: first estimates of infiltration and attenuation storage.
• BS EN 752 Drainage Guide: the European standard behind Approved Document H’s alternative approach.
• Manning’s Equation Explained: the velocity and capacity formula behind self-cleansing gradients.
• Reading a Drainage Plan: how invert levels and gradients are shown on drawings.