Drainage Falls & Pipe Sizes: UK Guide

Why Drainage Falls Matter

Get the fall wrong on a drainage run and you'll have problems. Too shallow and waste sits in the pipe, solids build up, and the drain blocks. Too steep and the liquid runs ahead of the solids, leaving them stranded in the pipe — which also blocks. The sweet spot is set out in Building Regulations Approved Document H, and every builder doing groundwork needs to understand it.

What Is a Drainage Fall?

The fall (or gradient) is the slope of the pipe from the highest point to the lowest. It's expressed in three ways:

• As a ratio — e.g., 1:40 (the pipe drops 1 unit for every 40 units of length)
• As a percentage — e.g., 2.5% (the pipe drops 2.5mm for every 100mm of length)
• As mm per metre — e.g., 25mm/m (the pipe drops 25mm for every metre of length)

These all describe the same thing in different ways:

| Ratio | Percentage | mm per metre | |---|---|---| | 1:40 | 2.5% | 25mm/m | | 1:60 | 1.67% | 16.7mm/m | | 1:80 | 1.25% | 12.5mm/m |

Approved Document H Requirements

Approved Document H sets minimum and maximum gradients for different pipe sizes and drainage types.

Foul Drainage (Toilets, Sinks, Baths)

| Pipe Diameter | Minimum Gradient | Maximum Gradient | Notes | |---|---|---|---| | 75mm (single appliance waste) | 1:40 (25mm/m) | 1:10 | Short runs only | | 100mm (main foul drain, 1+ WC) | 1:40 (25mm/m) | 1:10 | Most common domestic size | | 100mm (with fewer than 5 dwellings) | 1:80 (12.5mm/m) | 1:10 | If self-cleansing velocity is achieved | | 150mm (shared drain, 5+ dwellings) | 1:150 (6.7mm/m) | 1:10 | Larger developments |

The 1:40 rule for 100mm pipe is the one you'll use most on domestic work. It's the safe default for foul drains serving houses and extensions.

Surface Water Drainage

Surface water pipes (rainwater, gullies) can generally run shallower because they carry water without solids:

| Pipe Diameter | Minimum Gradient | |---|---| | 100mm | 1:100 (10mm/m) | | 150mm | 1:150 (6.7mm/m) |

What About the 1:80 Rule?

You can use a shallower gradient of 1:80 for 100mm foul drains if the pipe serves at least one WC. The theory is that the flush from a WC provides enough flow to carry solids along at the shallower gradient. However, many building control officers prefer 1:40 as it gives more margin for error. When in doubt, use 1:40.

The Drainage Calculator lets you enter your pipe run length and desired fall, then checks it against Approved Document H requirements. It shows the gradient as a ratio, percentage, and total drop.

How to Calculate the Fall

Finding the Total Drop

Total drop (mm) = Pipe length (m) × Fall (mm/m)

For a 10m foul drain at 1:40 (25mm/m):

10 × 25 = 250mm total drop

This means the invert (bottom of the pipe) at the far end must be 250mm lower than the invert at the starting point.

Finding the Fall from Two Known Points

If you know the invert levels at each end (from a survey or existing manholes):

Fall (mm/m) = (High invert - Low invert) ÷ Pipe length

For example, if the invert at the house is 98.500m AOD and the invert at the manhole 8m away is 98.300m AOD:

(98.500 - 98.300) = 0.200m = 200mm

200 ÷ 8 = 25mm/m = 1:40

That's spot on for a 100mm foul drain.

Pipe Sizes for Domestic Work

Waste Pipes (Above Ground)

| Appliance | Minimum Pipe Size | Minimum Fall | |---|---|---| | Basin | 32mm | 1:20 | | Bath/shower | 40mm | 1:40 to 1:80 | | Kitchen sink | 40mm | 1:40 | | Washing machine | 40mm standpipe | Vertical | | WC | 100mm (or 110mm) | 1:40 to 1:80 |

Underground Drainage

| Use | Minimum Pipe Size | |---|---| | Single dwelling foul drain | 100mm | | Shared foul drain (2–5 properties) | 100–150mm | | Surface water drain | 100mm | | Combined drain | 100–150mm |

In practice, 110mm uPVC (which is the standard underground pipe — it's actually 110mm OD but called "100mm" in the regs) is used for almost all domestic foul and surface water drainage.

Laying Drainage: Practical Steps

1. Set Out Your Levels

Use a laser level or dumpy level to establish the invert level at each end of the pipe run. Mark these on profile boards.

2. Excavate the Trench

Trench width should be at least the pipe diameter plus 300mm (150mm each side for bedding and surround).

Trench depth depends on your invert levels plus:

• 100mm granular bedding below the pipe
• 300mm cover above the pipe crown (minimum, more in trafficked areas)

For a 110mm pipe at 1:40 fall over 10m:

• Starting depth: crown + 300mm cover + 100mm bedding = roughly 600mm minimum
• End depth: 600mm + 250mm drop = 850mm

3. Bed the Pipe

Lay 100mm of pea gravel (10mm single-size) in the trench bottom, compacted level. Lay the pipe on this bed with the sockets pointing uphill.

4. Surround and Backfill

Fill around the pipe with pea gravel to 150mm above the pipe crown. Then backfill with excavated material in 150mm layers, compacting each one. Don't drop heavy lumps of clay or stone directly onto the pipe.

5. Test the Drain

Before backfilling completely, carry out an air test or water test:

• Air test: Seal both ends, pressurise to 100mm water gauge, and hold for 5 minutes. A drop of no more than 25mm is acceptable.
• Water test: Seal the lower end, fill the drain with water to 1.2m head above the invert at the upper end. Top up for 2 hours, then measure loss over 30 minutes. Maximum loss: 0.05 litres per metre of pipe per hour.

Building control will want to witness this test.

Manholes and Inspection Chambers

You need an inspection chamber or manhole at every change of direction, change of gradient, and at junctions. For domestic work:

| Chamber Type | Typical Use | Depth | |---|---|---| | Shallow inspection chamber | Domestic drains, garden | Up to 600mm | | Deep inspection chamber | Domestic drains | 600mm–1.2m | | Manhole | Shared drains, highways | Over 1.2m |

Position chambers so that every part of the drain can be reached with standard drain rods (typically 1m rods, so maximum 25–30m between chambers in practice).

Connecting to the Sewer

Before connecting to the public sewer, you need a Section 106 agreement (for foul water) from your local water authority. You can find your local water company and apply via the Water UK network. For surface water connections, you may also need their approval.

Key rules:

• The connection must be made by an approved contractor
• You cannot connect surface water to a foul sewer (or vice versa) without permission
• A disconnection chamber or back-inlet gully is often required at the boundary

If you're working with gravel for drainage bedding or soakaway fill, the Gravel Calculator can work out tonnage for pea gravel, clean stone, and MOT.

Common Mistakes

• Using the wrong fall. 1:40 for foul, 1:100 for surface water. Mixing these up causes problems.
• Not checking the existing invert levels. If the manhole you're connecting to is shallower than you expected, you might not achieve the required fall. Always check levels before you dig.
• Bedding on sharp stone. Angular aggregate can puncture the pipe under load. Use rounded pea gravel or purpose-made pipe bedding.
• Running foul and surface water into the same pipe without checking. Combined drainage is only permitted in certain areas. Check with your local authority building control team. Many newer estates have separate systems. Check with the water authority.
• Forgetting about tree roots. Pipes near trees (within the root protection area) should be in ducting or root-resistant material. Clay pipes with rubber seals are worst affected.
• Not testing before backfilling. Once the trench is filled, finding a leak means digging the whole lot up again.

Quick Summary

1. Choose your pipe size (110mm for most domestic work)
2. Calculate the gradient (1:40 for foul, 1:100 for surface water)
3. Work out the total drop: pipe length × fall rate
4. Excavate with 100mm bedding below and 300mm cover above
5. Lay on pea gravel, test, then backfill
6. Get building control to inspect before covering

Use the Drainage Calculator to check your falls against Building Regulations and calculate the total drop for any pipe run.