Drainage Fall Calculator: Getting the Right Gradient
A 110 mm foul drain pipe needs a minimum fall of 1:80 (12.5 mm per metre) and a recommended fall of 1:40 (25 mm per metre). The gradient creates the flow velocity needed to carry solid waste through the pipe — too flat and solids settle; too steep and liquid outruns solids, leaving them stranded. The target is a self-cleansing velocity of 0.7 m/s at one-third pipe capacity, which a 1:40 gradient achieves reliably for domestic 110 mm drainage.
Getting the drainage gradient right is the single most important aspect of underground pipe installation. Everything else — pipe material, bedding type, joint method — is secondary. A perfectly installed pipe at the wrong gradient will block. A basic installation at the right gradient will flow freely for decades.
Quick-Reference Fall Table
| Pipe Size | Application | Minimum Gradient | Recommended Gradient | Fall Per Metre | Fall Per 10m |
|---|---|---|---|---|---|
| 110 mm | Foul (domestic) | 1:80 | 1:40 | 25 mm | 250 mm |
| 110 mm | Surface water | 1:100 | 1:60 | 17 mm | 170 mm |
| 160 mm | Foul (multi-dwelling) | 1:120 | 1:60–1:80 | 12.5–17 mm | 125–170 mm |
| 160 mm | Surface water | 1:150 | 1:80–1:100 | 10–12.5 mm | 100–125 mm |
How to Calculate the Fall
The Formula
Total fall = Length of pipe run × Gradient
Or rearranged:
Gradient = Total fall ÷ Length of pipe run
Worked Examples
Example 1: New drain from extension to existing chamber
- Pipe run: 8 metres
- Desired gradient: 1:40
- Total fall = 8,000 mm ÷ 40 = 200 mm
- The outlet end must be 200 mm lower than the inlet end
Example 2: Checking an existing gradient
- Pipe run: 12 metres
- Measured fall (invert at building minus invert at chamber): 180 mm
- Gradient = 12,000 ÷ 180 = 1:66.7
- This is between 1:60 and 1:80 — acceptable but not ideal for foul drainage
Example 3: Limited depth — what gradient is achievable?
- The building’s soil pipe exits at 450 mm below ground
- The existing chamber invert is at 750 mm below ground
- Available fall: 750 - 450 = 300 mm
- Pipe run: 15 metres
- Achievable gradient = 15,000 ÷ 300 = 1:50 — excellent
Why the Gradient Matters
Too Flat (Below 1:80)
- Water moves slowly through the pipe
- Solid waste (faecal matter, paper, food debris) settles on the pipe invert
- Grease and fat solidify in the slow-moving water
- Blockages become frequent
- Standing water in the pipe produces foul odours
Too Steep (Above 1:20)
- Water rushes through too quickly
- Liquid outruns solid waste, leaving solids stranded
- Solids accumulate and eventually block the pipe
- The turbulent flow can erode pipe joints over time
- Waste reaches the sewer connection in sudden surges, which can cause backflow
The Sweet Spot (1:40 to 1:60)
- Water and solids move together at a consistent pace
- Self-cleansing velocity (0.7 m/s minimum) is achieved naturally
- Solids are carried fully through the pipe to the sewer
- No standing water, no accumulation
Self-Cleansing Velocity Explained
Self-cleansing velocity is the minimum flow speed that keeps the pipe clear. At 0.7 m/s, the turbulence at the bottom of the pipe lifts and carries particulate matter rather than allowing it to settle.
| Pipe Size | Gradient for Self-Cleansing (foul) | Flow Rate at Self-Cleansing |
|---|---|---|
| 110 mm | 1:40 to 1:60 | ~2.5–3.5 l/s |
| 160 mm | 1:60 to 1:80 | ~5.5–8.0 l/s |
For domestic drainage, the key insight is: a single WC flush (approximately 6 litres in 3–4 seconds = ~1.5 l/s) does not fill a 110 mm pipe. The pipe runs at partial capacity. At 1:40, the flow velocity at this partial fill is around 0.8 m/s — just above self-cleansing. At 1:80, the velocity drops to around 0.6 m/s — below self-cleansing.
This is why 1:40 is recommended and 1:80 is the minimum. The numbers are tight at partial domestic flows.
How to Set the Gradient on Site
Method 1: Laser Level (Most Accurate)
- Set up a rotating laser level on a tripod near the trench
- Place a laser receiver on a staff at the starting invert level (building end)
- Calculate the fall needed at the far end
- Set the receiver to the target height at the far end
- Work along the trench, adjusting the bed depth until the receiver reads the correct level at each point
Method 2: Dumpy Level
- Set up the dumpy level with a clear line of sight along the trench
- Take a reading at the starting invert
- Calculate the target reading at the far end
- Take readings at intermediate points and adjust the bed accordingly
Method 3: Spirit Level and Straight Edge (Short Runs)
For runs under 5 metres, a 1.2 m spirit level on a straight edge can check the gradient:
- At 1:40, a 1.2 m straight edge should show 30 mm of fall
- At 1:60, it should show 20 mm of fall
- At 1:80, it should show 15 mm of fall
Use a pre-cut spacer block under one end of the straight edge to check the exact fall.
Method 4: Water Level
A simple water-filled tube with the water line visible at each end gives a perfectly level reference. Mark the required fall above this level at the high end and below it at the low end.
What If the Site Doesn’t Allow 1:40?
Sometimes site conditions limit the achievable gradient:
| Problem | Solutions |
|---|---|
| Sewer is too shallow | Dig the trench deeper at the building end (lower starting invert) |
| Building pipe exit is too low | Use a mini pumping station to lift waste to a higher level |
| Long run with minimal fall available | Accept 1:60 or 1:80 with Building Control approval; may need more frequent rodding |
| Rock or high water table | Shallower trench with SN8 pipe for strength; consider alternative route |
| Obstruction in the way | Route around the obstruction (with inspection chambers at bends) |
If the achievable gradient is flatter than 1:80 for a 110 mm pipe, the options are:
- Increase the pipe size to 160 mm (achieves self-cleansing at flatter gradients)
- Install a pumping station (lifts waste to a higher level for gravity flow)
- Redesign the route to achieve a steeper gradient
Gradient for Different Drain Types
| Drain Type | Typical Pipe Size | Recommended Gradient | Notes |
|---|---|---|---|
| Domestic foul | 110 mm | 1:40 | Single dwelling |
| Domestic surface water | 110 mm | 1:60 to 1:100 | Rainwater only — no solids |
| Shared foul (2–10 dwellings) | 110 mm | 1:40 | Higher flows help self-cleansing |
| Shared foul (10+ dwellings) | 160 mm | 1:60 to 1:80 | Higher flow volume |
| Land drain | 100–160 mm | 1:100 to 1:200 | Perforated pipe, no solids |
Frequently Asked Questions
What fall does a drain pipe need?
A 110 mm foul drain pipe needs a fall of at least 1:80 (12.5 mm per metre), with 1:40 (25 mm per metre) recommended. For surface water drains, 1:60 to 1:100 is standard. The gradient must achieve a self-cleansing velocity of 0.7 m/s to prevent solids from settling.
Is 1:80 gradient enough for a drain?
It is the absolute minimum for 110 mm foul drainage. At 1:80, the self-cleansing velocity is marginal, and blockages are more likely than at steeper gradients. Building Control may accept 1:80 in situations where site constraints prevent a steeper fall, but 1:40 is always preferred.
What happens if a drain is too steep?
If the gradient exceeds approximately 1:20, water flows too fast and separates from solid waste. Solids are left behind in the pipe and accumulate into blockages. Extremely steep drains (1:10 or steeper) should include backstop or rest bends to slow the flow.
How do I check the gradient of an existing drain?
Use a drain camera (CCTV survey) to measure the internal gradient, or rod the drain and measure the depth at two inspection chambers. The gradient equals the difference in invert depths divided by the distance between them.
Can I use a steeper gradient for surface water drains?
Yes. Surface water carries no solids, so the self-cleansing velocity is not critical. Gradients up to 1:10 are acceptable for surface water. However, very steep surface water drains can deliver water to the sewer too quickly during storms, potentially causing surcharge — attenuation may be required.
