Above-ground drainage moves waste from appliances down to the point where it leaves the building. Underground drainage takes it from there — carrying foul water to the sewer and surface water away from the property. Level 2 doesn't expect you to install underground drainage systems (that's Level 3 ground work), but you do need to know the three main system types, how gullies work, and why the distinction matters when connecting above-ground pipework to what lies beneath.
This is the fourth post in the Level 2 drainage sub-cluster. For the others, see the traps, stack systems, materials and jointing, rainwater and guttering, and testing and maintenance posts.
What counts as "underground" drainage
Underground drainage is the pipework below ground level that carries:
- Foul water from WCs, basins, sinks, baths, showers and washing machines — anything that's been used inside the building
- Surface water from roofs (via rainwater pipes), driveways, and yard gullies
How these two streams are handled — separately, combined, or a bit of both — defines the three underground system types.
The three underground drainage systems
Combined system. The most common arrangement in older UK properties. Foul water and surface water both go into the same sewer pipe, which takes everything to the sewage treatment works. In this system, every rainwater pipe must have a gully trap at the bottom to stop sewer gases escaping back up through the rainwater pipework.
- Simpler underground pipework (just one set of pipes)
- All water, including rainwater, ends up at the sewage treatment works
- In heavy rainfall, the combined sewer can be overwhelmed, which is one reason this arrangement has fallen out of favour for new builds
Separate system. Two separate sets of underground pipes: one for foul water, one for surface water. The foul water goes to the sewer and on to treatment. The surface water — which is relatively clean — discharges to:
- A stream or river, or
- A soakaway (a gravel-filled pit that lets the water soak into the ground)
The advantage: the sewage treatment works only deals with actual sewage, and rainwater doesn't overwhelm the system during heavy downpours. Surface water is also rightly considered a different kind of waste from foul water and doesn't need processing in the same way.
Partially separate system. A hybrid. Some of the surface water goes into the main sewer along with the foul water; the rest discharges separately (to a soakaway, stream, or surface water sewer). Typically seen where site constraints make a fully separate system impractical — for example, a front roof discharging to the sewer via the property's combined drain, and a back roof discharging to a soakaway.
Key differences summary
| Feature | Combined | Separate | Partially separate |
|---|---|---|---|
| Foul water route | Main sewer | Main sewer | Main sewer |
| Surface water route | Main sewer (with gully trap at base of each downpipe) | Separate pipe to stream, river, or soakaway | Mixed — some to main sewer, some separate |
| Underground pipes | One set | Two sets | Hybrid |
| Most common in | Older properties | Newer properties | Sites with constraints |
Gullies and gully traps
A gully is a fitting that sits at ground level and connects above-ground pipework to the underground drainage. It serves three purposes:
- Provides an access point for rodding and clearing blockages
- Contains a water seal (gully trap) to prevent sewer gases coming back up
- Allows certain appliances (sinks, washing machines, baths, showers) to discharge outdoors rather than into a soil stack
Which appliances can discharge to a gully?
- Most waste appliances can — basins, sinks, showers, baths, washing machines, dishwashers
- WCs and urinals cannot discharge to a gully — they must connect to the soil stack and go directly to the underground drainage
How to connect to a gully. The waste pipe must discharge:
- Below the gully grid — so debris doesn't splash out
- Above the water seal — so the pipe isn't sitting submerged in waste water
Too high and splashing causes staining and smells. Too low and the pipe sits in the water, which causes slow drainage, smells, and potential siphonage of the trap seal.
Types of gully
Two common types at Level 2:
Back inlet gully. Has a dedicated connection point (the "back inlet") for a waste pipe to enter the gully below the grid level but above the water seal. This is the modern standard — the waste pipe goes straight into the gully from the side, with the grid still available on top for surface water.
Direct connection to the grid. Older installations sometimes discharge straight over the grid. This works but has disadvantages: splashing, debris, and the visual mess of waste water discharging in an open gully.
Most modern installations use back inlet gullies for waste, with the grid reserved for surface water only (like rainwater from a downpipe above).
Rainwater pipes and gully traps
In a combined system, every rainwater pipe must have a trap at the bottom — either a dedicated gully trap or a gully with a water seal. Why? Because the rainwater pipe is connected to the combined sewer, and without a trap, sewer gases would rise up the rainwater pipe and discharge at the top of the gutter.
In a separate system, the rainwater pipes connect to the surface water sewer, which doesn't carry sewage and therefore doesn't produce the same gases. Rainwater pipes in a separate system don't need traps, though some installers fit them anyway as a belt-and-braces measure.
Why this matters for connecting pipework
When you're working on above-ground pipework, you need to know which underground system it connects to:
- Working on a combined system: every rainwater pipe needs a trap. If you're replacing a downpipe, make sure the gully trap at the base is still functional.
- Working on a separate system: make sure foul water is going into the foul drain and not the surface water drain — cross-connecting waste into a surface water sewer is a serious pollution offence because surface water drains typically discharge untreated to watercourses.
- Working on a partially separate system: know which bits go where before you connect anything new. Schematics or site drawings are worth finding before the first cut.
Common exam traps
Trap 1: Combined vs separate. Combined = one pipe for everything (with gully traps on rainwater pipes). Separate = two pipes, surface water going to streams/rivers/soakaways. Questions test the distinction directly.
Trap 2: Surface water system traps. Surface water drains in a separate system don't need traps on rainwater pipes. Questions sometimes test this by asking "which of these is NOT fitted with traps." A separate surface water system is the answer.
Trap 3: Gully discharge position. Below the grid, above the water seal. Both parts matter. This is also covered in the stacks post because it's a connection rule more than an underground one.
Trap 4: WCs and urinals. These cannot discharge to a gully. They must connect to the soil stack or directly to the below-ground drain. Only "waste" appliances (basins, baths, sinks, etc.) can use a gully.
Quick revision summary
Before the mock test, five things you need to be able to produce from memory:
- Three underground systems: Combined, Separate, Partially Separate
- Combined: foul + surface water in one pipe; every rainwater pipe needs a gully trap
- Separate: two pipes; surface water to stream, river, or soakaway
- Gully discharge: below the grid, above the water seal
- WCs and urinals do NOT discharge to a gully — soil stack or direct to drain only
📝 10-Question Mock Test
Click an option to see whether you got it right. Explanations appear instantly — no submitting at the end.
Foul and surface water in the same pipe is the defining feature of a combined system. Separate (A) uses two pipes. Partially separate (C) is a hybrid. "Spate" (D) isn't a real drainage system type — a distractor that shows up in some exam papers.
In a separate system, rainwater pipes don't need traps because the surface water sewer doesn't carry sewage and doesn't produce the gases that would require sealing off. The other options are incorrect: foul water pipes can be any size, foul systems don't normally use soakaways, and interceptor traps aren't a universal feature of surface water systems.
In a combined system, the rainwater pipe connects to a sewer that also carries foul water — and sewer gases. Without a trap, those gases would rise up the rainwater pipe. A gully trap (either a dedicated trap or the water seal in a gully) stops that happening. Options A, B, and D don't solve the gas problem.
The hybrid arrangement where some surface water goes to the main sewer and some discharges separately is called partially separate. Combined (A) has everything in one pipe; Separate (C) has everything in two pipes.
The point of a separate system is to keep relatively clean surface water out of the sewage treatment works. Options A and B describe combined systems or a malfunction of a separate system. Option D is nonsensical.
Both halves matter: below the grid prevents splashing out; above the seal keeps the pipe clear of waste water. Covered in more detail in the stacks post — worth locking in both parts of the rule.
WCs and urinals must connect directly to the soil stack or to the underground drain — they carry solids that a gully wouldn't handle properly, and the discharge rate is too high for a gully's water seal to cope with. Basins, baths, sinks and washing machines can all discharge to a gully.
The back inlet is a separate connection point built into the gully body, specifically for waste pipes, positioned below the grid but above the water seal. The grid on top is then reserved for surface water. Back inlet gullies still have water seals (A is wrong) and are standard domestic fittings (C is wrong).
The definition of a separate system — two sets of pipes. Option A describes a combined system (everything in one drain). Option B describes a specific gully type, not a drainage system type. Option D describes a gully fitting detail.
A soakaway is a purpose-built pit that lets surface water soak into the ground. Acceptable outlets for surface water in a separate system are streams, rivers, and soakaways. Options A and B would be cross-connecting surface water into the foul system, which defeats the purpose of a separate system. Option D is nonsensical — hopper heads are above-ground components.
How PlumbMate puts this into practice
Underground drainage is a shorter topic than most, but the specific facts (which system has traps where, what discharges to what) are exam staples. Spaced repetition handles them quickly.
- Flashcards, not essays. One prompt, one answer — the format that research has consistently shown works best for active recall.
- Wrong answers are logged. Every question you get wrong goes into a dedicated collection that resurfaces more frequently in future sessions.
- The 3× rule. You need to get a question right three times before it clears — one lucky guess isn't enough.
- Explanations on every question. Like the ones above, but on every single question in the app.