What are drain gullies and how are they used?

Drainage gullies – also referred to as gully traps – are ground-level drains that rainwater or grey waste discharge into. There are numerous types of drainage gully on the market, but the principal function remains the same.
Drainage gullies are designed to allow waste and stormwater to pass through easily but stop bad odours and rodents from escaping from out of the drains. They also catch roof and plant debris, which prevents it from entering the mainline system and causing blockages.


What types of drain gullies are there?

The two main types of drainage gullies are the P trap and bottle gully. Both are used to collect water from a variety of surfaces and benefit a property by trapping gases and foul odours. However, the bottle gully is designed to be cleaned out with a drainage rod, which makes it easier to clean and maintain than the P trap.

Road gullies are for a similar purpose but are designed to cope with the demands of high traffic areas, such as airports, motorways and car parks. Road gullies are fitted with a trap that prevents sediment from entering the drainage system while stopping unpleasant odours from escaping.

P trap drainage gullies

The P trap gully works by letting water into an inlet, called a gully hopper, that leads to a curved pipe before joining the main underground drainage system. The P trap gully is named after this curved section, which bears a resemblance to the letter P. Water is trapped in this part of the gully and forms a barrier, preventing odours and gases from escaping into the atmosphere.

There are two types of P trap gully; the 45-degree P trap gully and the 90-degree P trap gully. Both operate on the same principle and neither is roddable – the main distinction from a bottle gully. Both types of P trap gully require a drainpipe coupling to connect the gully to the drainpipe.

Bottle trap drainage gullies

Bottle gullies – also known as yard, compact or universal gullies – work on largely the same principles as P trap gullies. However, the main and most distinctive difference is that bottle gullies are roddable.

The central chamber within the trap is removable, which makes it easier to clean the gully. This means that it’s not always essential to call an expert to clean or remove blockages from a bottle trap gully, as it’s something the homeowner can undertake.

Road drainage gullies

Although much larger than residential gullies, road gullies work in much the same way. As a guide, they tend to have 150mm-diameter outlets and can drain a maximum area of 250m2. Modern road gullies use lightweight plastic units, which act as a liner for concrete casting. Once all the necessary pipework is connected, concrete is used to provide strength and resist deformation.

Road drainage gullies are larger than yard gullies and capable of handling a much heavier flow of water from roads. All road gullies must be held securely in place with concrete and each gully requires a specific type of grating.


Which grating do I need for my drainage gully?

While bottle gullies often come with the specified grid attached, P traps and road gullies tend to require a separate grating. Grates are classified according to intended weight and the anticipated flow regime:

  • Class A: Pedestrians and cyclists only (test load: 15kN)
  • Class B: Car parks and residential driveways – light, slow-moving traffic only (test load: 125kN)
  • Class C: Small commercial/industrial car parks, kerbside and service stations (test load: 250kN)
  • Class D: Carriageway use and all public highways (test load: 400kN)
  • Class E: Commercial and industrial heavy loads (test load: 600kN)
  • Class F: Exceptional loads, freight yards, airports and military (test load: 900kN)

The average patio drainage fitting can use anything from Class A upwards, while carriageway grates require at least a Class C. These heavy-duty coverings are typically manufactured from ductile or cast iron and can be hinged or completely removed from their frames when inspections are required.

Double-triangle gratings are recommended for use in high-speed traffic areas. This is due to their sturdy properties, making them less likely to cause problems.


Gully trap hoppers

Whether you’re using a P trap gully or a bottle gully, you’ll need a hopper to receive the water from above and funnel it into the trap. It’s possible to join the hoppers to the gully traps with a length of pipe, though this depends on the depth the traps are set to.

Gully trap hoppers usually come with a grate attached, this prevents debris from entering the drain, reducing the risk of blockages. Gully tray hoppers come in a variety of shapes, including square, round and rectangular. The shape is commonly chosen to fit with the outdoor design or paving, with square being the most popular design.


How to Unblock a Drainage Gully?

Unblocking a P trap gully

The most significant drawback of using a P trap gully is that a blockage can’t be cleared by rodding. Typically, a gully trap can be unblocked using a flexible rod that is fed into the drain. This can either break up the blockage or push it towards a manhole cover where it can be manually removed. Due to the shape of a P trap gully, rods are not suitable. So a drainage specialist will be required to unblock it using electro-mechanical pipe cleaning.

This method uses a flexible device that can be guided around the bend of the gully. It’s powered by electricity and has a rotating cutting head which can disperse blockages.

Unblocking a bottle trap gully

A bottle gully typically gives you much better access to the outlet pipe than a P trap gully. Once the inner chamber has been removed, a drainage rod is inserted into the pipe to disperse any blockages in close proximity to the bottle trap gully.

If a blockage has formed deep in the system, it may be necessary to remove it using water jetting. A drainage specialist will be required in this situation. They’ll guide a hose through the pipe to dislodge any accumulated material using high-pressure water.


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