Mactar · Tarmac · Asphalt · Civils · Duleek, Co. Meath

Road Gully Types & Specification Guide

The gully types, grating classes and sizes used on Irish roads and driveways and how to specify the right unit for the job.

Road gullies are specified by four things: type (trapped, untrapped or silt box), pot size and depth, outlet diameter (100mm domestic or 150mm road), and the grating load class (from A15 for pedestrian areas up to D400 for carriageways). Matching all four to the setting is what keeps a gully working and safe under load.

Road Gully Types & Specification Guide

Road gully specifications are often assumed rather than checked, which is how the wrong grating ends up under a driveway or a footpath unit ends up on a carriageway. This guide sets out the gully types, sizes, outlets and grating load classes used on Irish roads and driveways, based on Mactar's experience as a Duleek, Co. Meath civils and drainage contractor.

1. Gully Type

The first choice is the type of gully, driven by whether you need to block odours and how much silt the run carries.

• 1
  Round trapped gully Has a water-seal trap that stops drain smells rising. Used where odour matters.
• 2
  Untrapped square gully Simpler surface-water collection where no trap is needed.
• 3
  Silt box Sits in the run to catch grit and silt before the pipe; supplied with one pre-tapped hole and three that can be tapped out on site.

2. Size, Depth and Outlet

Gully pots come in a range of diameters and depths; deeper pots hold more silt between cleans. The outlet diameter must match the pipework and the catchment: domestic gullies commonly run a 100mm outlet, while road gullies use a 150mm outlet and drain up to around 250m² of carriageway. Larger road pots run up to 450mm in diameter and are heavy enough to need craning into place.

3. Grating Load Class

The grating (and its frame) must carry the traffic that will run over it. Gratings are classified by load class — the right class depends entirely on where the gully sits, from footpaths to trafficked carriageways:

4. Installation Requirements That Affect the Spec

How a gully is installed feeds back into what you should specify. All gulliesincluding pre-cast concrete are bedded and surrounded in mass concrete to resist deformation. Modern installs often use a lightweight plastic liner as a mould for in-situ concrete. The grating sits on regulating masonry or cover slabs and is set before the final surfacing; where a smaller grating meets a larger 450mm pot, brickwork is corbelled inwards, with no course rising more than 50mm above the one below.

How to Specify — A Quick Checklist

• 1
  Setting and traffic Footpath, driveway, car park or carriageway → determines the grating load class.
• 2
  Odour control needed? Decides trapped vs untrapped.
• 3
  Silt load in the run? Add a silt box.
• 4
  Catchment area and pipework Determines pot size, depth and outlet diameter (100mm vs 150mm).
• 5
  Access for installation Whether larger 450mm units are practical on site.

Specification FAQs

• What grating class do I need for a driveway?

  A B125 grating is typically specified for domestic driveways and car parks. Carriageways and trafficked roads need D400. If a gully sits where heavier vehicles will cross it, specify up rather than down.
• What size outlet does a road gully have?

  Road gullies generally use a 150mm outlet, versus the 100mm outlet common on domestic drainage gullies, because they drain far larger catchments — up to around 250m² of carriageway.
• Trapped or untrapped gully - which should I choose?

  Choose a trapped gully where drain odours could be a problem, as the water seal blocks smells. An untrapped gully is fine for straightforward surface-water collection where odour is not a concern.