Measure the internal diameter of the pipe below the grating — not the grating, and not the gully body. That one measurement decides the size. The grating is always wider than the pipe beneath it, and the wider upper chamber of a gully is not the reference either. It is the single commonest reason an order comes back as the wrong size.
On a site with dozens of drains across wet rooms, plant rooms and a commercial kitchen, getting this right at survey stage saves a second visit. The procedure below takes about a minute per drain.
How to measure
- Lift the grating. Most lift out, or release with a single screw.
- Look past the gully body. There is often a wider chamber below the grating; what matters is the pipe below that.
- Measure the internal diameter wall to wall through the centre — a tape, a caliper or the measuring tag.
- Record it in millimetres and match it to the range in the table.
Between two sizes, specify the smaller one. The silicone ribs expand against the pipe wall; an oversized seal will not enter, while a slightly smaller one still seals.
Size table
| Measured | Size | Typical application |
|---|---|---|
| 32 mm | 1.25″ | Offices, ice machines, cold rooms |
| 38 mm | 1.5″ | Wet rooms, en-suites, utility rooms |
| 50–58 mm | 2″ | Hotel bathrooms, hospital wards |
| 55–65 mm | 2.5″ | Older building stock |
| 75–85 mm | 3″ | Commercial kitchens, hotels — the most common size |
| 80–89 mm | 3″/3.5″ | Refurbished healthcare facilities |
| 89–100 mm | 3.5″ | Industrial kitchens, bakeries |
| 88–102 mm | 3.5″/4″ | Food processing |
| 102–110 mm | 4″ | Meat processing, breweries, distilleries |
| 120–135 mm | 5″ | Car parks, plant rooms |
| 150–156 mm | 6″ | Ports, heavy industry |
Across Irish commercial stock the 3″ size covers most kitchen and back-of-house gullies; wet rooms and en-suites are usually 1.5″ or 2″.
Gratings, channels and square gullies
The seal sits in the pipe, not in the grating, so the shape above it is irrelevant. Round and square gullies, channel drains in kitchens and shower drains all take the same seal, provided there is a round pipe below for it to seat in. With a channel drain, measure the outlet.
Surveying a whole building
For a multi-drain site it is worth measuring a representative drain in each area — kitchen, wet rooms, plant room, car park — rather than every drain individually. Most buildings use two or three diameters throughout. We can review a drain schedule or a set of photographs and return a quantity breakdown by size; send it through and we will mark it up.
The measuring tag is a keyring with the ranges printed on it — useful for a maintenance team doing the survey themselves.
Installation
No tools, no work on the pipework, no downtime: lift the grating, drop the seal in, replace the grating. About 30 seconds per drain, which is why a full building is typically done in a single shift. Nothing is glued or cut, so units can be removed for cleaning with the extraction tool.
If the drains in question are in a kitchen or food area, the HACCP guidance for drains covers the documentation side; for closed or seasonal buildings see why traps dry out in empty buildings.