The complete guide
How To Choose And Fit A Replacement Gully Grating
The problem a missing gully grid causes
A drain gully sits quietly outside the back door or at the foot of a downpipe, doing its job, until one day the grid over it cracks. Maybe a spade caught the edge. Maybe a mower wheel dropped in. Maybe frost got into a hairline crack that was already there.
Once the grid is gone, three things start to go wrong. Leaves and grit wash straight into the trap and build up until water backs onto the paving. The open square becomes a genuine trip hazard, especially at night or when the gully is right beside a step. And you get a small dark hole that slugs, mice and the occasional frog treat as an invitation.
None of that's dramatic. It's just a small annoyance that grows into a blocked drain and a Saturday spent with a rod and a bucket. A replacement grating stops the whole chain of events for the price of a takeaway.
What this grating actually is
This is the Osma replacement gully grid, a square black plastic cover measuring 217mm x 217mm with a height of 17mm. The full product dimensions are listed as 21.7 x 21.7 x 1.7 centimetres. The model name and number is 4D900 and the part number is 4D906.
It's made to suit the Osma 4D900 and 4D910 bottle gully. That matters more than it sounds. Gully frames aren't standardised across brands, and a grid that's two millimetres out will either refuse to seat or rock underfoot every time someone walks past. Buying the part that was designed for the frame removes that guesswork.
The material is plastic and the colour is black. There's no metal in it, so there's nothing to rust.
Why plastic instead of metal
Cast iron and pressed steel gratings are still common, and they're strong. They also rust. On a light grey patio or a white rendered wall, a rusting grid leaves orange streaks that never really wash off. If your gully is set into a decorative surface, that stain is the thing you'll notice every day.
Plastic sidesteps all of it. It doesn't corrode, it doesn't stain the surround, and it stays light enough to lift with two fingers. That last point is quietly important. A heavy iron grid seized into its frame is a job you put off. A light plastic one comes out in a second, so you actually clear the silt when you should. Ease of lifting is one of the honest advantages of a good plastic gully grid.
The trade-off is load capacity. This record gives no stated load class, so treat it as a domestic foot traffic cover for paths, patios and doorways. If a car or a delivery van crosses the spot, ask your merchant for a grating with a published load rating instead.
Measuring before you order
Do this before anything else, because it takes thirty seconds and saves a return.
Take a tape measure and check the opening in the gully frame, edge to edge along one straight side. You're looking for something close to 217mm. The listing also quotes item length as 22cm and item width as 21.5cm, which are rounded external figures from the packaging, so don't be thrown by the small differences. The 217mm square is the number that counts.
Then check the depth of the rebate the old grid sat in. This grating is 17mm tall. If your frame has a shallower seat, the new grid will sit proud and catch shoes.
If your old cover is still in one piece, even in fragments, measure that instead. It's usually easier than measuring an awkward frame at ground level.
Fitting it
There are no tools, screws or sealant involved. That's the whole appeal.
Start by clearing the seat. Silt, moss and small stones collect on the ledge the grid sits on, and they're the single most common reason a new grating won't go home properly. Scrape the rim with an old screwdriver or a stiff brush, then sweep the debris out.
Lower the grid in square rather than dropping it in at an angle. Press down gently and check it sits level with the surround. Put a foot on it and shift your weight. If it rocks or clicks, lift it out and clean the rim again. Nine times out of ten that's all it needs.
While the gully is open, it's worth glancing down at the trap. If you can see standing water and a clear route through, you're fine. If it's packed with leaf mulch, scoop it out with a gloved hand or a trowel before you refit. Doing that once now is far easier than rodding a blocked drain in December.
Keeping it working
Maintenance on a drain cover is close to nothing, but the little you do makes a real difference.
Sweep the grid whenever you notice leaves collecting, which in autumn might be weekly. A grating covered in a mat of wet leaves is functionally a solid lid, and water will pool around it in heavy rain. Thirty seconds with a stiff brush clears it.
Two or three times a year, lift the grid out entirely, hose it down, and scrub the underside where silt and greasy film build up. Warm soapy water and a stiff brush handles it. Rinse, then drop it back in square. Avoid solvents and strong chemical cleaners, and don't lever the grid out with a sharp metal tool, as chipping the edge is how the last one probably started failing.
In frost, treat the plastic gently. Cold plastic is more brittle than warm plastic, so lift by hand rather than hammering or prying.
Who this suits
It's the right buy if you've an Osma bottle gully with a broken, cracked or missing cover and you want the matching part rather than a compromise. It also makes sense as a spare to keep in the shed, because gully grids tend to get damaged by garden tools rather than by wear, and that damage always happens at an inconvenient moment.
It isn't the right buy if your gully is round, if it's a long channel drain, or if the opening measures noticeably different from 217mm. It's also not the right choice where vehicles cross, since no load class is published for it.
What we've not been told
We would rather leave a gap than fill it with a guess. The packed weight, country of origin, GTIN and any load or UV rating aren't stated in the supplier record for this item. If any of those figures are critical to your job, particularly a load rating, confirm them with a supplier who can give you the manufacturer datasheet.
The short version
You need a grid that matches your frame, sits flush, and comes out easily when you want to clear it. This one is 217mm square, 17mm deep, black plastic, made by Osma to suit the 4D900 and 4D910 bottle gully. Measure your opening, clear the seat, drop it in, and give it a rinse a couple of times a year. That's genuinely the entire job.