The complete guide
How To Choose A Gully Grid For Your Outside Drain
The drain nobody looks at until it floods
There's a small square drain at the bottom of most downpipes. It has a grate over the top, water goes in, and that's the last thought anyone gives it. Then one wet autumn the water starts pooling on the path instead of going down, and you go out with a torch to find the grate has rusted through, cracked in half, or vanished entirely.
Once the cover is gone, the gully underneath fills fast. Leaves, moss washed off the roof, grit, small stones, the odd bottle cap. All of it drops into the trap, and a trap full of silt can't take the water from a heavy shower. The result is a puddle by the back door and, in bad cases, water finding its way somewhere it shouldn't.
A PVC gully grid solves the problem at the surface, which is the cheapest place to solve it. Debris stays on top where a brush can shift it. Water goes through. The trap below stays clear.
What a gully grid actually does
The grate is a filter, not a lid. Its job is to keep the solid stuff out while letting the wet stuff through. The slots need to be wide enough that rain drains fast during a downpour, and narrow enough that leaves and twigs bridge across rather than slipping between them.
The second job is safety. An open drain recess is a hole in your path. It's a shin-scraper at best and a proper trip hazard at worst, particularly for older visitors or anyone carrying shopping. A cover seated flat in the recess removes that.
The third job is access. A good grid lifts straight out. When the gully does eventually need a clear, or when you want to run a rod down the pipe, you want to be able to pull the cover off with one hand rather than chipping mortar away with a bolster.
Plastic versus metal
Traditional grates were cast iron, and they look right on an older property. The trouble is that they sit in a wet recess all year. Iron in standing water rusts, and rust does two things you won't enjoy. It bleeds orange staining onto the stone or concrete around the drain, which is very stubborn to remove. And it expands, which welds the grate into its frame so the next person to lift it has a fight on their hands.
Plastic sidesteps both. A plastic gully grate doesn't corrode, so no staining and no seizing. It stays light, which sounds trivial until you're crouched on wet paving in November trying to free something. It's also cheap enough that replacing it in five or ten years is no hardship.
The trade-off is strength. Cast iron takes a beating. Plastic is fine for people walking over it, but it isn't a vehicle cover. If a car crosses that exact spot, you need a load-rated grate designed for it, not a domestic cover.
Getting the size right
This is where most orders go wrong, and it's easily avoided. This grid is 15 x 15 cm, a nominal 150mm square, and 1.1 cm deep.
Measure the inside of the recess, not the outer edge of the surrounding stone. Put the tape across the opening in both directions, because older recesses aren't always perfectly square. Then check the depth of the rebate the cover has to sit in. A shallow rebate with a deep cover leaves the grid standing proud, which catches toes and brush bristles. A deep rebate with a thin cover leaves it sitting low, which is usually fine but collects a little more silt around the rim.
If your measurements land between standard sizes, look at what's actually holding the cover. Sometimes the frame itself is the problem and replacing that gives you a clean, standard opening to work with.
Round openings need round covers. It sounds obvious, but plenty of people order a square grid for a circular drain and only notice on the doorstep.
Fitting it
There's not much to it. Clear the recess of silt, leaf mulch and any fragments of the old grate. A trowel and a stiff brush do the job. Check the seating is reasonably level, because a cover rocking on a lump of debris will rattle every time someone walks over it.
Drop the grid in. Press down at each corner to confirm it sits flat. That's the whole installation.
Some people prefer to bed the grid on a thin ring of mortar, particularly where the recess is oversized or irregular. It gives a snug, quiet fit. Just remember you're trading away easy removal, so only do it if you're happy to break the mortar next time the drain needs rodding. For most homes, loose is better.
Keeping it working
A grid only helps if the slots are open. Silted-over slots shed water off the side of the drain instead of into it, which is worse than no cover at all.
The routine is simple. Every few weeks, sweep the surface. Every couple of months, lift the grid out, rinse it under a hose, and scrape any build-up out of the recess below while you've the access. A stiff brush and warm soapy water clears anything stuck in the slots.
Autumn is the season that matters. Leaf fall is the single biggest cause of blocked domestic gullies, and a weekly sweep through October and November saves a great deal of bother in December. If your drain sits under a tree, make it part of the same round as clearing the gutters.
In freezing weather, don't lever a frozen cover out. Pour a little warm water around the rim and let it release on its own. Plastic handles the cold fine, but nothing likes being forced.
Who this suits
It's the right choice for a domestic surface water drain in a pedestrian area. Beside a patio, at the foot of a downpipe, along a side passage, by a back door, in a garden path. If you're replacing a rusted iron grate on an older house, this is the straightforward swap.
It isn't the right choice where vehicles cross, where the drain is a foul waste chamber needing a sealed cover, or where a specific load class is required by regulation. In those cases you want a properly rated product and, if you're unsure, a word with a drainage contractor.
Most homes have more than one gully. If you're ordering, it's worth walking round the house first and counting them, because the second and third grate are almost always the same age and condition as the one that just failed.
The short version
Measure the inside of your drain recess in two directions. If it comes out at roughly 15 x 15 cm, this grid is made for it. Clear the recess, drop the cover in, and check it sits flat.
After that, sweep the surface now and then and lift it for a rinse every couple of months, more often in autumn. It's a small, cheap part doing a job that saves you standing water, a smelly trap and an afternoon with a set of drain rods.