At first, the question of whether it matters if a drainage grate has larger or smaller openings sounds like a silly one. Of course, it would matter as larger openings would allow items to fall into the grate that could clog the drain, right? But drainage grate openings are not that big, to begin with. The difference in size between larger and smaller openings might be a centimetre or so, meaning that a lot of things that would clog up drains wouldn't be able to flow through the grate and down the drain anyway. So, knowing that the difference is very small, does opening size matter?

Avoiding Clogged Grates

A smaller opening will catch more debris, but it will also clog up easily with debris. If the grate will be in an area where you can access it easily and clean it frequently, smaller openings would be fine. But if the grate will be in a remote area where you don't really go that often, slightly larger openings would be better so that debris doesn't clog up the grate and lead to severe flooding. 

Allowing Faster Drainage

If you need a grate that allows water to flow into a drain very quickly, larger openings in the grate are better. Maybe you need a grate for a warehouse where you routinely have to hose down the floor, such as a rug-cleaning company that power-washes rugs in an internal work area. You wouldn't want the water to pool on the floor or have dirt clump up on the grate, so you'd need a large grate with larger openings that allow the water and dirt to flow in easily.

Preventing Hair and Other Solids From Entering the Drain

Smaller debris can often enter small openings in a drainage grate anyway, but you still want to try to block that debris. That means getting grates that have very small openings. If the openings are small, you do want to have more of them; think of a mesh drain net used over a tub drain to catch hair instead of letting it flow down the drain. You'll need to clear the debris away from the grate frequently, though.

Any Size Requires Your Help

No matter the size of the openings in the grate that you get, you'll still need to do what you can to prevent items that shouldn't enter the drains, from entering the drains. In other words, clean up yards so that leaves don't fall or flow into storm drains; try not to let hair run down into a shower or sink drain. If you need a grate for a location where a particular type of debris is a problem, contact a grate manufacturer to discuss having a custom grate made.