Questions about trench grate open area continue to pop up. And similar related question is clogging factor as related to trench drain grating. To answer this question we need to look at the perspective of a typical design engineer. When looking at a floor drain this is a very relevant question because the grate open area is often similar in size to the area of the pipe. If it is less the drain cannot fill the pipe to capacity and the design is flawed. Also, if debris clogs the drain then the same thing happens.
With a trench drain the grating is continuous over a long length providing many times excess in open area as compared to the size of the outlet piping. Many times a trench drain grate can be 80-90% clogged and still deliver more fluid to the outlet pipe than that pipe can take. So to boil this all down the open area of a trench grate is often a mute point.
There are three cases where the open area of a trench drain grate may need be evaluated.
1. Very steep cross slopes with narrow trenches (in excess of 5%).
2. When using a heel proof grate (especially in areas with heavier debris loadings that can clog the small openings).
3. When the trench drain is very short (ie more similar to a floor drain).