One of the most basic of percussion rudiments, the single-stroke roll is simply one stick (or mallet, or whatever) after the other, plained in a steady beat:

R-L-R-L-R-L . . .

It sounds easy. Okay, it is easy, kind of. Even though it uses the term "roll", there's no particular tempo implied; you could play a single-stroke roll at 60 BPM if you wanted to.

However, to play even a slow, steady roll at a constant dynamic level is harder than it sounds. I often play a roll like this for several minutes to warm up. Just like everything else in percussion, dynamic variance is everything. Even a single-stroke roll becomes hard if you accent it such that it implies a 7/8 or 9/8 time signature. Harder still if you think in 4/4 and still play the 7/8 accents, or try playing in subdivisions of 5, where for every count you say, you play five strokes, R-L-R-L-R.

The moral of the story? Practice your rudiments.