I've found that when using a large rock/stone to make the deadfall, there's not much of a problem with the weight rolling off the support, because there's usually enough of an edge on the end that's against the ground that it can't move, it works more like it's on a hinge. With a log however, as was mentioned, you usually need to do something like put a couple rocks on either side of the end of the log that's on the ground so it can't roll.