To Avoid Lap Marks...
To avoid lap marks, roll the full height of the wall and keep a wet edge
Lap marks are those ugly stripes caused by uneven layers of paint buildup. They occur when you roll over paint that's already partly dry. (In warm, dry conditions, latex paint can begin to stiffen in less than a minute!) The key to avoiding lap marks is to maintain a "wet edge," so each stroke of your roller overlaps the previous stroke before the paint can begin to dry.
To maintain a wet edge, paint an entire wall all at once. Start at one end, running the roller up and down the full height of the wall, moving over slightly with each stroke. Move backward where necessary to even out thick spots or runs. Don't let the roller become nearly dry; reload it often so that it's always at least half loaded. Keep the open side of the roller frame facing the area that's already painted. That puts less pressure on the open side of the roller, so you're less likely to leave paint ridges.