The installation of door sills requires careful work. Many builders mark all rough corridor walls 36" (915 mm) above the finished floor lines. These lines are called "bench marks." Once a sill is set and grouted in, it is very difficult and expensive to change its position.

Many times in steel structures, it is necessary to run lines between building columns, and then extend a level line across the front of the hoistway at each landing. In this way, the finished floor location is established. On masonry structures, you will generally find that the builder has marked the guideline on the wall at each landing.