When building and installing your own door or maybe just installing a store-bought door, it is important to know the difference between the door threshold and the door sill. There are those that believe that the two are the same and possibly use the terms interchangeably. There are even certain dictionaries that define the terms interchangeably. Where it has been convenient to make a distinction, most standard definitions separate the two terms as follows.
On a door frame, the sill is the portion of the door frame that runs along the bottom and sits directly on the foundation of your floor. It is the cross piece that actually completes the pre-installed door frame. The sill is actually underneath your threshold.
The door seal needs to be sealed to prevent water damage. To seal it, you can use epoxy paint and cover gaps with silicone caulk. A screen door can also help against bad weather.
In order to replace a sill, you will need to first remove the threshold. The sill itself can be removed using a crowbar and a hammer.
The threshold of your door can be made of either wood or metal and it is the sloped cross piece that extends across the bottom of your door area. It is slightly raised to keep water from flowing in and often is fitted with a piece of weather stripping to help insulate the area underneath your door. The threshold sits on top of the sill.
Many thresholds come pre-fixed to fit in a doorway. Before being installed, a new threshold will need to have notches on its ends that match that of the old threshold. Once in place, you should caulk the area between the threshold and the sill to form a seal against cold and hot weather.