How can you tell a door sill from a threshold?
Door sills and thresholds are located close to one another at the bottom of an exterior doorframe, but they’re not the same part.
Door sills are a structural element, and thresholds direct the flow of water to prevent leaks.
Sills are made of wood, while thresholds are made of aluminum.
It’s easy to confuse these two terms because a door sill and a threshold are installed in the same location at the bottom of a door. The differences between these two parts of a door lie in their function and the order in which they’re installed. The sill acts as a structural element, while the threshold protects the sill and the home's interior from water. Read on for what you need to know to differentiate between a door sill and a threshold.
Door sills and thresholds are located in nearly the same spot, so it can be tricky to tell the difference. The doorframe rests on the door sill, and the threshold sits on the sill and between the door jambs. It helps to think of these parts in layers.
The sill is the first layer that rests on the foundation, deck, or porch. Once the doorframe is installed, the threshold is tucked between the two door jambs and sloped to direct water away from the door to prevent leaks from getting through.
| Difference | Sill | Threshold |
|---|---|---|
| Location | Bottom of the door | On top of the sill |
| Materials | Metal, wood, concrete | Metal, fiberglass, wood |
| Function | Base of the doorframe | Transition piece that prevents water from entering |
If you need a new door sill or threshold, a door installer can help get your door working like new.
While the sill and the threshold are installed at the bottom of the door, their location is substantially different. The sill is located at the bottom of the doorframe between the studs and acts as a base for the door and the frame. The threshold sits on top of the sill and helps stop water from leaking into the structure's interior.
As the base of the doorframe, the sill can be made from wood, metal, or concrete. The threshold of an exterior door is made of metal, such as aluminum, brass, or stainless steel, but it can also be made from fiberglass or include vinyl or wood elements.
The sill and the threshold work together to keep moisture out of the house, but they do so in different ways. The threshold is the first line of defense against the elements, and its slanted design allows water to drain away from the door. Because it’s made of a durable material like metal or fiberglass, it’s also effective at protecting the sill from damage. If the threshold itself becomes damaged, it can be replaced relatively quickly.
The sill supports the frame and door casing and creates a layer of protection between the foundation and the door. To add more protection from water, you can also install a sill pan between the sill and the threshold.
The installation process for sills and thresholds have some similarities, but there are different challenges to tackle with each. The sill is installed with the original door, which gives plenty of room to work.
If you need to replace a rotted or damaged sill, you’ll have to remove the threshold. You’ll have to cut the sill out while keeping the doorframe and casing intact. Then, you’ll cut a new sill to size, notching the corners to fit under the door, and make sure it sits firmly against the studs on either side of the frame.
If you’re installing a pre-hung door, it might come with an installed threshold, so all you have to do is set it in the opening and secure it with screws or nails. If you’re installing a new threshold on an existing door, you’ll have to remove the old one and cut the new one to size, notching the corners so it sits snugly against the stops inside the doorframe. A local door repair company can install these elements so your door functions properly.
Sometimes, these two terms are used interchangeably, but they are separate parts of a door, and each serves a different function. The sill helps ensure the doorframe is secure, and the threshold helps protect the sill and direct water away from the door to prevent leaks.
Yes, you need to install a sill for an exterior door. A sill offers structural support for the doorframe and creates a place to install the threshold, which is essential for keeping water from seeping underneath the door and into the home. Door sills are not necessary for interior doors.
