The purpose of weatherstripping, or “gasketing“, around a door opening is pretty straight-forward. There are gaps around the door so that the door can be opened and closed without binding on the frame. These gaps, however, can allow air flow, sound, light, and smoke transmission. Weatherstripping effectively seals up the gaps all the way around the frame as well as the gap at the bottom of the door to prevent the transmission of unwanted elements when the door is closed. Weatherstripping can be used at exterior or interior openings.

Today’s blog post will cover the ways the gap at the bottom of the door can be sealed. Thresholds are a common piece of hardware installed on the floor beneath the door to seal the gap. But usually there is some space left so that the door doesn’t get stuck on the threshold as it’s closing. This leftover space means there will still be some transmission of elements mentioned previously. This is where door sweeps and door bottoms come in.

Door Sweeps

Standard thresholds will usually still have about 1/8″ gap left between the bottom of the door and the top of the threshold. The simplest way to seal that gap is with what is called a door sweep.

A door sweep is a long strip of metal that holds a seal made of a material such as neoprene, silicone, or a nylon brush. This long strip of metal is surface mounted to the bottom of the door. Ideally, the sweep should be installed so that is comes in contact with the bevel on the threshold but where the sweep will not touch the flooring material (i.e. carpet, tile) as the door swings open and closed. (These aren’t meant to sweep the floor!)


Door sweeps can be ordered in a range of sizes and lengths, depending on the gap at the bottom of the door.

Automatic Door Bottoms

To get a better seal at the bottom of the door you can use automatic door bottoms instead. An automatic door bottom is adjustable and often provides a better seal. An automatic door bottom is bulkier than a door sweep but the reason is because of what is on the inside. The “automatic” part is a rod that protrudes slightly on one end that acts almost like a button.

When the door is closed, the button (usually located on the hinge side of the door) is pressed in by the soffit of the frame which causes the seal to drop and close the gap at the bottom of the door. When the door is opened, the activator “button” releases and the seal material is pulled up inside the door bottom which allows the door to swing open and closed without the seal material dragging along the floor.

The distance that the seal drops can be adjusted to create the best seal possible.

There are three ways to mount an automatic door bottom: surface, half mortise, and full mortise. Fire ratings may determine which type of door bottom you can use. Auto door bottoms are often used for STC ratings and can also be lead lined for X-ray rooms.

Other Door Bottoms

Another type of door bottom is called a “door shoe”. A door shoe will have seal material underneath the door and will either mount direct into the bottom of the door or will wrap the bottom of the door and fasten to the face of the door on one or both sides of the door. The undercut of the door needs to be coordinated with these types of door bottoms so that they fit correctly. Some will fill the standard 3/4″ undercut while others can be 1/4″ to a 1/2″ tall.


These are only some of the ways to seal the gap at the bottom of doors. There are diverse ways and the most important thing is to coordinate with the conditions at the opening to ensure the proper seals are used. Using weatherstripping can provide cost savings for the building owner, it can prevent the transmission of light and sound as well as smoke in the case of a fire. Contact Beacon CDL to speak with a sales representative about the correct solution for your openings.