A drop-down threshold is an automatic seal installed in the lower part of a door. When the door closes, it lowers and closes the gap between the leaf and the floor. When the door opens, the seal automatically retracts into its profile. Depending on the model, the profile can be recessed into the leaf or mounted on its surface.
A properly selected solution can reduce the passage of sound, air, light, dust and insects, and in selected models also smoke or water – without the need to install a fixed threshold on the floor.
A drop-down threshold, also called a drop-down seal or automatic door seal, is used wherever effective sealing of the space under the leaf is needed while maintaining a flat and convenient passage.
How Does a Drop-Down Threshold Work?
When the door is closing, the activating button contacts the frame and triggers the mechanism. The internal profile lowers the seal along the full width of the leaf, closing the gap above the floor. When the door opens, the seal automatically retracts into the door leaf. Its drop range can be adjusted to the size of the gap
The operating method depends on the type of mechanism used:
- CHRONOSOFT – the seal lowers gently and quietly, and full sealing occurs after about 5–8 seconds;
- CHRONOCLICK – after the door is closed, the seal drops with a quick, step-like movement accompanied by a characteristic click.
In both solutions, the seal lowers only after the door is closed. As a result, it does not rub against the floor or carpet while the leaf is moving, which reduces the potential damage.
What Does Using a Drop-Down Threshold Provide?
The basic task of a drop-down seal is to close the gap under the door leaf. Depending on the model and door construction, the solution can reduce the passage of:
- sound;
- drafts and cold air;
- dust and fine contaminants;
- odors;
- smoke;
- light;
- water;
- insects
This does not mean, however, that every seal provides all of the properties listed above. If the door must meet specific requirements for acoustic insulation, smoke control, fire resistance, or protection against water, the relevant parameters must be confirmed by testing and by the documentation appropriate for the given solution.
What Types of Doors Can Drop-Down Thresholds Be Used In?
Automatic seals are available in variants designed for different types of constructions. Depending on the specific model, they can be used in doors made of:
- wood;
- PVC
- aluminium
- steel
- glass
- entrance;
However, it is not enough to select a seal only for the material from which the door is made. It is also necessary to consider the construction of the leaf, its width, available installation space, the size of the gap above the floor, and the requirements for the entire door set.
Where Are Drop-Down Seals Used?
Drop-down thresholds are used in residential construction as well as in commercial facilities and public buildings.
Houses and Apartments
In residential buildings, the seal can be used, among other places, in exterior as well as interior doors: to a bedroom, study, or utility room (basement, garage). It helps reduce airflow and the passage of sound, smoke and light through the gap under the door. The absence of a fixed threshold can also be beneficial in places where continuity of the floor is important, for example with soft carpet or a historic floor.
Offices, Hotels, and Consulting Rooms
In offices, hotels, conference rooms, and consulting rooms, acoustic comfort and privacy are among the most important reasons for using a drop-down threshold.
The gap under the leaf is one of the places through which sound can pass. Properly sealing it can therefore support the acoustic properties of the whole door set, even up to Rw 52 dB.
Specialist Doors
Drop-down seals can be part of fire-rated, smoke-tight, or acoustic doors. In such applications, the product should not be selected solely on the basis of dimensions. It is necessary to check whether the specific model is intended for the given construction and whether its use is consistent with the relevant documentation.
Selected drop-down seal models have been tested for fire resistance. For specified variants, it is up to 120 minutes, according to APPLUS certificate no. 20/22763-1240.
Selected models also carry the cULus Listed mark and have been evaluated according to UL 10C, the standard concerning fire testing of door assemblies under positive pressure. This is particularly important in projects carried out for the American and Canadian markets.
If you’re wondering which solution will work best for a specific type of door, be sure to check out our guide on the different types of drop-down seals and their applications.
What to Consider When Choosing a Seal
The selection of the right seal should result from the door construction and the conditions in which the door will be used. Individual models can differ significantly in dimensions, drop range, and achieved parameters.
1. Size of the gap under the Door
Depending on the model, drop-down seals can close gaps from about 1 to 20 mm high.
For example, CHRONOSOFT and CHRONOCLICK models are designed for gaps up to about 12 mm, while selected self-adjusting solutions can work with gaps up to 20 mm. The exact drop range depends on the specific product.
2. Seal Length
The drop-down threshold should be matched to the width of the leaf. Depending on the model, seals are available in lengths from about 330 to 1830 mm.
For example, CHRONOSOFT and CHRONOCLICK models in the 15 × 30 mm version are available in lengths from 330 to 1530 mm, usually in 100 mm increments. The permissible trimming range depends on the starting length and, in these models, is from 10 to 150 mm.
The seal should be shortened only within the range and on the side indicated by the manufacturer, so as not to damage the activating button or the internal mechanism.
3. Installation method
Depending on the stage of seal installation, the following seals are used:
- recessed into the lower part of the leaf – installed during production;
- surface-mounted – installed on already mounted doors, for example during repair or renovation without needing to replace the entire door.
It is worth considering the installation method already at the door design stage. This makes it easier to prepare the right space in the leaf and reduces the need for later modifications.
4. Type and evenness of the foor
The seal should press against a properly prepared surface. Significant floor unevenness can make contact with the floor insufficient across part of the door width.
Selected models use a self-leveling spring mechanism that helps achieve even pressure from the seal.
5. Required Door Properties
Before choosing a seal, it is necessary to define what task it should perform. Requirements differ for standard interior doors and for constructions that are:
- acoustic;
- smoke-tight;
- fire-rated;
- required to protect against water.
For doors with specific technical requirements, the documentation appropriate for the specific solution should always be checked.
Traditional threshold vs. drop-down threshold – How do they differ?
| Traditional threshold | Drop-down threshold |
| It is a fixed element of the passage | The mechanism is mounted in or on the leaf |
| It remains visible after the door is opened | The seal retracts after the door is opened |
| It can create an obstacle in the passage | It allows a flat passage to be maintained |
| The leaf closes against a fixed threshold | The seal automatically lowers after the door is closed |
| It requires matching the threshold to the floor | It requires proper selection of the drop range and appropriate adjustment |
Is it worth using a drop-down seal?
A drop-down seal combines the possibility of sealing the door with maintaining a flat and convenient passage. This solution is worth considering already at the door-construction design stage, because it allows the leaf to be prepared properly and the right seal model to be selected.
Selected models can also be installed in existing doors, which increases the range of possible applications. This applies to both exterior and interior doors, including glass constructions.
The final choice should take into account not only the dimensions of the seal, but also the type of door, the size of the gap, the installation method, operating conditions, and the required properties of the entire door set.
Do you have questions or need help choosing the right seal? Contact us – we will be happy to help select a solution suitable for your project.
