Flush Threshold Doors with Minimal Windows

Flush threshold doors are an increasingly important feature of contemporary architectural design. From modern extensions and new-build homes to accessible residential projects, a flush threshold creates a smooth, step-free transition between internal and external spaces. 

When flush threshold doors are open, the boundary between the home and garden can feel almost completely removed. Continuous floor finishes, minimal framing and level access help merge indoor and outdoor spaces, creating a stronger connection between the architecture and its surroundings. 

For sliding glass doors, the threshold detail is a key part of achieving this seamless design. The minimal windows® range offers several threshold and base options designed to create flush, step-free transitions while maintaining the weather performance required for residential architecture. 

What are flush threshold doors?

Flush threshold doors are designed with little or no change in floor level between internal and external spaces. The door frame and threshold are integrated into the floor construction, allowing the internal and external floor finishes to meet with a smooth and continuous transition. 

With a flush threshold sliding door, the base frame is recessed into the floor structure so that the threshold does not project significantly above the finished floor level. 

How do you achieve a flush threshold door?

A flush threshold detail can be achieved by first specifying a sliding door system designed with this detail as part of its base configuration. The internal and external floor finishes must then be designed to sit at the same level. 

When the sliding door frame is integrated into the building structure, the top of the aluminium base frame should sit in line with the internal and external floor finishes. Achieving this detail requires close collaboration between the architect, builder and architectural glazier. 

There must also be sufficient space below the finished floor level for the aluminium base frame to be recessed, allowing the top of the frame to sit level with the surrounding floor finishes. 

What is the difference between a low threshold and a flush threshold?

A flush threshold patio door is different from a low threshold patio door. 

The term 'low threshold' is often used to describe bi-folding doors or typical aluminium sliding systems. However, a low threshold is not fully flush and still involves a change in level between the internal and external spaces or across the frame. For these types of systems, this level change is often required to achieve the necessary weather performance. 

Premium sliding door systems such as minimal windows® do not require this level change to achieve the high levels of weather resistance required for residential projects. All base options for minimal windows® are designed with a fully flush threshold, creating a step-free and level transition from inside to outside. 

What are the advantages of flush threshold doors?

There are various advantages to specifying a flush threshold detail for a patio door. From a design perspective, the absence of a raised frame or structure at the base of the glass doors allows the connection between internal and external spaces to continue even when the doors are closed. In many projects, it can be difficult to tell from images alone whether the glass doors are open or closed. 

This continuation of building and living space from inside to outside is a desirable feature for many contemporary home projects. 

In terms of usability, a flush threshold creates an easier transition between the internal and external living spaces of a building. This can be particularly beneficial on swimming pool projects where the threshold of the sliding door is likely to be walked over in bare feet. A flat threshold that sits level with the floor finishes can provide a more comfortable transition for users. 

Flush thresholds also have an important role in accessible and inclusive building design. The minimal windows® sliding door thresholds have been tested and certified for barrier-free accessibility, demonstrating their suitability for wheelchair users and accessible environments. A flush threshold allows wheelchairs and other mobility devices to move across the frame without a step, level change or obstruction. 

This detail can be seen to great effect in the accessible building design for Claywood House. All exterior doors incorporated a flush threshold to provide wheelchair access through the minimal windows® doorways, allowing the architects to create an inclusive home design for the wheelchair user and their family. 

How do flush threshold doors maintain weather resistance?

When specifying a flush threshold patio door, it is important to ensure that the system maintains its weather resistance despite the level base detail. Sliding door systems have an advantage over bifold doors in this area due to the recessed position of the sliding glass panels. 

The sliding glass panels within minimal windows® systems sit recessed within the base frame of the doors. This overlapping relationship between the frame and glass helps create the robust weather resistance expected of high-performance architectural glazing. 

This allows systems such as minimal windows® to achieve high levels of water and wind resistance while maintaining a completely flat threshold detail. The minimal windows® 4+ system, for example, uses a flush threshold detail and has been tested to hurricane levels of wind and water resistance. 

Do flush threshold doors require drainage?

Yes. Drainage is an important consideration when designing flush threshold doors. 

When floor finishes continue from inside to outside, the design must ensure that surface water and external water run-off are managed correctly. A slim sliding door system should incorporate built-in drainage within the base frame itself, alongside appropriate drainage outside the flush threshold. 

The external drainage solution will depend on the requirements of the individual building project and the wider construction details. Various drainage options are available for minimal windows® systems and can be designed to maintain a flush transition between the internal and external spaces. 

Whether using a hidden slot drain or a higher-volume external drainage channel, the transition from inside to outside can remain flush while providing the drainage required for high levels of rain resistance. 

How do I specify flush threshold doors for my project?

If you are looking to include a flush threshold patio door in your project, the first step is to ensure that the sliding door system has been designed to accommodate this detail. All minimal windows® sliding door systems and base options have been designed and tested with a flush threshold configuration. 

The integration of a flush threshold patio door should be planned as early as possible. Your builder will need access to the base details of the sliding glass door system to ensure there is sufficient space below the finished floor level for the frame and drainage requirements. 

By addressing these interfaces early in the project, it is possible to achieve a flush and continuous transition from inside to outside. 

All minimal windows® sliding door systems have been designed to accommodate flush threshold details. If you would like to include a flush threshold patio door in your project, contact the team at minimal windows® to find out how a premium sliding door can be integrated into your project.