Flush Threshold Detail & Drainage: A Complete Guide

A flush or level threshold is when the flow between interior and exterior spaces occurs with no step and no change in level – it effectively merges the spaces by removing that level change or ‘lip’.

For example, Sky-Frame Plain builds the framework into the floor. Offering a frictionless motion, the panes run on slim tracks – which leaves a 10mm slot – fully sunk into the floor. Products such as the Sky-Frame Plain show how flush thresholds can seamlessly integrate into the structure and design of the building.

It’s worth noting that the track changes in depth depending on whether you’re planning for two, three or four panel doors.

As attractive as this design element is to modern residential design, flush thresholds can present an architectural challenge when it comes to drainage. You’ll need to ensure that the chosen opening flush threshold doors are fully detailed and capable of dealing with this crucial element of performance. 

With any flush or level threshold, it’s important to allow rainwater falling down the doors to be collected and then channelled away via the appropriate gulley . When the floor effectively flows from outside to inside, any openings need to be designed to prevent the external elements from crossing that level threshold. 

This is an area where architects have generally been presented with a one-size-fits-all approach. This approach is based on the need to remove water from large areas of concrete (such as public car-parks) and leave a lot to be desired when it comes to the finesse expected in residential and commercial patio areas. There are solutions that are more refined, but these need to be expertly designed and detailed to work on larger openings, which might require multiple joints and loose covers.

Sliding glass doors can offer the highest level of weather sealing whilst creating that desired level threshold, if done correctly. As the sliding door panels extend down into the floor, it helps create that weather seal, enabling these large elevations of sliding glass to have very high testing results for water and air tightness.

This is where a collaborative approach to design between the contractor, architect, and glazing specialist is crucial. The tricky bit is the underfloor detailing; on our projects, for instance, Cantifix typically develops the detail between the gutter and the door seal, using any combination of machined drainage slots, seals, 3D-printed spigots, and profile grooves – but getting these in place effectively requires teamwork with the contractor. 

The contractor on the project will be responsible for sorting the water-proof membranes that line most building openings, which will impact how the specialists then design and detail the track, and any other elements of the design. Nurturing and facilitating a good working relationship between all of the various companies should be high on the priority list for architects, as well as choosing specialists who approach these kinds of projects collaboratively. 

Of course, you don’t always need a low-level threshold on sliding doors – many installations continue to use the standard arrangement, which includes a suitable track height for most client requirements. Raised door sills are often only 44-50mm high. However, it’s an increasingly popular feature for modern architectural designs, as it looks aesthetically pleasing and completes a frameless glazing solution, with visibly reduced aluminium sightlines compared to a regular sliding door track.

With more clients wanting to connect the patio area to the indoors, a low track and flush threshold forms part of the design, and creates a more fluid transition from the inside to the outside space.

It also has a range of practical benefits, like ensuring that guests don’t trip on an awkward step, or elderly relatives using a walking stick or wheelchair find the lower step easier. Even at 45-50mm high, a raised threshold can pose a trip hazard, particularly for the elderly and the very young. A flush threshold eliminates this risk.

In order to achieve a flush threshold, there are four main things to consider:

• Concept and design: as you plan the project, incorporate the design of the flush threshold from the beginning. This will avoid potentially costly or complicated changes later in the project.
• Choose the right sliding door: Not all sliding door systems allow you to create a flush threshold. Research to find the system which best suits your plans, from threshold, panel size, thermal performance and frame sight lines.
• Confirm flooring types and levels: To achieve the flush threshold, you’ll need to have confirmed full details about the room’s flooring for the internal and external areas where the sliding doors will be installed.
• Choose a glazing specialist: Find a reliable team to make the design and installation a smooth process, from enquiry through to survey, installation and aftercare. (This is where Cantifix can help!).

Flush thresholds are a challenge, but one that with the right approach can certainly be overcome. These kinds of finishes are the things that can set a project apart aesthetically, and by factoring in all of the above – and approaching your project in a collaborative, creative, and innovative way, you can ensure the level of practical and visual performance that your clients expect. 

If you’d like to talk about your own project, our experienced in-house team are always on hand. Simply head over to our contact page and fill in the form, and the Cantifix team will be in touch with you directly.

Alternatively, if you’d like some inspiration for your next project, visit our case studies page.

We look forward to hearing from you.