The Expert’s Guide to Bifold Doors: Thresholds, Traffic Doors & Technical Specs

Everything you need to know about weather ratings, configurations, and building regulations before you buy.

Understanding the technical components of a bifold—like thresholds and traffic doors—is crucial before ordering.

Most bifold door buying guides stop at “they let the light in and look nice.” But if you are planning to spend thousands of pounds on a major home improvement or extension, you need the actual technical details.

At KJM Group, installing hundreds of doors across Hampshire every year, we know that the success of a bifold door relies entirely on how it is specified. You need to understand how the water drainage works, how the doors will physically stack, and how to stay fully legal with local Building Regulations.

We want you to be an informed buyer. This technical guide covers the three critical decisions you need to make before ordering: Thresholds, Configuration, and Regulations.

1. The Threshold Dilemma: Flush vs. Weathered

The most common request we get in our showroom is: “I want a completely flush floor from my kitchen straight out to my patio, with no step at all.”

While this looks incredible in architectural magazines and on Instagram, there is a harsh reality to face in the UK: True flush thresholds are not fully weather-proof.

Because a flush threshold is sunk completely level into the floor, it has no “upstand” or rebate. This means there is no physical barrier for the rubber door seals to compress against when the door is closed. They rely entirely on brush seals, which offer very little resistance to wind-driven rain, resulting in drafts and potential water pooling inside your home.

2. Configuration Logic: The “Traffic Door” Secret

If you have a wide aperture, the configuration—meaning how many doors there are and which way they slide—is critical for daily usability. The golden rule of bifold design is aiming for a Traffic Door.

A Traffic Door (sometimes called a lead door or master door) is a single panel that opens independently on a standard hinge, exactly like a normal back door. This allows you to pop out to the garden quickly without having to unlock and push back the entire heavy, multi-panel folding mechanism.

To guarantee a traffic door, you generally need to specify an odd number of panels swinging in one direction (e.g., all 3 doors sliding left, or 5 doors sliding right).

3. Solving the “Goldfish Bowl” Problem

Bifold doors are essentially an entire wall made of glass. In the height of summer, a south-facing bifold acts exactly like a greenhouse, leading to severe solar gain (overheating). In winter evenings, with the lights on inside, it can feel like you are living in a goldfish bowl with zero privacy.

Traditional heavy curtains often get caught in the folding mechanism, and standard blinds flap wildly when the doors are opened. This is why we strongly recommend specifying Integral Blinds. These are sleek, Venetian-style blinds permanently sealed inside the argon gas cavity of the double-glazed unit. They never need dusting, they cannot get damaged, and they fold seamlessly with the door panels.

👉 Learn more about KJM Integral Blinds technology here.

4. Building Regulations (Part L & F)

Since the major national update to UK Building Regulations in 2022, compliance for new doors has become significantly stricter.

• Part L (Conservation of Fuel & Power): Any new door installed in an existing dwelling must achieve a highly efficient thermal U-Value of 1.4 W/m²K or better. Our premium Aluminium systems (including Smart Visofold and Korniche) easily meet these strict thermal targets via advanced polyamide thermal breaks.
• Part F (Ventilation): If you are replacing older patio doors that had trickle vents, the new bifolds must also have them. Furthermore, even if the old ones didn’t, if the room lacks adequate background ventilation, Building Control will usually require us to install discreet trickle vents into the upper bifold frame to ensure healthy indoor air quality and prevent black mould.

5. Hampshire Local Installation Advice

6. Frequently Asked Questions