Bifold Doors for Seamless Indoor–Outdoor Transitions
Bifold doors designed to create a level, flush transition between internal floors and external paving are commonly referred to as low-threshold, flush-threshold, or level-access bifold doors. Where accessibility compliance is required, they may also be described as Part M compliant thresholds.
These systems are engineered to minimise the upstand at the door track, allowing a near-seamless connection between inside and outside spaces. They are frequently specified for patios, terraces, and contemporary residential extensions.
Due to the susceptibility of water ingress and damp issues with level thresholds, it is essential that the correct bifold doors are chosen and paving is laid to meet the following criteria.
Weather Performance Requirements
External bifold doors in the UK are tested under BS EN 14351-1 (External Pedestrian Doorsets) and classified for performance in the following areas:
- Air Permeability – BS EN 12207
- Minimum recommended: Class 3
- Preferred: Class 4
- Water Tightness – BS EN 12208
- Minimum recommended: Class 7A
- Preferred for exposed elevations: Class 8A or 9A
- Highly exposed sites may require enhanced ratings such as E750 or higher.
- Wind Resistance – BS EN 12210.
- Minimum recommended: Class C3.
- Preferred for exposed locations: Class C4.
Technical Considerations for Flush Thresholds
Reducing the threshold upstand can increase the risk of water ingress during wind-driven rain. To maintain performance, quality systems incorporate:
- Integrated drainage channels within the frame.
- Multi-chambered seals and gaskets.
- Rebated track systems.
- External linear slot drains where necessary.
Proper external paving design is critical. Surfaces must fall away from the building at a minimum gradient of 1:60, and damp protection measures must be maintained.
Regulatory Considerations
Where bifold doors provide primary access to a dwelling, thresholds should comply with Approved Document M (Access to and Use of Buildings).
Finished external ground levels should normally remain at least 150 mm below internal floor level (damp-proof course), unless alternative damp-proof detailing is incorporated.
Correct specification of threshold type, drainage provision, falls on paving and weather rating is essential to ensure durability, compliance, and long-term performance in the UK climate.
