Doors, windows and glazing

6.7.3In-service performance

Doors, windows and glazing shall be designed, specified, performance tested and classified to ensure adequate in-service performance.

  1. weathertightness
  2. fire safety
  3. thermal break
  4. strength
  5. resistance to movement, shrinkage and the effects of moisture.


Doors and windows should be installed correctly to ensure adequate in-service performance. Windows and external doors exposed to wind-driven rain should be constructed and detailed to ensure they remain weathertight, including at interfaces with the structure.

BS 6375 contains guidance on suitable levels of performance for windows and doors to be used in the UK.

Window and door frame assemblies contained within a single storey should be:

  • designed as an engineered system to support it's self weight and imposed loads safely to the supporting structure,
  • designed with a dual sealed approach to joints comprising outer seals as the primary water barrier, inner seals providing air and secondary water barriers and a drained cavity to remove any water that by passes the outer seals,
  • formed as a system using products and materials fully specified by the manufacturer for required service life performance and durability.

Air and watertightness testing of a ‘prototype’ assembly should be carried out in accordance with BS EN 1435-1 Windows and doors. Product standard, performance characteristics. Part 1. Windows and external pedestrian doorsets. The assembly tested should be of a similar size and configuration as intended to be used on the building and include all jointing details.

Wind resistance, serviceability and safety testing should be carried out in accordance with BS EN 1435-1. When tested for wind load resistance the main structural members of the assembly shall satisfy class B for which the allowable deflection is span/200 under the design wind load in both positive and negative directions.

Further guidance is also contained in CWCT Technical Note 95 Weathertightness of windows, doors, window assemblies and curtain walls.

Vertical and horizontal DPCs should be provided around the frame in accordance with Chapter 6.1 ‘External masonry walls’ and Chapter 6.2 ‘External timber framed walls’.

DPCs should:

  • be correctly installed
  • extend approximately 25mm into the cavity
  • be continuous for the full height of the frame
  • proprietary insulated cavity closures that function as a DPC should be in accordance with manufacturers guidance and third-party certification.

When placing frames for external elements in openings, ensure:

  • the head of the frame is protected by the lintel
  • throatings in sill members are not obstructed by the wall face.

Additional precautions include:

  • setting the frame back from the facade
  • building a projecting porch
  • providing a rain check groove to inward opening external door frames
  • fixing weatherboards and water bars to external doors, but ensuring the threshold is accessible where appropriate.

In Scotland, Northern Ireland and areas of very severe exposure, ‘check’ reveals should be used, and an appropriate sealant applied between door/window frames and the structure.

Fire safety

Fire-resisting doors and positive self-closing devices should be fitted where they are required by building regulations.

Thermal break

Metal windows should incorporate a thermal break.


Door frames, windows and their fittings should be adequate to withstand operational loads.

Structural loads should be carried on lintels, beams or appropriate structural elements. Where frames are required to carry structural loads, they should be designed accordingly.

Resistance to movement, shrinkage and the effects of moisture

Doors and windows should be designed to:

  • avoid significant distortion, such as twisting and bowing during use
  • take account of timber shrinkage
  • be moisture resistant, including window boards.