Common Building Regulation Failures With Windows & Doors

Why Do Windows Fail Building Control?

Most problems aren’t caused by terrible windows.

They happen because a perfectly good window was wrongly specified for its location.

These are the issues builders should catch before manufacture.

1. Wrong U-Value

The specification asks for a particular whole-window Uw.

The supplier quotes the glass Ug.

Nobody notices until documentation is requested.

Avoid it: Ask specifically for the whole-window Uw.

2. Ordering Before the Energy Assessment

The frames are manufactured.

Then the SAP/energy assessor requires better thermal performance.

Avoid it: Finalise performance before ordering.

3. Missing Trickle Vents

The builder copies the existing windows and assumes:

“No old vents = no new vents.”

That is not a valid ventilation assessment.

Under the current Part F guidance, replacement-window ventilation needs to be assessed rather than simply copied visually from the old frame.

4. Wrong Vent Equivalent Area

The supplier says:

“It’s got a 400mm vent.”

But the specification is based on equivalent area, not the physical length of the vent.

Avoid it: Obtain the manufacturer’s EA figure.

5. Customer Refuses the Vents

The customer doesn’t like their appearance and asks the installer to remove them.

Avoid it: Customer preference does not replace applicable Building Regulations compliance.

6. PAS 24 Product Isn’t Actually Covered

The profile manufacturer advertises PAS 24 capability.

But the actual oversized sash or bespoke doorset may not be within the supplier’s evidence.

Part Q addresses resistance to unauthorised access in new dwellings, so the exact product configuration matters.

Avoid it: Request evidence for the exact configuration.

7. Laminated Glass Assumed to Equal PAS 24

Someone adds laminated glass and declares:

“Now it’s PAS 24.”

Wrong.

Security performance relates to the complete product.

8. Escape Window Too Small

The overall window looks large enough.

The actual opening is not.

Approved Document B’s relevant dwelling guidance specifies an unobstructed openable area of at least 0.33m², with minimum 450mm height and width.

Avoid it: Measure the actual unobstructed opening.

9. 450mm × 450mm Escape Opening

This misunderstanding refuses to die.

450 × 450mm = 0.2025m², not 0.33m².

It therefore does not meet the cited 0.33m² escape-opening area by itself.

10. Standard Hinges Reduce the Escape Opening

The sash theoretically has enough area but the hinge geometry blocks the route.

Avoid it: Specify appropriate egress hardware before manufacture.

11. Wrong Safety Glass

A low-level window or door sidelight is supplied in non-safety glass.

Part K contains the current glazing-safety guidance for critical impact locations.

Avoid it: Mark all safety-glazing locations on the survey.

12. Finished Floor Level Was Ignored

The window is surveyed from slab level.

Then the builder installs:

• Insulation
• Screed
• Underfloor heating
• Tiles

The relationship between glazing and floor level changes.

Avoid it: Survey from the intended finished floor level.

13. Toughened Glass Assumed to Be Fire Glass

Toughened safety glass is not automatically a fire-resistant glazing product.

Avoid it: Treat safety performance and fire resistance as separate specifications.

14. Fire Glass Put Into an Ordinary Frame

This is one of the most serious mistakes.

A fire-rated window is a classified/tested assembly.

Changing only the glass does not automatically give the frame the same fire classification.

15. EI30 and E30 Treated as the Same

They’re not.

E relates to integrity.

EI includes integrity and insulation.

Avoid it: Obtain the exact classification from the fire strategy.

16. Fire-Rated Product Made Too Large

A system has evidence for a particular size range.

The contractor enlarges it because:

“It’s only another 150mm.”

That can put the product outside the relevant evidence.

Avoid it: Check permitted dimensions before manufacture.

17. Wrong Fire Stopping

The door/window is correctly specified.

The installer then fills the perimeter with whatever foam happens to be in the van.

That can undermine the intended system.

Avoid it: Follow the tested/classified installation detail.

18. Solar-Control Glass Specified Too Late

Large west-facing sliders are ordered with standard glazing.

Then the Part O assessment requires a lower g-value.

Part O applies to new residential buildings and addresses overheating mitigation.

Avoid it: Obtain g-values before glass manufacture.

19. G-Value Confused With U-Value

The builder thinks a low U-value means the glass won’t cause overheating.

Wrong measurement.

U-value = thermal transmission.

G-value = solar gain.

20. Acoustic Glass Ruined by a Standard Vent

The project spends heavily on acoustic laminate but fits a basic ventilator facing a busy road.

Part F guidance specifically allows consideration of noise-attenuating background ventilation where external noise is significant.

21. Restrictor Conflicts With Escape

Someone fits a window restrictor after installation without checking whether the window provides emergency escape.

Avoid it: Coordinate Parts B and K.

22. Restrictor Conflicts With Part O

An overheating calculation relies on a large opening.

The installer then restricts it.

Avoid it: Check the approved opening strategy before selecting hardware.

23. Missing Glass Markings

The inspector asks whether low-level glazing is safety glass.

Nobody can establish it easily.

Avoid it: Check product markings and documentation before installation.

24. Unsupported Site Modifications

Examples include:

• Changing locks
• Changing closers
• Drilling additional holes
• Changing fire seals
• Swapping glazing
• Changing threshold
• Removing reinforcement

These can affect tested/certified product performance.

25. No Evidence File

This is the avoidable one.

The windows may actually comply, but nobody can produce:

• U-value calculations
• PAS 24 evidence
• Glass specification
• Fire classification
• Installation photographs

Months later everyone is searching old emails.

Avoid it: Create a compliance folder when the order is placed.

The £10,000 Question

Before pressing ORDER, ask:

“If Building Control asked me tomorrow to prove the thermal, ventilation, security, safety and fire performance of these products, could I?”

If the answer is no, the specification isn’t finished.

Builder’s Golden Rule

Do not manufacture first and prove compliance later.

The correct sequence is:

Drawings → Regulatory requirements → Performance specification → Product selection → Evidence → Manufacture → Installation → Handover.

That one process prevents a large percentage of window and door compliance problems.

Related Guides

• Building Control Checklist
• New-Build Window Regulations
• PAS 24 Explained
• Escape Window Regulations
• Toughened Glass Regulations
• Fire-Rated Windows Explained
• Trickle Vent Regulations
• Window U-Values Explained
• G-Values Explained

Need Us to Check a Window Schedule?

Send Trade Tech:

• Window/door drawings
• Specification
• Energy requirements
• Security requirements
• Ventilation schedule
• Fire requirements
• Overheating specification

before you place the order.

We can flag obvious glazing and window-specification issues at quotation stage, when they are far cheaper to correct.