Part B Building Regulations Explained – Fire Safety, Escape Windows & Fire-Rated Glazing
What is Part B?
Part B deals with fire safety.
For window and door contractors, it can affect:
- Means of escape
- Emergency escape windows
- Fire-resistant doors
- Fire-resistant windows
- Glazed screens
- Compartmentation
- External fire spread
- Fire-service access
This is one area where substituting products because they “look equivalent” can be extremely dangerous.
Emergency Escape Windows
Where an emergency escape window is required by the applicable guidance, the window needs to provide a genuinely usable escape route.
The current Approved Document B guidance for relevant dwelling situations states that an emergency escape window should provide an unobstructed openable area of at least 0.33m², with both the height and width of the opening at least 450mm, and the bottom of the openable area no more than 1100mm above floor level. The window should also be capable of remaining open without being held.
This means a window measuring 450 × 450mm does not automatically comply.
450mm × 450mm provides only 0.2025m².
The minimum dimensions and minimum area have to be achieved.
Don’t Use Overall Frame Size
This is one of the easiest mistakes to make.
Building Control is interested in the unobstructed openable area, not simply the overall dimensions of the frame.
The actual clear opening is affected by:
- Frame profiles
- Sash profiles
- Hinges
- Opening angle
- Restrictors
- Hardware
A seemingly large casement can therefore provide a surprisingly small escape opening.
Egress Hinges
Egress hinges can increase the clear opening available from a casement window.
Where a window forms part of an escape strategy, tell your supplier before manufacture.
Don’t assume ordinary friction stays will provide the necessary opening.
Window Restrictors
Restrictors create an important interaction between Part B and Part K.
A window may need restriction to reduce falling risk while also being intended for emergency escape.
Where both requirements apply, the hardware and opening arrangement need to be designed accordingly.
Do not fit arbitrary restrictors to an escape window without checking the consequences.
Fire-Rated Windows
Fire-resistant windows are completely different from ordinary windows containing “fire glass.”
A fire-resistant glazed element is a system.
Performance can depend upon:
- Frame system
- Glass
- Beads
- Gaskets
- Setting blocks
- Fixings
- Anchors
- Intumescent materials
- Maximum dimensions
- Supporting construction
- Installation method
Putting fire-resistant glass into a standard aluminium frame does not create a certified fire-resistant window.
EI30 and REI30
You’ll see classifications such as:
E – Integrity
Ability to resist passage of flames and hot gases.
I – Insulation
Ability to restrict temperature rise on the non-fire side.
R – Loadbearing capacity
Relevant where the element is required to maintain structural loadbearing performance.
Therefore:
EI30 generally means integrity and insulation performance for 30 minutes.
REI30 adds loadbearing capacity where that classification is applicable to the element.
Do not use EI30 and REI30 interchangeably simply because both contain “30”.
We’ll cover this properly in the dedicated REI30 vs EI30 guide.
Fire Doors
A fire door is also a complete doorset.
Its performance can depend upon:
- Frame
- Leaf
- Glass
- Hinges
- Locks
- Closers
- Seals
- Threshold
- Panic/emergency hardware
- Fixings
- Installation substrate
Changing one component can potentially take the installation outside the tested or classified scope.
Site Substitutions
This is where contractors can create serious problems.
Examples:
“We couldn’t get the specified closer, so we used another one.”
“The glass was delayed, so we fitted an equivalent.”
“We changed the hinges.”
“We used different expanding foam.”
“The opening was larger, so we made the frame bigger.”
Any of those changes could matter on a fire-rated assembly.
Installation
The installation itself forms part of achieving the intended performance.
Pay attention to:
- Opening construction
- Frame-to-wall gaps
- Fixing positions
- Fixing type
- Fire stopping
- Sealant
- Packers
- Threshold details
- Perimeter materials
Follow the evidence and manufacturer’s installation instructions.
Photographic Records
For commercial fire-rated installations, create a photographic record.
Photograph:
- Opening before installation
- Product labels
- Glass markings
- Fixings
- Perimeter gaps
- Fire stopping before covering
- Threshold
- Hardware
- Completed installation
This gives the main contractor and Building Control a much better evidence trail.
Common Part B Mistakes
Mistake: “It’s fire glass, therefore it’s a fire window.”
False.
Mistake: treating EI30 and REI30 as identical.
They aren’t.
Mistake: changing hardware without checking certification.
Potentially serious.
Mistake: measuring escape windows using overall frame size.
Wrong measurement.
Mistake: covering fire stopping before recording it.
Poor evidence management.
Mistake: assuming any competent window fitter can improvise installation details.
Fire-rated systems should be installed strictly in accordance with applicable evidence and instructions.
Real-World Example
A nursery project specifies an EI30 glazed aluminium doorset.
The contractor cannot simply buy an aluminium door, install EI30 glass and declare the completed product EI30.
The frame, glazing, hardware, seals, fixings and installation need to correspond with appropriate fire-performance evidence.
Before installation, the contractor should obtain the relevant documentation and installation instructions and retain records of the completed work.
Frequently Asked Questions
Does every bedroom need an escape window?
No. Requirements depend on building layout, storey and escape strategy.
What size is an escape window?
Where the cited Approved Document B provision applies, the unobstructed openable area should be at least 0.33m², with minimum height and width of 450mm and the bottom of the opening no more than 1100mm above floor level.
Does 450 × 450mm comply?
No. It doesn’t achieve 0.33m².
Can I fit fire glass into a normal aluminium frame?
That does not automatically create a fire-rated window.
Can hardware be changed on a fire-rated door?
Only where the proposed configuration remains supported by the relevant evidence.
Related Guides
Part B should internally link to Escape Window Regulations, Fire-Rated Windows, Fire-Rated Doors, EI30 vs REI30, Fire-Resistant Glass, Part K and the Building Control Checklist.
Trade Tech Technical Support
For fire-rated enquiries, ask the customer/main contractor for the required classification, fire duration, opening size, supporting construction, door/window configuration, hardware requirements and project specification before quoting.
That protects both supplier and installer.
Important: Fire safety is project-specific. This page should never be used as a substitute for the fire strategy, Building Control requirements, product classification/test evidence or manufacturer’s installation instructions.