Toughened Glass Regulations Explained

Where Do I Need Toughened Glass?

This is another question that builders frequently oversimplify.

The regulation is not simply:

“Anything below 800mm must be toughened.”

Part K deals with protection against impact with glazing and identifies critical glazing locations.

In these locations, appropriate measures are required to reduce the risk of injury if somebody collides with the glass.

Using safety glass that breaks safely is one common solution.

That may include:

• Toughened safety glass
• Laminated safety glass

depending on the application and required classification.

The Quick Rule for Builders

The critical locations most window installers need to recognise include glazing:

Within doors: up to 1500mm above floor level.

Adjacent to doors: within 300mm horizontally of the door opening and up to 1500mm above floor level.

Other low-level glazing: below 800mm above floor level.

These dimensions should be understood properly rather than reduced to “all low glass needs toughening.”

Critical Location 1 – Glazing in Doors

Doors are high-risk locations because people naturally move towards and through them.

Examples include:

• Entrance doors
• French doors
• Patio doors
• Bifold doors
• Internal glazed doors
• Commercial doors

Relevant glazing in these critical zones needs appropriate safety performance.

Critical Location 2 – Glazing Next to Doors

Sidelights are another common critical location.

If glazing is within 300mm horizontally of a door opening and within the relevant height zone up to 1500mm above floor level, it falls within the critical-location guidance.

This is why front-door sidelights frequently require safety glazing.

Critical Location 3 – Low-Level Windows

Glazing in walls with its lower part below 800mm above floor level falls within the low-level critical zone described by Approved Document K.

This commonly affects:

• Full-height windows
• Low-cill windows
• Fixed glazing
• Glazed screens

Finished Floor Level Matters

This catches builders out.

The critical height is assessed relative to floor level.

Suppose a window is surveyed before the floor is finished.

The builder later installs:

• 100mm insulation
• Screed
• Underfloor heating
• Tiles

The relationship between the glass and final floor level changes.

Use the correct finished floor level when determining glazing requirements.

Does It Have to Be Toughened Glass?

Not necessarily.

Approved Document K provides more than one route for glazing in critical locations.

Depending on the situation, glazing may:

• Break safely
• Be sufficiently robust
• Use qualifying small panes
• Be permanently protected

Toughened safety glass is extremely common, but don’t publish the misleading statement:

“Building Regulations say every critical pane must be toughened.”

Laminated safety glass can also be appropriate.

What Is Toughened Glass?

Toughened glass is thermally processed to increase its strength and alter its breakage characteristics.

When broken, it is designed to fragment differently from ordinary annealed glass, reducing the risk of large dangerous shards.

It is commonly used in:

• Doors
• Bifolds
• Patio doors
• Low-level windows
• Shower screens
• Commercial glazing

where the appropriate specification requires it.

Toughened vs Laminated Glass

This deserves its own article because the two products behave differently.

Toughened

When sufficiently stressed, it typically fragments into many relatively small pieces.

Laminated

An interlayer holds broken glass fragments together.

Laminated glass may be selected where additional requirements exist for:

• Security
• Acoustic performance
• Residual containment
• Other project-specific performance

Neither should be treated as universally superior.

Use the product appropriate to the application.

Toughened Glass and PAS 24

Safety and security are different performance requirements.

A pane being toughened does not mean it satisfies every security requirement.

Similarly, laminated glass should not automatically be described as PAS 24 glass without reference to the tested product configuration.

For PAS 24 applications, follow the glazing specification covered by the relevant product evidence.

Toughened Glass and Part Q

Part Q deals with security in new dwellings.

Part K deals with protection from collision and impact.

A pane may need to satisfy both requirements.

Don’t solve one and ignore the other.

Toughened Glass and Fire Resistance

Ordinary toughened safety glass is not automatically fire-resistant glazing.

This distinction is critical.

A pane can be:

Safety glass

without being:

Fire-resistant glass.

Fire-rated windows and doors require appropriately tested/classified systems.

Do not substitute ordinary toughened glass into a fire-resistant assembly.

Toughened Glass and Acoustic Performance

Toughening ordinary glass doesn’t automatically create high acoustic performance.

Acoustic specifications may instead use:

• Different pane thicknesses
• Laminated acoustic glass
• Asymmetric sealed units
• Specialist interlayers

Safety requirements still need to be met where applicable.

Safety Markings

Safety glazing products should carry appropriate permanent identification/marking in accordance with the applicable product standard and specification.

Installers should not casually assume an unmarked pane is safety glass because:

“The supplier said it was toughened.”

Check the product and documentation.

Can Toughened Glass Be Cut on Site?

No.

Once glass has been toughened, you should not attempt to:

• Cut it
• Drill it
• Grind significant material from it

The glass needs to be manufactured to the correct dimensions and with required holes/notches before the toughening process.

Accurate surveying matters.

Nickel Sulphide Inclusions

Very occasionally, toughened glass can suffer spontaneous breakage associated with nickel sulphide inclusions.

Where a project specification identifies a requirement for heat-soak testing or other measures, follow that specification.

Do not automatically promise that all toughened glass is heat-soak tested unless that has actually been specified and supplied.

Doors With Small Glass Panes

Another misconception is that every tiny piece of glass in every door must always be toughened.

Approved Document K contains provisions for robust glazing and small panes in certain circumstances.

However, don’t improvise.

For modern window and door supply, using appropriately classified safety glazing in recognised critical locations is often the straightforward route.

Conservatories and Glazed Extensions

Large areas of floor-to-ceiling glazing make safety-glazing assessment particularly important.

Check:

• Doors
• Door sidelights
• Low-level fixed glazing
• Internal screens
• Glazed returns

Don’t assume that because the unit is double glazed it is automatically safety glass.

Schools and Nurseries

Schools and nurseries deserve additional consideration because of the way occupants interact with doors and glazing.

Project specifications may exceed the minimum Building Regulations guidance.

Check:

• Safety glazing
• Manifestation
• Restrictors
• Guarding
• Security
• Fire requirements

before manufacture.

Commercial Glazing

Commercial buildings can also require manifestation so that large transparent glazed areas are apparent to building users.

This is separate from the glass simply being toughened.

A perfectly safe toughened pane can still present a collision risk if occupants don’t realise it is there.

Common Safety-Glass Mistakes

Mistake 1 – “Everything below 800mm must be toughened.”

Oversimplified.

Mistake 2 – Forgetting sidelights.

Glazing adjacent to doors can be a critical location.

Mistake 3 – Measuring from unfinished floor level.

Use the correct final relationship.

Mistake 4 – Assuming laminated glass and toughened glass are identical.

They aren’t.

Mistake 5 – Assuming toughened means fire rated.

It doesn’t.

Mistake 6 – Assuming toughened means PAS 24.

It doesn’t.

Mistake 7 – Cutting toughened glass on site.

You can’t treat toughened glass like ordinary annealed glass.

Real-World Example 1 – Front Door

A builder orders a new entrance door with a full-height glazed sidelight.

The sidelight is only 150mm from the door opening.

The glazing extends almost to floor level.

This should immediately trigger a safety-glazing assessment because the glass sits within a critical impact location.

Real-World Example 2 – Kitchen Window

A kitchen window is surveyed while the room is stripped out.

After underfloor heating, screed and tiles are installed, the finished floor is substantially higher.

The bottom of the glazing is now within the low-level critical zone.

Had the final floor level been considered during surveying, the appropriate glass could have been specified initially.

Real-World Example 3 – Bifold Door

A four-panel bifold is almost entirely glazed.

The contractor asks:

“Does it need toughened glass?”

Rather than answering solely from the fact that it is a bifold, the supplier should consider the applicable safety-glazing requirements and any additional:

• Security
• Thermal
• Solar
• Acoustic

requirements.

The final glass specification may need to satisfy several performance criteria simultaneously.

Builder’s Safety-Glazing Checklist

Before ordering, identify:

• Glass in doors
• Glass within 300mm of door openings
• Glass below 800mm from floor level
• Full-height glazing
• Finished floor levels
• Schools/nurseries
• Guarding requirements
• PAS 24 requirements
• Fire requirements
• Acoustic requirements
• Any project-specific glass specification

Frequently Asked Questions

When is safety glass required?

Part K identifies critical impact locations including relevant glazing in doors, adjacent to doors and low-level glazing.

Is all glass below 800mm required to be toughened?

Critical low-level glazing requires appropriate protection, but toughened glass is not the only possible route recognised by Approved Document K.

How close to a door is considered a critical location?

The Approved Document diagram identifies glazing within 300mm horizontally of the door opening, within the relevant height zone.

What height applies to door glazing?

The critical zone extends up to 1500mm above floor level for doors and adjacent panels as shown in Approved Document K.

Is laminated glass safety glass?

Appropriately classified laminated glass can provide safety performance.

Is toughened glass fire rated?

Not automatically.

Is toughened glass PAS 24?

PAS 24 applies to the performance of the complete tested/certified window or doorset, not simply the glass.

Can toughened glass be cut after manufacture?

No. Required processing should be carried out before toughening.

Should safety glass have a permanent mark?

Appropriate safety glazing should carry the identification required by the applicable standard/product specification.

Related Guides

• Part K Explained
• Laminated Glass Guide
• PAS 24 Explained
• Escape Window Regulations
• Fire-Rated Windows
• Building Control Checklist

Trade Tech Technical Support

When requesting a quotation, identify any:

• Low-level glazing
• Doors
• Sidelights
• Full-height screens
• Schools/nurseries
• Security glazing
• Fire glazing

on the drawings.

That allows the glass specification to be checked before manufacture rather than after Building Control identifies a problem.

Important: This guide explains the principal Part K safety-glazing considerations in England. Individual projects may require additional safety, guarding, fire, security or specialist glazing measures.