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CROSS Safety Report

Fire protection considerations for roof structures in building design

Report ID: 1249 Published: 20 February 2024 Region: CROSS-UK


Overview

The reporter highlights concerns that Approved Document B (ADB) is being misapplied or misinterpreted when applying structural fire protections to a roof structure.

Additionally, the reporter highlights the need for design teams and clients to prioritise adequate fire protection measures for roofs, especially when they serve additional functions beyond merely supporting self-weight.

Key Learning Outcomes

For designers and engineers:

  • A fire in a compartment below the roof should not result in the rapid collapse of the roof due to the addition of loads upon it. Fire resisting construction for a time relevant to the building's nature (i.e. ADB Appendix B table B3) would likely be regarded as adequate

  • A fire on the roof should not threaten the means of escape of any roof occupants (for example, workers) or those in compartments below the roof. This might mean fire-resisting construction from the roof into the building is required or improved fire detection

For government and those writing building regulations guidance:

  • Guidance should clarify when the removal of structural fire resistance is acceptable given the changing nature of roof space usage, particularly regarding photovoltaic (PV) and battery storage systems

For fire and rescue services:

  • During firefighting, structural stability should form a critical part of any dynamic risk assessment with particular attention paid to: 1.) Any known fire resistance 2.) The effect of any imposed loads and 3.) The extent of fire involvement

Full Report

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The reporter highlights the increasing use of roof spaces, particularly for photovoltaic panels and air-handling equipment, and examples of schools and other large developments where the roof provides valuable space for such installations. The reporter suggests climate change drivers will continue to increase the use of these roof spaces for such equipment for new buildings, as well as additions to existing ones.

Fundamentally, this trend adds a combination of risk factors to what was previously often a sterile area, including:

  • Fuel loads

  • Ignition sources

  • Imposed loads

The reporter is concerned that designers are using ADB inappropriately to avoid adding structural fire resistance to a roof that supports plant and equipment, often using guidance as follows:

Firstly using Approved Document B Volume 2 Appendix A Elements of structure definitions:

Image
ADB Volume 2 Appendix A Definition of element of structure

Then using clause 7.3 of Approved Document B Volume 2 to omit the fire rating of the roof structure, arguing that the roof is not a floor and no structural fire protections are required.

Image
ADB Volume 2 Clause 7.3a Exclusions from the provisions for elements of structure

The reporter suggests this is a misapplication of the guidance and if the roof is intended to support additional functions beyond merely supporting self-load and structural stability (ADB clause 7.2a), then it should be designed as a floor and have no less fire resistance than the other elements of structure (see ADB Table B4).

a fire could start unnoticed in or on a roof and lead to premature collapse, particularly if heavy plant equipment is involved, causing death or serious injury to people below

The reporter states a fire could start unnoticed in or on a roof and lead to premature collapse, particularly if heavy plant equipment is involved, causing death or serious injury to people below. In addition, the reporter highlights the difficulties for firefighters carrying out firefighting operations under or on an unprotected roof.

The reporter, a fire engineer, advises design teams and clients that a roof structure is to be fire-protected when it clearly performs the function of a floor using the following examples:

  • A roof terrace/ amenity space

  • A roof that provides a means of escape route for day-to-day occupants

  • A roof that supports plant equipment (unenclosed), of which the roof is providing stability (for example, air conditioning units, smoke ventilation systems, generators, heavy PV installations and others)

  • A roof that supports plant equipment and installations which are part of the permanent construction (and therefore part of the day-to-day operation and running of the building)

The reporter considers that the intent of ADB is that fire protection to a roof can only be omitted where it supports only the roof itself and no other equipment, uses, or the stability of elements of structure and fire resistance.

Expert Panel Comments

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Intent of Building Regulations

The Building Regulations 2010 indicate that "the building shall be designed and constructed so that, in the event of fire, its stability will be maintained for a reasonable period".

The Panel acknowledge the concerns raised and agrees with the reporter that "the intent of ADB is that fire protection to a roof can only be omitted where it supports only the roof itself and no other equipment, uses or the stability of elements of structure and fire resistance."

Where the guidance is followed as intended, there should not be a problem. It is when the standards are not applied, understood, or misinterpreted that problems arise

Furthermore, the Expert Panel indicates that there is a valid concern here if there is a misapplication of ADB, or if ADB doesn't cover 'common building situations' in terms of what roof spaces/structures are now often being used for (for example, PV arrays) and how this might impact the means of escape in a fire.

This goes back to interpretation and due diligence by the designer and installer. Where the guidance is followed as intended, there should not be a problem. It is when the standards are not applied, understood, or are misinterpreted that problems arise.

Design considerations for existing buildings 

Little thought is given to the fire resistance of a roof after the initial construction has been completed and how the installation of new systems throughout the life of a building will have an impact on fire safety. 

The addition of PV panels on top of a roof requires the supplier to complete a survey of the roof space and roof members to assess the suitability of their addition. The Panel suggests design objectives should consider the effect of adding additional imposed load, fuel load and ignition sources onto a roof and the effect this may have on means of escape for relevant persons on the roof and below if a fire occurs in the installation. 

the compartmentation within the roof voids following the installation of new services should be adequately fire stopped by a competent person

Many existing buildings are also found where the internal roof space has no fire protection at all (often exposed timbers), yet now supports an increasing array of electrical cables, junction boxes, batteries, plant, water tanks and other items. It is foreseeable that a fire starting in this area could break into common escape routes or apartments below and should be considered in a fire risk assessment. 

In addition, the compartmentation within the roof voids following the installation of new services should be adequately fire stopped by a competent person.

Firefighting

The Expert Panel highlight there are substantial risks that firefighters face when fighting a fire at height or where there are risks to the structure from an incident. A good dynamic risk assessment is essential and defensive tactics should be considered.

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