Guide to Conversion of Traditional Buildings

2.3 Structural protection

Standard 2.3

Every building must be designed and constructed in such a way that in the event of an outbreak of fire within the building, the load-bearing capacity of the building will continue to function until all occupants have escaped, or been assisted to escape, from the building and any fire containment measures have been initiated.

2.3.1 Type of standard

Mandatory Standard

In the case of conversions, as specified in regulation 4, the building as converted shall meet the requirement of this standard (regulation 12, schedule 6).

2.3.2 Commentary

For new build, this is a relatively straightforward standard that presents little difficulty to the designer. However, the conversion of a traditional building can raise complex issues relating to the level of fire protection offered by an element of structure. Modern building materials comply with relevant British and European standards and fire test certifications. However, traditional historic materials may not be as well understood and often lack such certification. A designer or verifier who lacks experience in converting historic buildings might adopt a conservative approach in the absence of test data, potentially leading to over-designing structural fire protection. This could have a negative impact on the building’s character and historic fabric.

It is essential, therefore, to understand the way in which materials have been combined to form an element of structure, and to appreciate how both the individual materials and materials used in combination affect the fire performance of the element. Such an understanding will consider the effects of age, physical and chemical degradation, applied treatments and any other forms of degradation that will have an impact on fire performance. It is also important to know the likely impact that any applied protective coatings or finishes may have on the historic character of the element. More detailed advice on the performance of historic materials in fire is available in Historic Scotland’s Guide for Practitioners 7: Fire Safety Management in Traditional Buildings (2010).

It is clear, therefore, that before embarking on the design of a historic building conversion it will be necessary to conduct a full survey and fire risk assessment of the structural elements and their likely performance in fire. In the absence of such a survey, it will be impossible to determine the structural performance of the building in fire, or the extent to which upgrading of the elements – or the introduction of other measures - will be necessary. However, experience of real fires in historic buildings suggests that, in many cases, the load-bearing capacity of structural elements will be maintained for 30 minutes or more. For example, timber is a combustible material, but the degree of fire resistance increases with thickness; the formation of a char layer further inhibits the effect of fire. However, the condition of the timber is vital to the assessment of performance as fungal decay and insect attack, for example, will have a significant effect. Traditional lime or gypsum plasters have an inherent fire resistance when in sound condition, but where the plaster is not firmly adhering to the substrate – especially keying through the laths – its ability to resist fire is reduced significantly.

Failure of a structural element may not be due simply to the combustibility of the materials – the effects of heat can cause distortion resulting in the collapse of supported elements. For example, the expansion, buckling or other deformation of steel columns or beams in the initial stages of a fire can cause the collapse of supported floors. The performance of cast iron15 depends on how quickly the heat rises and the applied load.

For single storey and multi-storey buildings, the Tables 2.1 & 2.2 to clause 2.1.1 in the Non-Domestic Technical Handbook and Table 2.1 to clause 2.3.1 in the Domestic Technical Handbook should be referred to for the fire resistance durations required for elements of structure. For example, a multi-storey building with no floors at a height of 7.5m above the ground could (depending on its use and compartment area) trigger a requirement for short, or medium or long fire resistance duration for its elements of structure.

2.3.3 Issues to be considered

Issue - Risks to historic / traditional buildings

1. Elements of structure Traditional buildings of limited height and size with no floor more than 7.5 metres above the ground will only need a short fire resistance duration, which can usually be accommodated without significant disruption to historic finishes and materials. The exceptions are separating walls and floors between dwellings, adjoining walls and floors between domestic and non-domestic or common areas in different occupation, and care homes, all of which require medium fire resistance. Buildings with a floor height between 7.5 and 18 metres above the ground, more than medium fire resistance duration is required. Most traditional buildings will not achieve structural Fire Protection of this order without improvements to the fire resistance which may damage historic materials and finishes. Specialist advice is necessary. Buildings with floors at a height more than 18 metres above the ground are required to have a long fire resistance duration. To improve structural fire protection, it is likely there will be an extreme loss of historic fabric. Specialist advice is essential.

2. Combustibility It is normal to find combustible elements of structure in traditional buildings. Their presence is acceptable providing the element achieves the appropriate fire resistance. Structural timber, while combustible, may have appropriate fire resistance duration for the given situation. Unnecessary improvements to fire resistance of timber can be damaging to its historic character.

3. Openings and service penetrations Although unlikely to present a risk to structural stability, past alterations to structural floors may have adversely affected their fire resistance. Further interventions to improve fire resistance may be harmful to historic fabric.

4. Junctions Fire stopping at junctions of elements of structure might be required where weaknesses occur. Fire stopping may result in damage to historic fabric.

a) Elements of structure.

Traditional buildings of limited height and size with no floor more than 7.5 metres above the ground will only need a short fire resistance duration, which can usually be accommodated without significant disruption to historic finishes and materials. The exceptions are separating walls and floors between dwellings, adjoining walls and floors between domestic and non-domestic or common areas in different occupation, and care homes, all of which require medium fire resistance.

Where a building has floors at heights between 7.5 metres and 18 metres, more than a medium fire resistance duration is required. Most traditional buildings will not achieve structural Fire Protection of this order without improvements to the fire resistance which may damage historic fabric and finishes. Specialist advice is necessary.

Buildings with a floor at a height of over 18 metres are required to have a long fire resistance duration. To improve structural fire protection, it is likely there will be an extreme loss of historic fabric. Specialist advice is essential.

b) Combustibility.

It is normal to find combustible elements of structure in traditional buildings. Their presence is acceptable providing the element achieves the appropriate fire resistance. While combustible, structural timber may have appropriate fire resistance duration for the given situation. Unnecessary improvements to fire resistance of timber can be damaging to a building’s historic character.

c) Openings and service penetrations.

Although unlikely to present a risk to structural stability, past alterations to structural floors may have adversely affected their fire resistance. Further interventions to improve fire resistance may be harmful to historic fabric.

d) Junctions.

Fire stopping at junctions of elements of structure might be required where weaknesses occur. Fire stopping may result in damage to historic fabric.

2.3.4 Recommendations to meet the standard

It is recommended that specialist advice for design and construction should be sought from those with appropriate heritage and fire expertise. It may be appropriate to install an automatic fire suppression system (see guidance to standard 2.15) and an enhanced automatic fire detection and alarm system, for example a Category L1 to BS 5839- 1 2017, to ensure the earliest possible warning in the event of an outbreak of fire.

a) Fire risk assessment

Although most main structural features have some inherent fire resistance, a comprehensive fire risk assessment and hazard reduction exercise can demonstrate an adequate level of safety. Limiting amounts of fuel present in a building, combined with limiting ignition sources and good management practice in accordance with BS 9999 2017 and BS9991 2024 Section 9: Managing Occupied Buildings may in some cases be sufficient to satisfy the authorities having jurisdiction on this matter, as part of a holistic based approach to the buildings safety.

Additionally, occupancy factors, including the use of the premises for sleeping or non-sleeping purposes, should be considered when evaluating the sufficiency of existing materials. Finally, in some cases it may be necessary to carry out a full fire engineering analysis in accordance with BS 7974 2019 or CIBSE Guide E Fire safety engineering 2019.

b) Improve fire protection

Even after carrying out a thorough fire risk assessment, it may still be necessary to improve the quality of fire protection to existing elements of structure and there are many ways of doing so without damaging historic features. Methods include:

• using non-combustible materials, such as mineral fibre, to clad structural members,
• enclosing existing historic structures within new structures, such as floors, which can, at some future date, be removed without damaging historic features,
• using intumescing or Poly Phosphor based paints to treat iron, steel and other metals. Intumescent pillows and ablative mineral wool batts are also available and useful for ensuring fire-stopping around columns and beams.

c) Install active fire-defence systems

Active systems such as fire suppression and alarms may also be used to compensate for weaknesses in structural fire protection or where confidence in the quality of existing measures is low. Some of these circumstances are already recognised within the Technical Handbooks:

• Where suppression is present, the guidance in the Technical Handbooks sets out where the fire resistance of external walls may be reduced. Suppression can also influence distance to boundary calculations, particularly for assembly, residential, residential care, office and open sided car park buildings,
• smoke outlets need not be provided where the basement storey has an automatic fire suppression system and is ventilated by a mechanical smoke and heat extraction system incorporating a powered smoke and heat exhaust ventilator which has a capacity of at least 10 air changes per hour and conforms to BS 7346: Part 2: 1990 with a class D level of performance or more detailed guidance can be found in the BS EN 12101 series of standards.