Guide to Conversion of Traditional Buildings

2.4 Cavities

Standard 2.4

Every building must be designed and constructed in such a way that in the event of an outbreak of fire within the building, the spread of fire and smoke within cavities in its structure and fabric is inhibited.

2.4.1 Type of standard

Mandatory Standard

In the case of conversions, as specified in regulation 4, the building as converted shall meet the requirements of the standard in so far as is reasonably practicable, and in no case be worse than before the conversion (regulation 12, schedule 6).

2.4.2 Commentary

The spread of fire and smoke in concealed spaces is a primary concern for fire safety design in all buildings as it can spread undetected very rapidly. The risks are heightened in traditional buildings where cavities and other pathways for fire and smoke may be concealed within the construction, and their presence might not be discovered without thorough investigation. It is not uncommon to discover significant quantities of combustible materials in older buildings, such as off cuts of timber, bird's nests, guano, soot, and other forms of combustible refuse that have accumulated within the wall or roof cavities over the years.

A further consideration is that traditional buildings stay dry and in good condition from airflow through the structure. This means that most cavities will be ventilated, thus raising the potential for rapid fire spread. The undetected spread of fire within cavities has been responsible for many serious fires in historic buildings – especially because they are notoriously difficult to fight and extinguish once they have reached the roof. Eventually the roof timbers collapse, and the fire is spread downwards floor by floor.

When a fire occurs in a compartment there will be a heat gradient that is hottest at ceiling level and coolest at the floor. There will be a corresponding pressure gradient as the gases in the room (smoke and air) expand by 1/273 per degree rise in temperature. As the expansion occurs within the walls, pressure rises. A small hole could allow hot gases to escape, converting pressure to velocity and entraining air. If the air mixed with any highly flammable constituent of the hot gases is within its upper and lower flammable limits, auto-ignition may occur.

The most common internal wall finish in Scotland’s traditional buildings stock is timber lath and plaster fixed into to the masonry using timber dooks. This method of construction leaves a small, ventilated gap behind the lath and plaster, which frequently provides a pathway for the products of combustion into other ventilated spaces within floors and roofs. Sealing off these cavities to prevent the spread of fire and smoke may seem the simplest solution, but cutting off ventilation pathways and the process of inserting cavity barriers can be damaging to historic fabric.

The standard recommends that cavity barriers should be installed around the edges of cavities. Barriers, for example, are required around the head, jambs and sill of an external door or window opening and at a wall-head between a wall cavity and a roof space cavity. However, the standard acknowledges that this may present difficulties where through ventilation has to be maintained. Designers of conversions must recognise that appropriate solutions will require a thorough understanding of the location of cavities; the risks posed by fire spread and by reduced ventilation within construction. It may be that alternative strategies to reduce the risks to occupants will have to be employed.

For the purposes of this standard, a roof space is also classed as a cavity, as it is often a high-risk area that can be quickly consumed by fire passing into the space through adjoining cavities. Often the cellular arrangement of walls on lower floors does not extend into the roof void, creating a large volume roof (refer to Standard 2.1 Compartmentation, for further information). Large volume roofs, as encountered for example in the conversion of churches or public buildings, are vulnerable to fire spread. Domes, which are often double skinned and have a timber supporting structure, also have little fire resistance and allow rapid fire-spread within their construction.

2.4.3 Issues to be considered

Issue - Risks to historic / traditional buildings

1. Cavity barriers The introduction of cavity barriers into traditional buildings creates two issues: -

• Sealing off a cavity may reduce ventilation and may promote dampness within the void.
• Inserting fire stopping materials into historic fabric may cause damage.

The recommendation to provide fire barriers around the edges of the cavity such as around window openings is impracticable where existing finishes are retained.

2. Roof space cavities The roof space is a cavity. Many traditional roofs are likely to have ceilings constructed from lath and plaster, which may be capable of providing a degree of fire resistance when in sound condition. However, upgrading a ceiling to provide additional fire resistance may result in loss of historic features.

3. Cavities above ceilings in residential buildings (Except residential care buildings and hospitals). Where a ceiling void extends over rooms intended for sleeping, a cavity barrier above the wall separating the rooms is required. Where this is not practicable, fire resisting ceilings can be installed to each room. However, each of these solutions can lead to damage to historic fabric. Existing ceilings may need to be disturbed to provide access hatches to the roof space and upgrading the fire resistance of the ceiling may result in the loss of important features.

4. Fire resisting ceilings These may be used in place of cavity barriers in appropriate situations.

5. Combustibility Cavity barriers and fire resisting ceilings provided as an alternative to cavity barriers generally need to be constructed of non-combustible material. No additional risk to historic fabric is posed by this clause.

6. Supporting structures No additional risk posed by this clause.

7. Openings and service penetrations No additional risk posed by this clause.

8. Junctions Cavity barriers are required to fit tightly to rigid construction. This may not be possible at a junction with slates, tiles or similar materials in which case the junction should be fire stopped. For rooms contained within roofs, access for fire stopping may be destructive to finishes. When a separating wall or floor meets a structure containing a cavity, such as in lath and plaster constructions, a cavity barrier should be installed to continue the line of the structure. This requirement can pose challenges in traditional and historic buildings, as installing the barriers can be damaging to the fabric and sealing cavities in these areas will impact the ventilation of voids.

2.4.4 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.

There is likely to be a limited degree of intentional compartmentation and/or separation in many buildings of traditional construction. All roof spaces should be carefully checked and assessed as part of the overall fire risk assessment. It is not easy to determine what is present without damaging historic fabric and a specialist survey using a combination of ground penetrating radar, endoscopes or fibre optic cameras may be the only way of determining what lies inside a wall or under a floor.

1. The vulnerable locations are:

• areas around chimneys and flues should always be carefully checked and any waste combustible material and rubbish removed
• breaches in walls, ceilings, and floors, especially around service penetrations like electrical cables, pipes, ventilation ducts, grills, and light fittings where seals or fire stopping must be applied
• areas where light wells penetrate through the building

2. In addition to traditional materials, a wide variety of modern proprietary products are available for the installation of fire barriers including:

• fire barrier curtains, both intumescent and mineral fibre, for fixing inside roof-spaces or floors and separating large voids. These should extend across the sarking to at least the next common rafter to stop fire spreading across the top of the barrier (if roofing fire break membranes are not provided)
• intumescent putty for PVC/metal electrical pattress boxes in ceilings and walls
• fire resisting covers/caps for recessed light fittings and downlighters
• intumescent soffit vent grilles
• roofing fire break membranes (for fitting over separating or compartment walls and preventing fire spread across a roof)
• intumescent pads and pillows for inside metal electrical trunking in ceilings and walls

3. Where the building fabric cannot be disturbed or the installation of barriers is not practicable, consideration should be given to improving the level of automatic fire detection or even installing an automatic fire suppression system.