Guide to Conversion of Traditional Buildings

2.2 Separation

Standard 2.2

Every building, which is divided into more than one area of different occupation, must be designed and constructed in such a way that in the event of an outbreak of fire within the building, fire and smoke are inhibited from spreading beyond the area of occupation where the fire originated.

2.2.1 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).

2.2.2 Commentary

This standard should be read in conjunction with Standard 2.1, Compartmentation. However, 2.1 applies to non-domestic buildings only, while 2.2 applies to both domestic and non-domestic buildings. The essential differences between the two standards are:

• The guidance to Standard 2.1 defines the maximum compartment areas for different non-domestic building uses, and the minimum fire resistance duration for compartments and elements of structure, to inhibit fire and smoke spread and thus enable occupants to escape from the compartment and fire-fighters to enter.
• The Standard is concerned with the need to prevent fire spread from one dwelling to another (domestic buildings) or from buildings, or parts of a building, in different occupations (non-domestic buildings). Its purpose is to reduce the risk where parts of buildings are in different occupation and separate management. It covers the design and construction of separating elements which must provide a complete barrier to the products of combustion.

The requirement of Standard 2.2 to provide specified periods of fire separation of spaces poses significant demands on design and construction when converting traditional buildings. The crucial elements that might need upgrading – or alternative strategies adopted – are the doors, ceilings/floors, walls, penetrations through floors and walls, and roof voids.

The presence of multiple flues in traditional buildings is a specific problem that may be present in existing walls that are to become separating walls. Most rooms will have had a fireplace, and redundant flues may be unlined and in poor condition. Mortar undergoes degradation with age, and gaps can occur which permit the transfer of the products of combustion across separating elements.

Similarly, ends of beams and floor joists built into separating walls may be a source of weakness that, if not fire-stopped, can permit the passage of fire and smoke across the wall. Lime mortar is suitable for firestopping and if joints are excessively wide or there are large areas of mortar, it is recommended to use pinning stones (offcuts of stone, brick or pieces of slate) to pack out the mortar. See ‘Generic specifications for lime mortar’, Scottish Lime Centre Trust.

Traditional construction will normally contain hidden voids but may also contain redundant spaces that have been covered up. These include cavities behind lath and plaster that permit the free circulation of air to control the moisture content of timber within these spaces. In addition, most historic buildings are likely to have undergone a degree of alteration over the years, such as the introduction of bathrooms, kitchens and service voids. Often, the careless installation of mechanical and electrical services over many years can create pathways within the construction that will destroy its effectiveness as a fire barrier.

Identifying existing voids in the design of a conversion is essential to maintaining fire (and sometimes sound) separation between parts of buildings in different occupation. Understanding the role of these voids in a historic building is crucial, as the thoughtless sealing up of such voids can adversely affect how such buildings ‘breathe’, possibly leading to fungal decay and deterioration of porous plaster finishes and the like. Balancing the need for fire integrity while maintaining air movement presents a significant challenge to designers.

Separating walls are required to extend into roof spaces and through basements to the solum forming a complete barrier to prevent fire and smoke spread between dwellings, or between one part of a building and another. It is always worth remembering that, while improving fire resistance of elements may affect a building’s historic character, the option to ‘do nothing’ is not appropriate, as weaknesses in the fire resistance could lead to the loss of life and even complete loss of the building.

2.2.3 Issues to be considered

Issue - Risks to historic / traditional buildings

1. Separating walls and floors Traditional lath and lime plaster ceilings and walls in good condition can provide adequate fire separation but cannot be relied upon to provide the necessary period of fire resistance when in poor condition. Upgrading timber floors may cause loss of historic materials and finishes.

2. Combustibility Traditional buildings with stories at a height of more than 18 metres must be constructed with separating floors and walls of non-combustible materials as evacuation and fire-fighting operations may take longer than with lower buildings. Major replacement of structural materials may be required with consequent loss of historic finishes unless alternative strategies are employed. Traditional buildings under 18m high often use combustible materials for floors and walls. Whilst the replacement of structural material may not be needed, upgrading of fire resistance might result in the loss of historic finishes unless alternative strategies are used. (Refer to section 2.2.4 below).

3. Openings for doors, service penetrations, chimneys or flue pipes. Upgrading historic doors to meet the standard of fire resistance required may lead to damage or loss of features of historic interest. Holes created for the passage of pipes and cables should be effectively fire stopped, but this is unlikely to pose a serious risk to historic fabric. Existing chimneys and flues in masonry walls where the condition of stonework and pointing is poor will affect the fire resistance. Improving the fire resistance by raking out and re-pointing should be carried out in a sympathetic manner.

4. Junctions

Traditional buildings with separating timber floors may have cavities between the floor and the wall. Fire stopping these cavities may reduce air movement and increase the risk of decay.

2.2.4 Recommendations to meet the standard

a) General recommendations

Some traditional buildings may already have a degree of fire separation. Wall scanners and cable detectors can be used to find hidden cables and pipes close to the surface. Thermal imaging can identify heating pipes during the winter when the heating is on but ground penetrating radar (GPR) is more effective. Ground Penetrating Radar can locate and distinguish a wide variety of metallic and non-metallic materials. GPR works best when there is a big difference in the electromagnetic properties of the materials being surveyed, so metallic objects make ideal targets. Some of the more common target materials include:

• Metal
• Air pockets or voids
• Plastics
• Changes in wall strata and embedded timber
• Masonry

A specialist may conduct an intrusive survey to validate the results of the Ground Penetrating Radar (GPR). This could involve the use of endoscopes or fibre optic cameras to examine the interior of a wall or the area beneath a floor. A hole between 8mm and 10mm will need to be drilled to insert the camera and this should be fire-stopped on completion of the survey.

The greater part of a building or structure may be flammable, and even where non-combustible materials (such as slates) are present, these will be supported by combustible timber and sarking and roofing felt. However, the following guidance may prove helpful:

i. Traditional lath and plaster will give a period of fire resistance that depends on its thickness, condition, strength of keying and adequate fixing of the laths to the joists. Although intumescent coatings are available, they only increase what is already the good standard of insulation provided by the plaster, but cannot improve the condition, strength of fixings or keying which determines the integrity of the element.

ii. Close-boarded floors on substantial joists with lime plaster ceilings below may provide longer periods of fire resistance than is normally anticipated. There will be many floors that will provide 30 minutes fire resistance, and some will provide at least 60 minutes resistance (Chatham Row Fire Test)¹². Where pugging (or deafening) in the form of rough plaster, cinder ash or crushed seashells has been introduced this will increase the resistance to fire.

iii. Existing doors may not offer any formal evidence of fire resistance, but this does not mean that they provide no resistance. Historic England’s “Guide to the Fire Resistance of Historic Timber Panel Doors”¹³ gives some examples of upgrading solutions but does not include intumescent paints and varnishes as no improvements more than a few minutes have been demonstrated.

iv. Where the proper standard of fire separation between different occupancies cannot be achieved, a fire alarm system giving early warning to occupants of all the different occupancies including the occupancy of fire origin should be provided. This may, in certain circumstances, provide adequate compensation as part of a holistic risk-based approach. While automatic fire detection must be installed in all domestic properties, it will not usually be linked to any common or neighbouring alarms in purpose-built blocks of flats with a stay put policy. This should be installed in accordance with BS 5839 Part 6 2019: Grade A type LD1 or LD2. Similarly, the presence of automatic fire suppression, such as an automatic fire sprinkler system designed and installed to either BS 9251 2021, or a water mist system conforming to BS 8458 both for domestic applications: or BS EN 12845:2015+A1:2019 (currently under review) for commercial applications, may, in certain circumstances, provide adequate compensation. However, in a domestic setting, the issue of annual servicing and maintenance will clearly be an important matter requiring careful consideration. Where converting buildings to domestic properties, such as flats, automatic fire suppression systems may be a requirement in any case. Refer to Mandatory Standard 2.15.

v. Good ventilation and airflow is normally important to maintaining the good health of traditional buildings. However, vents in walls, doors or floors that are also required to inhibit the spread of fire and smoke can be acceptable if suitable products are installed that will seal in the event of fire e.g. automatic fire/smoke dampers or intumescent grilles.

b. Upgrading fire resistance of ceilings and floors

There are various methods of upgrading the fire resistance duration of existing timber separating ceilings and floors. The most appropriate method will be determined by the required period of resistance, the historic importance of the floor and its construction and condition.

In the image below indicates a 17th century house, redeveloped by Robert Adam in 1760. The original Georgian floor had overlong and undersized joists. When the room was later converted to a music room, a new more substantial floor was inserted above. This led to a change in level with steps to reach it.

For these floors, the following methods of upgrading performance could be considered. The examples illustrated below are reproduced, in part, from BRE Digest 208 1988 Increasing the fire resistance of existing timber floors, where more detailed information on specification and fixings can be obtained. This document is out of print, but scanned copies can be obtained from the BRE Group. However, the gap between the internal lath and plaster finish and the external wall will need to be retained to allow ventilation. It is therefore imperative that no weaknesses exist between the room and the gap that will allow fire to penetrate.

i. Applying a layer or layers of plasterboard to the underside of the existing floor. Note this is likely to be inappropriate where the existing ceiling is of significance to the historic character of the building, or where ornate finishes, ceiling roses and coving are present.

ii. Installing a fire-resisting suspended ceiling system beneath the existing floor may be appropriate when high fire-resistance duration is required and where the floor to ceiling height permits. The existing historic ceiling may then be preserved within the new construction.

iii. Installing a lightweight underfloor fire-resisting barrier within the depth of the floor is useful when the existing ceiling and its fixing to the joists are in good condition (Figures 2.2.4(b) and 2.2.4 (c)). In addition to the examples illustrated, proprietary systems are available. Usually, such systems require floorboards to be lifted and, for example, wired mineral wool installed between the joists and fixed with metal clamps. This system allows the historic ceiling to be retained while possibly achieving in excess of 60 minutes of fire resistance.

iv. One of the least damaging methods of improving the fire resistance of a separating floor to achieve half-hour fire protection while retaining the existing ceiling is by adding a hardboard or plywood layer to the surface of the existing floor.

v. Where existing joists are exposed, it is possible to upgrade the floor by adding plasterboard protection below the flooring to achieve half-hour protection (Figure 2.2.1(d)). The plasterboard will need sealing around the edges and over the fixings and skimmed. The joists will need to be thick enough that charring of the timber will not result in the loss of structural stability for the duration of the fire. Charring rates range from about 38mm/hour for oak to 66mm/hour for spruce. These charring rates could be reduced if the timber receives a fire-retardant impregnation. However, the advice of a structural engineer should be sought to make sure the floor will support the applied load when the joists have been exposed to fire conditions.

vi. Although intumescent paints and papers are available, they only increase what is already the good standard of insulation provided by the plaster, and cannot improve the condition of the ceiling, strength of fixings or keying through the laths which ensures the integrity of the element.

When upgrading the fire resistance duration of a historic timber floor, it is essential to provide fire resistance to any holes in the ceiling (deliberate or otherwise) and to the junction between the floor and the wall. However, sealing the gap between the floor and the wall will prevent the ventilation of voids behind lath and plaster wall finishes. For further information on this issue, refer to the advice contained in part II, Standard 2.4 Cavities.

c. Upgrading the fire resistance of doors in separating walls

Upgrading the fire resistance duration of an historic door in a separating wall is discussed in Section 2.0.3 (b) of this chapter. Further information can be found in the latest edition of the Historic England and Institution of Fire Engineer’s “Guide to the Fire Resistance of Historic Timber Panel Doors”.

i. For a framed door with solid panels, it is important to identify whether a softwood or hardwood is used to enable an estimate to be made of the potential burn-through time for the thinnest timber within the door construction. The thickness of the frame will influence the distortion of the door as the wood will shrink on the fire side of the door bowing inwards, creating gaps in the top corners. Additionally, door shrinkage or splitting can affect burn-through time. The thickness will allow a decision to be made on how best to upgrade the performance of the door. Methods can include:

• facing the door on both sides with non-combustible boards, which can be removed in the future with minimal damage, but this adds to the weight and will be unsightly
• sealing all cracks and gaps with intumescent paste, or in the case of a split panel, removing it and glueing the edges before fastening with butterfly or bow ties
• applying intumescent veneer to protect vulnerable areas such as fielded panels
• fitting proprietary intumescent strips and flexible cold smoke seals to the door edges or frame.

ii. In the case of a door with a glazed panel, the normal approach will be to replace the existing glass with fire resisting glass. However, this may mean that important historic glass is lost. In such a case it may be possible to apply secondary fire glazing to the door (using a suitable fire resisting glass in an intumescent channel) which retains the historic glass in place and allows the door to be returned to its original state. The beading should be cross pinned in place to support the intumescent seal when it expands.

iii. The gap between the door leaf and frame in a historic door opening is frequently wider than the 4mm recommended maximum. The door and frame may have distorted, or the hinges dropped over the years leaving a variable gap. In such cases, the hinges should be adjusted or changed. If gaps remain, hardwood lipping, pinned and glued to the door leaf edges may be necessary, pinned to allow the fitting of intumescent strips and cold smoke seals in a routered groove. Where a sloping gap has formed at the top of the door, a sloping fillet of wood should be glued and pinned to the top rail.

iv. The gap at the bottom of the leaf is not as vulnerable to fire and smoke spread as the other gaps because clean air prevails at floor level. This is because the heat and pressure decrease in a gradient within a room until it reaches the neutral plane where there is ambient temperature and pressure. Underneath the neutral plane, air outside of the room on fire will invariably be drawn into the room, preventing smoke from escaping. The smoke layer, as it descends to the item/s burning starts to suffocate the fire due to lack of oxygen and this lowers the temperature and pressure allowing the smoke layer to lift, keeping the floor level tenable.

Illustrations 15 and 16 below show the descending layer of smoke in a BRE fire test.

A sloping floor inevitably leads to a sloping gap underneath a door to allow it to open fully without grounding. A judgement must then be made on the how important it is to minimise the gap. It might be achieved by introducing a sloping threshold if it does not become a trip hazard or creating a ramp up to the door or straightening the floor. The two photographs below from inside and outside of two different rooms that have suffered from fire illustrate this phenomenon.

This phenomenon does not apply to doors at the top of a shaft that sit within the anticipated hot smoke layer of smoke, so a competent fire engineer must make this judgement.

v. It is not just the door leaf that will require attention. For the whole door installation (including the door frame construction and the frame to wall junction) to achieve at least a short fire resistance duration, the following steps to improve performance should be implemented.

• Repair damaged frames, including tightening of mortice joints where these impact fire performance
• If it is a masonry wall and the architraves have a combined thickness greater than the door frame. They will protect the gap, but if in doubt, architraves should be carefully removed to check that no voids occur between the frame and surrounding wall and thus allow the passage of combustion products. Any voids should be filled with lime plaster or lime mortar, intumescent material or tightly packed mineral wool insulation.

vi. Existing historic door ironmongery cannot always be assumed to be suitable for use on a fire-resisting door. Where this is the case, historic door furniture may need to be replaced with suitable approved fittings. British Standard BS EN 1935 grades hinges from 7-14 which refers to the hardness and durability of the hinges. Each hinge has an eight-digit classification code and only hinge grades 10-14 are suitable for fire doors. In the case of a listed building a copy of an original ornate hinge might be manufactured, although this may be a matter for listed building consent. Alternative options relating to the use or management of the building may have to be explored where the fittings are important to the character of the building. Similarly, alterations to the door furniture over the life of the door may have resulted in gaps or reduced door thicknesses that should be filled with a timber insert glued and pinned into place.

vii. It will be very difficult to remove the cinched nails without damaging a door, so any repairs to the knuckles of the hinge should be undertaken by removing the hinge pins. The repairs might involve fitting bushes or sintered bronze washers and replacing the pin with a new stainless steel one.

viii. Self-closing devices will be required in many circumstances. These include floor springs with pivots, overhead closers, spring or hydraulics closers fitted inside the mid rail and concealed overhead closers. The different devices offer a choice between: -

• preserving the appearance of the door but damaging its fabric by inserting the device into the timber or removing part of the top rail to fit a sliding device, or
• being visually intrusive, but reversible.