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Exposure Hazards – Adjoining and Surrounding Property

INTRODUCTION


A key element of any property survey is that of the assessment and reporting of the fire/explosion exposure hazards presented to the property being surveyed from any third party property. Such ‘property’ may take the form of buildings and facilities, plant and/or storage in the open, and includes surroundings buildings as well as other occupants within the surveyed property, should it be in multi-tenure occupancy.


Some Insurer Clients that RiskSTOP undertake surveys for may provide guidance on this subject and this MUST be followed; others may provide structured EML instructions which may give Consultants guidance on acceptable separation distances or fire compartment wall standards to assume fire will not spread. Such guidance may be useful in assessing whether there is significant exposure hazard from surrounding occupancies. 


Assessing the Exposure hazard is not an exact science, it requires experience and a degree of subjective judgement by the Consultant. The extent of reporting required varies according to Client and template being used; if in doubt however the Consultant should err on the cautious side when reporting. Where possible, the name of the occupant and nature of occupancy, plus details of separation are to be provided.

 

COMMUNICATING AND ADJOINING BUILDINGS


Insurers adopt varying standards when assessing whether the fire separation between adjoining buildings is ‘adequate’ to provide independent premium rating/risk assessment for different premises. Traditionally the required separation has been a minimum of 240 minutes compartment fire walls with the need for structural metalwork within or abutting the wall to be protected to at least the same level of fire resistance.


The presence of door or other openings in compartment walls will normally result in the wall being considered as potentially compromised even if the doors are fitted with ‘approved’ fire doors or shutters.


Approved Document B of the Building Regulations (England and Wales) ‘requires’ compartment walls separating buildings to be of a minimum 60 minutes fire resistance to allow for life safety/fire escape, and may not necessarily seek fire protection to supporting metalwork. Consequently, many compartment walls encountered in the field may not have a high degree of fire resistance – for example it is quite common for terraced units on out of town retail parks to have 60 minutes dividing walls to the underside of roofs, with or without fire protection to the supporting steelwork at roof level.  


Despite the existence of what at face value appears to be a reasonable fire compartment wall, there may be other factors which can compromise the separation, examples being:


  • Cracks in the compartment wall as a result of impact or other damage;

     

  • Fire doors or shutters which, although having a certified fire resistance for integrity, have little or no insulation properties;

     

  • Unbroken combustible cored external composite panels and other combustible external wall claddings or roofing materials;

     

  • The absence of a fire resisting “protected zone” on one or both sides of a compartment wall;

     

  • Cavities which exist between the compartment wall and the adjacent external wall or roof claddings which have inadequate fire stopping (often referred to as “fire safing”), enabling the compartment wall to be bypassed;

     

  • Valley gutters and opposing roof pitches, particularly where combustible roofs or linings are involved, or where roof lights are likely to be breached during a fire;

  • Poorly protected service apertures;

     


  • Exposure from adjoining buildings, in the case of multi-storey properties, from upper floors overlooking the roof of an adjoining building of lower height, particularly when combustible roof construction is involved;


When assessing the fire resistance of common compartment wall construction, reference should be made to guidance within the RISCAuthority Categories of Building Construction and the FPA Design Guide for the Fire Protection of Buildings, (see Appendix 1 of this Technical Bulletin).


Apart from where constructed of conventional brickwork, laid in a header and stretcher pattern, it is appreciated that identifying the nature and designed fire resistance of compartment walls is not always an easy task and occasions will arise when intelligent assumptions may have to be made based on the available evidence. Where this is the case, a cautious approach should be taken.


When assessing exposure hazard, whether from an adjoining or neighbouring facility, it is important that due consideration is taken not just of damage resulting from thermal radiation or the spread of fire from an exposing building, but also of the potential for smoke damage, such as would arise, for example, in the case of a food risk.

        

SURROUNDING BUILDINGS AND FACILITIES


Assessment of the exposure hazard presented from the surrounding buildings and facilities involves the consideration of the following key features:


  • Spatial separation between buildings;  

  • Height and construction of both the exposed and exposing buildings;

  • Presence of opposing doors, windows and other openings;

  • Presence of external storage and use of the intervening space between buildings;

  • Occupation of the exposing buildings with particular reference to inherent hazards and fire load;

  • Exposure from hazardous installations;

  • Sloping ground presenting a potential risk from burning liquids, molten materials and extinguishment water;

  • The effect of high winds in areas exposed to the elements;

  • The effect of structural collapse of the exposing building;

  • Presence of vegetation between buildings.


10m clear space between the surveyed property and surrounding property is the minimum required to assess that a low (or ‘normal’) exposure hazard exists, but this may need to be considerably increased (to 30 metres or more) in certain circumstances. 


Consultants must exercise caution over high risk occupancies, for example external (or even internal) waste storage facilities will often create unfavourable exposure hazards.


The Consultant will also need to note any exposure to local high hazard facilities, even when not immediately adjacent, for example the approximate distance from a petroleum storage site (such as Buncefield).

 

MULTI TENURE RISKS


Consultants should give due consideration to the exposure hazard presented by other occupants/tenants in a surveyed property. The assessment will be based on the fire risk presented by the occupiers and of the fire separation between tenancies – for example a timber floored building where the survey is for a policyholder on one floor would likely be assessed as a high (or unfavourable) exposure hazard from other tenants. 


Caution needs to be adopted when assessing the Exposure hazard within buildings which have been converted from their original use and are now in multi-tenure occupancy. For example, it is common place for large old factory premises to be sub-divided into multi let units. It is not always easy to identify the fire resistance of dividing walls in such circumstances, and often compartment walls have limited fire resistance. In addition, the occupancy of other surrounding units cannot always be readily identified and is also subject to frequent change. When surveying an individual unit of a building in such a situation the Exposure hazard should be treated circumspectly.

 

ADDITIONAL INFORMATION


Further details on the subject of compartmentation can be found in the FPA Design Guide documents BDM 2, BDM6 and BDM7 located in ATLAS. 

 

APPENDIX


Concrete or Masonry Walls

FIRE RESISTANCE OF CONCRETE OR MASONRY COMPARTMENT WALLS

TYPE OF MATERIAL

Thickness of wall (mm) to achieve a fire resistance of:

 

Up to 90 minutes

120 minutes

240 minutes

Reinforced Concrete

95

100

180

Brickwork of solid clay bricks

95

100

170

Brickwork of solid concrete or sandlime bricks

95

100

200

Blockwork of solid pre-cast concrete blocks of dense aggregate

90

100

 

Blockwork of solid pre-cast concrete blocks of lightweight aggregate

100

100

 

Blockwork of solid autoclaved aerated concrete blocks of density 600kg/m3

95

100

215

 

1.      The thickness of walls given in this table relates to walls of 3m height. For walls of greater heights, these will need to be increased. Further guidance is in Part 6 of the Design Guide.

2.      For cavity walls, take the combined thickness of the two leafs to determine fire resistance.

3.     Since the above Table was prepared, a range of autoclaved, aerated concrete blocks (often referred to as “aircrete”) have been developed under brands such as “Thermalite” and “Celcon” with enhanced fire resistance - 100mm non-loadbearing walls up to 240 minutes, and load-bearing up to 120 minutes.

 

Dry Lined Studwork Walls

Fire Resistance

CALCIUM SILICATE BOARD (e.g. “Supalux”)

Integrity (min)

30

60

90

120

240

Insulation (min)

30

60

90

120

240

Example A

Timber stud construction, 6mm board on each face with 60mm thickness of 20 kg/m3 mineral wool in cavity

Steel stud construction, 9mm board on each face with 60mm thickness of 23 kg/m3 mineral wool in cavity

Steel stud construction, 9mm board on each face with 60mm thickness of 60 kg/m3 mineral wool in cavity

Steel stud construction, 9mm board on each face with two layers of 30mm thickness of 100 kg/m3  mineral wool in cavity

Steel stud construction, two layers of 9mm board on each face with two layers of 60mm thickness of 100 kg/m3 mineral wool in cavity

Example B

Steel stud construction, 12mm board on each side, no cavity insulation

 

20mm double layer, second layer screwed to first layer, fixed to perimeter frame

 

one layer of 20mm plus two layers of 15mm

 

 

1.      Where no second example is given, this indicates that different manufacturers specifications are the same or very similar.

2.      Where mineral wool is used in the cavity, it is often only necessary to increase the density of the mineral wool to raise the fire resistance.

3.     The data given above is intended to give only a few examples and specifications will vary from manufacturer to manufacturer.

 

 

 

 

Fire Resistance

GYPSUM BOARD (plasterboard)

Integrity (min)

30

60

90

120

Insulation (min)

30

60

90

120

Example A

steel stud construction, 15mm wallboard, 70mm air gap

(max height of barrier 3.3m)

steel stud construction, two layers of 12.5mm wallboard on each side

steel stud construction, two layers of 15mm wallboard each side, 146mm cavity, 30mm mineral wool of density 33kg/m3

steel stud construction, two layers of 15mm reinforced gypsum board on each side, 146mm air cavity

(max height of barrier 4.8m)

Example B

steel stud construction, 12.5mm wallboard, 60mm air gap

(max height of barrier 2.7m)

steel stud construction, single layer of reinforced gypsum board on each side

steel stud construction, inner layer of 12.5mm standard wallboard each side, 146mm cavity, plus outer layer of 12.5mm reinforced wallboard on each side

steel stud construction, two layers of 15mm reinforced gypsum board on each side, 50mm air cavity

(max height of barrier 3.6m)

 

1.      Reinforced gypsum board contains glass fibre reinforcement in the Gypsum core. This helps keep the board intact longer under fire conditions with consequent increase in fire resistance.

2.      The data given above is intended to give only a few examples and specifications will vary from manufacturer to manufacturer.

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