Cladding Remediation Programme: Single Building Assessment specification

Appendix G. Façade Configuration Risk Factors

The following images show Table N.1 of the PAS 9980 document which covers façade configuration risk factors.

Table N.1 gives a non-exhaustive list of examples of common factors influencing the likely speed and extent of fire spread based on:

• The extent to which the building is covered by combustible cladding;
• The presence or otherwise of continuous cavities;
• The extent of openings in the external building envelope; and
• The location of the cladding.

Note that no single row in Table N.1 of PAS 9980 gives a definitive answer on risk. Whether an entry is considered positive, negative or neutral is purely indicative of the potential influence it might have. A competent assessor uses their judgement when using the table to determine the actual relevance of each factor and its significance in the context of the particular building under consideration. Where numerical values are given they are only intended to be indicative as to the possible influence the particular factor might have in a risk-based assessment.

Where a risk factor is marked with an asterisk (*), this indicates that it is notably more of a positive influence.

Figure E.1 – Table N.1 of PAS 9980 – Façade Configuration Risk Factors

Where a risk factor is marked with an asterisk (*), this indicates that it is notably more of a positive influence.

N.1 Building height

Note These are commonly used trigger heights, but it is important that these are considered, along with all other pertinent factors in the round. Indeed, in risk terms, there is a reducing gradation in risk for heights below 18 m.

On buildings below 18 m in height, the extent of cladding is inherently limited by virtue of the number of storeys. For such buildings, traditionally, there have been no explicit restrictions on the combustibility of the external wall construction and, only in limited circumstances, any requirements relating to the reaction to fire classification of surfaces; it has still been necessary to provide cavity barrier protection, where applicable. It is therefore possible, and indeed likely, that rapid external fire spread would occur in buildings where elements of the external walls are combustible.

It is reasonable to expect that an assessment of the fire risk posed by external walls of low‑rise blocks of flats (buildings below 18 m in height) ought normally to place the building in the low‑risk category. However, with current knowledge of the burning behaviour of certain materials and how the configuration of these on the building can promote rapid fire spread at a rate much greater than previously anticipated for low‑rise buildings, it is possible that an external wall assessor might place the risk in the medium risk category, albeit still considering the risk tolerable. Where extremely rapid fire spread is likely (e.g. where Category 3 ACM is present or there is excessive use of timber or other combustible materials configured in such a way as to promote unusually rapid and extensive fire spread), this would suggest that fire spread would be at a rate far greater than previously considered acceptable for a low‑rise building, with the conclusion that the risk is unacceptably high. Issues around deficiencies in the construction of the walls might also lead an external wall assessor to conclude that further and more in‑depth technical assessment might be necessary to refine the risk. Concerns regarding effective intervention by the fire and rescue service might also lead to this conclusion; even in low rise buildings the difficulties of tackling a fire involving external wall construction when operating at ground level using typically available equipment need to be recognized (see Annex E).

Positive

<11 m

Neutral

<18 m

Negative

18 m to 30 m*

>30 m

N.2 Height of base of cladding above ground

Positive

>5 m

Note At this height, the likelihood of a fire originating externally (e.g. involving a parked vehicle or waste skip and started either accidently or deliberately) igniting the cladding is highly unlikely.

Neutral

2 m to 5 m

Note At this height, the scope for a fire originating externally (e.g. involving a parked vehicle or waste skip started either accidently or deliberately) igniting the cladding is considered possible, but not likely at an early stage in the development of the fire.

Negative

<2 m

Note At this height, the likelihood of a fire originating externally (e.g. involving a parked vehicle or waste skip started either accidently or deliberately) igniting the cladding is highly likely.

N.3 Extent of cladding

Note In the case of a high‑rise building with only partial cladding, the limited extent of combustible cladding might not be materially different, in terms of external fire spread, from the same extent of cladding on a low‑rise building. However, its location could lead to a situation that is very different in terms of overall risk because of the difficulty of fighting a fire involving the cladding at that height. This exemplifies the need for consideration of the potential for firefighting by the fire and rescue service. Also, in this situation, even when a high‑rise building only has combustible cladding on a limited number of lower floors, a fire involving that cladding could impact on fire protection measures, such as smoke control systems, required to protect the upper floors (see also Annex F).

Positive

Limited in extent and not vertically aligned, such as to delay significantly fire spread to windows and other openings on upper levels

Neutral

Limited in extent such as to delay fire spread over the external walls

Negative

Entire façade covered

Positive

No scope for a cladding fire to breach compartment wall and floor boundaries

Neutral

-

Negative

Scope for a cladding fire to breach compartment wall and floor boundaries significantly worsened by the nature and the extent of combustible material in the external wall construction

N.4 Cavities

Positive

No cavity

Neutral

-

Negative

-

Positive

Cavity not continuous, due to façade being only partially clad or broken by building features

(Examples include:

• projecting floor slabs that divide part of the wall and isolate sections of cavity from each other; and
• walls that project out or are set back, such as to limit the vertical extent of cavities)

Continuous vertically running cavity with cavity barriers or fire stops as appropriate

Neutral

Cavity limited in vertical extent, e.g. ventilated rainscreen that spans more than one floor level but not all floor levels

Cavity limited in extent and running horizontally only

Negative

Continuous vertically running cavity without cavity barriers or fire stops

Positive

Limited or no windows or openings in façade

Neutral

Openings in façade limited to ventilation outlets

Negative

Windows and other openings in line with vertical cavity

N.5 Infill/spandrel panels

Note 1 The above relates to panels that could, due to their combustible facing or content, contribute to fire spread. Non‑combustible panels might serve to divide a façade and positively reduce the scope for fire spread where other parts of the walls are combustible. Spandrel and infill panels are terms often used interchangeably for panels within a window or curtain wall framing system. However, spandrel can denote a panel that, by virtue of being between the sill of a window and the head of a window below it, spans a floor of the building. Thus, where floors are compartment floors, the significance of such panels lies in the potential for fire spread to bypass the compartment floor.

Note 2 The above is only indicative of some of the considerations relating to infill/spandrel panels. It does not address all potential situations where panels are present, such as in curtain wall systems also incorporating glazing.

Positive

Sufficiently remote from windows and not forming a continuous vertical section, such that fire and smoke spread into the building to give rise to secondary fires is unlikely and fire will only spread by cascading up panels*

Continuous vertical sections but sufficiently remote from windows such that fire and smoke spread into the building, causing secondary fires, is unlikely

Isolated areas of panels that do not cross compartment boundaries or cause a fire to cross a compartment boundary

Neutral

Adjacent to, but not in a vertical continuous line with, windows

Negative

In a vertical continuous line with windows such as to increase the likelihood of secondary fires

Where spanning a compartment boundary and in particular a compartment floor

N.6 Setbacks

Positive

Combustible cladding is set back from the wall edge, such that direct flame impingement on the cladding from a fire on a lower level is highly unlikely

(An example would be a penthouse flat constructed on the roof of an existing building)

Note 1 This depends upon the distance from the wall edge, the nature of the construction of the external wall below the set back and the proximity of the openings in the wall from which fire can spread. Consideration might need to be given to the use of the terrace and nature of the construction of the terrace itself if it is considered that there is a high likelihood that fire could spread due to the combustibility of the terrace, e.g. where there is timber decking in conjunction with exposed polymeric roof insulation below. Management controls are outside the scope of this PAS and are considerations for the building’s FRA.

Note 2 Fire engineering analysis and calculation might be able to assist by estimating the length of flame projecting from a window below and the level of radiant heat on the cladding from these flames.

Neutral

-

Negative

-

N.7 Overhangs and projections

Note 1 Overhangs, where a section of the façade projects forward from the section below, have the potential to divert flames horizontally under the soffit of the overhang and then for the flames to adhere to the vertical façade of the section above.

Note 2 This does not refer to balconies as projections (see N.11).

Note 3 The potential beneficial contribution of projections in terms of dividing cavities is referred to earlier in this table.

Positive

Projecting floor slabs that divide combustible cladding such as to divert flames away from the walls and protect the cladding above or slow the rate of fire spread

Neutral

-

Negative

Where fire spread under an overhang can give rise to extended flame lengths over the soffit and up the external wall beyond

Note This depends upon the size of the overhang and the distance for flames to spread before reaching the wall edge. It can occur whatever the construction of the overhang, but is exacerbated where this construction is combustible.

N.8 Proximity to windows and other openings to the accommodation

Positive

Remote from windows and openings, such that fire and smoke spread into the building, causing secondary fires, is not possible

(Typically, this occurs when a façade has no openings for windows and other unprotected openings; see N.9.)

Neutral

Horizontally adjacent to windows and openings, but not vertically in line with such openings, such that fire and smoke spread into the buildings, causing secondary fires, as a result of direct flame impingement, is possible, but only under adverse wind conditions

Negative

Horizontally adjacent to windows and openings, and vertically in line with such openings, such that fire and smoke spread into the buildings, causing secondary fires, as a result of direct flame impingement, is highly likely

N.9 Presence of vents or other openings for services in the façade

Positive

Where vents pass through a cavity, either:

• they are either protected by cavity barriers (including ADB “deemed to satisfy” alternatives); or
• the cavity is not a medium for fire spread between compartments (e.g. because it has adequate cavity barriers on compartment lines or does connect multiple compartments)

Neutral

Where vents pass through a cavity, either:

• the cavity does not include combustible materials; or
• the cavity is faced on either side by brick or concrete at least 75 mm thick and any combustible insulation in the cavity is not thermoplastic

Negative

Any other circumstances where vents pass through a cavity

N.10 Proximity of combustible elements of a façade to escape route windows and other openings

Positive

Remote from windows and openings, such that fire and smoke spread into the escape routes to give rise to untenable conditions is not possible*

(Typically, when a façade has no openings onto escape routes) Remote from windows and openings, such that fire spread into the escape routes to give rise to untenable conditions is remote

(Typically, when a façade has openings onto escape routes, but these are sufficiently separated by construction that would not support combustion)

Neutral

Adjacent to windows and openings onto escape routes, but the same fire could not spread to affect more than one escape route

(Typically, where there are two or more escape routes which can be used by occupants who all have access to multiple routes)

Negative

Adjacent to windows and openings, such that fire and smoke spread into the escape routes to give rise to untenable conditions is likely and there is only one escape for some or all occupants

(This includes vents that are part of a smoke control system, where there is the potential from an external fire to prejudice the effectiveness of the smoke control system)

Above doorways forming final exits from escape routes, such that burning material or debris from a fire involving the external walls above will pose a danger to escaping occupants

Note In this situation, burning material and debris from the fire above can also pose a danger to firefighters entering or leaving the building.

N.11 Attachments

[Covers:

• a balcony attached to an external wall;
• a device for reducing heat gain within a building deflecting sunlight, which is attached to an external wall (brise soleil);
• a solar panel attached to an external wall; • any other attachment which could present a fire risk]

Note A balcony approach to flats could potentially be considered an attachment if combustible, but other constraints regarding its construction and combustibility apply in the case of new buildings because of its use as an escape route. A combustible balcony used as a communal means of escape has the potential not only to impact on the fire behaviour of the external walls but also to lead to the means of escape being compromised in the event of fire.

Positive

Non‑combustible open balconies (Where these extend along a façade, they have the potential both to:

• interrupt a cavity; and
• deflect flames away from the building and away from the façade)

Neutral

Timber (or other combustible) balconies of limited extent

Timber decking with steel plate or concrete below

Negative

Timber balconies of large extent

Timber (or other combustible) balconies, with aggravating features

(For example:

• without protection from the underside; and
• adjacent to timber or other combustible wall panelling)

Combustible features such as brise soleil incorporating combustible material

Photovoltaic (PV) installations, especially if incorporating combustible elements

Note PV installations present an ignition hazard as well as a potential fire load.

Note 1 There is no current guidance relating to the risk posed by balconies with combustible elements and, in particular, timber decking. While it has always been possible for fire to spread vertically over the façade of a building by a fire igniting a balcony and spreading to the balcony above and then cascading up the building, the consequences of this have usually been limited. Some high‑profile fires which resulted in fire spreading into a large number of flats above by this mechanism have led some to take a very conservative approach. In practice, the scale and extent of such fires varies, and depends upon various factors, including:

• the size of the balcony;
• the extent to which more than the decking is combustible;
• whether the balcony is in line with similar balconies above/below, giving the potential for a fire to cascade upwards from balcony to balcony or cause ignition to balconies below;
• whether the balcony is staggered from others, reducing the potential for fire to cascade upwards or spread downwards;
• whether combustible material in the balcony is exposed from below, or is simply a lining on top of a metal or concrete deck, or underdrawn with an essentially non‑combustible material; and
• whether the likelihood of ignition is minimized by virtue of the limitations on what the balcony can be used for, by virtue of its size, or by management controls that can be placed on residents by the owner of the building, e.g. a prohibition on using barbecues. Management controls are primarily considerations for the building’s FRA, although recommendations relating to management controls could appear in the FRAEW report.

Note 2 Based on past experience of laminated glass in fires when used as part of balcony construction, replacement of laminated glass on balconies is not, at the present time, considered justified in relation to existing blocks of flats. Further consideration to balconies and laminated glass is given in Annex L.

N.12 Proximity of combustible elements of a façade to a neighbouring building

Positive

-

Neutral

-

Negative

Windows or other openings in adjacent or abutting neighbouring buildings that are sufficiently close that direct flame impingement from a fire in the neighbouring building is foreseeable

Note The potential for sufficiently high levels of radiant heat flux from unprotected openings in a neighbouring building could also be a negative risk factor, although it is recognized that there will be difficulties in determining this, given the need for information relating to the neighbouring building.