COMMENTARY ON CLAUSE 35

a) Fire Spread Restriction Methods:

· Direct Flame Impingement: Fire spreads directly from one building to another.

· Radiation: Fire spreads through radiative heat transfer, possibly with burning debris.

b) Factors Affecting Fire Spread:

· Distance and orientation between buildings (based on configuration factor).

· Extent of heat-transmitting building surfaces (fire-resistant constructions are assumed to prevent heat transfer).

· Intensity of radiative energy from the fire.

c) Assumptions for Recommendations:

· Fire is contained within the compartment of origin.

· The compartment has reached flashover.

· All unprotected areas radiate with equal intensity.

· Radiation intensity depends on fire load density.

· Automatic sprinklers halve radiation.

· Glazing and supporting structures fail unless fire-rated.

NOTE 1 A roof is not subject to the provisions in this clause unless it is pitched at an angle greater than 70° to the horizontal. Similarly, vertical parts of a pitched roof such as dormer windows (which taken in isolation may be regarded as a wall), would not need to meet these provisions unless the slope of the roof exceeds 70°. It is a matter of judgement whether a continuous run of dormer windows occupying most of a steeply pitched roof needs to be treated as a wall rather than a roof.

NOTE 2 Recommendations for the construction of walls common to two or more buildings is given in 31.3.1.2.

NOTE 3 The measures recommended in this clause will not necessarily protect a building from a fire in an existing building on an adjoining site. The property loss prevention aspects of the situation need to be assessed in each case.

35.1 Boundaries

35.1.1 Relevant boundary

NOTE A notional boundary can be a relevant boundary.

a) Relevant Boundary Definition:

The boundary used to measure separation distance.
Usually taken as the site boundary.

b) Exceptions:

If a wall faces a space unlikely to be developed (e.g., road, canal, river), the boundary can be assumed to be an imaginary line halfway across the feature.

A wall should be treated as facing a boundary if it makes an angle of 80° or less (see Figure 42).

35.1.2 Notional boundary

COMMENTARY ON 35.1.2

NOTE It is assumed in Figure 43 that building A is existing and building B is new/proposed.

A notional boundary is an imaginary line assumed to exist between two buildings. (See Figure 43.)

When the need for a notional boundary is determined, its location should be established according to Figure 43.

Where both buildings are new, one should be designated as existing and the recommendation for notional boundary assessed accordingly.

Figure 42 Relevant boundaries

Figure Notional boundaries

a) Notional Boundary Determination:

· Set based on the unprotected area of the existing building's façade.

· New buildings should follow proximity and unprotected area restrictions relevant to this notional boundary.

b) Both Buildings are New:

· Notional boundary assumed to be halfway between the two buildings.

c) Same Site, Same Organization:

· Separation between buildings can usually be ignored.

d) Higher Life Risk Buildings (Occupancy B and C):

· A notional boundary should be established unless both buildings are sprinkler-protected.

35.1.3 Property protection between buildings on the same site

The principle of a notional boundary can be applied to any building for property protection purposes. In these circumstances the recommendations in 35.1.2 should be met.

35.2 Unprotected area

35.2.1 General

The following factors should be taken into account when determining the extent of the unprotected area.

a) Unprotected Area Definition:

· Any external façade with fire resistance below the recommended level in Table 22 is counted as unprotected.

b) Combustible Surface Consideration:

· Sections of external walls with fire resistance but a combustible surface over 1 mm thick count as unprotected area, calculated as half the actual area of the combustible surface.

c) Restrictions on Unprotected Area:

· The amount of unprotected area must be limited based on the distance to relevant (or notional) boundaries.

d) Exclusions from Unprotected Area:

· External walls of stairways in protected shafts.

· External walls more than 30 m above ground in uncompartmented buildings (except where high rack storage is present).

· Small unprotected areas within an otherwise protected façade (see Figure 45).

e) Sprinkler Protection Impact:

· Unprotected area may be doubled, or the distance to the boundary halved, if automatic sprinklers are installed.

NOTE Small unprotected areas pose a negligible risk of fire spread.

Figure 44 Combustible surface material as unprotected area

Figure 45 Exclusion from unprotected area calculation

35.2.2 Canopies and open- sided car parks

a) Separation Distance for Canopies:

Separation distance can be measured from the wall, not the canopy edges, if canopy edges are at least 2 m from the relevant boundary.

b) Open-Sided Car Parks:

Same principle applies; separation distance from the wall is acceptable if canopy edges are 2 m from the boundary.
For life safety, the car park can be treated as having the same fire size/radiation intensity as a sprinklered building if it is at least 1 m from the relevant boundary.
For property protection, boundary separation may still be required.

35.2.3 External wall of portal frame buildings

COMMENTARY ON 35.2.3

a) Fire Resistance in Portal Frames:

· Single-storey portal frames may not need fire resistance if only supporting a roof, unless near a relevant boundary where external walls need fire resistance to prevent fire spread.

b) Design Guidance:

· Design method for portal frames is detailed in SCI publication P313, covering aspects including multi-storey types.

· If fitted with a sprinkler system per BS EN 12845 or BS 5306-2, foundation design to resist overturning is not necessary.

c) Reinforced Concrete Portal Frames:

· Can support fire-resisting external walls without special column base provisions to resist overturning.

d) Stability Considerations:

· If external wall stability is linked to the portal, columns and rafters need fire resistance to prevent premature wall failure.

· If not linked, foundations and connections should be designed to handle overturning from fire-induced collapse while maintaining wall stability and integrity.

35.3 Degree of separation

35.3.1 General

COMMENTARY ON 35.3.1

a) Ignition Threshold:

· Intensity required to ignite wood in still air is 12.6 kW/m².

b) Radiation and Unprotected Area:

· Ignition risk inside a building depends on the amount of unprotected area in its façade.

c) Boundary Distance:

· Measure separation from relevant boundaries instead of other buildings to allow development on neighbouring sites.

d) Separation Requirement:

· Buildings should be at least half the distance at which total thermal radiation intensity from unprotected areas would be 12.6 kW/m².

NOTE The recommendations in 35.4 also relate to the separation distance between a roof and a relevant boundary.

35.3.2 External walls within 1 m of the relevant boundary

a) External Wall Requirements (within 1 m of relevant boundary):

· Fire Resistance: Achieve appropriate fire resistance for both integrity and insulation (see Table 22).

· Unprotected Areas: Limited to small areas as per Figure 45.

· Flame and Radiation Resistance: Withstand direct flame impingement and high radiation levels.

· Surface Material: Use non-combustible surfaces.

· Barrier Effectiveness: Serve as an effective barrier to fire from both inside and outside the building.

NOTE 1 These recommendations are aimed at protecting neighboring buildings from direct flame impingement as well as radiation.

NOTE 2 See 35.5 for external wall materials and construction.

35.3.3 External walls 1m or more from the relevant boundary

a) Wall Requirements (at least 1 m from relevant boundary):

· Unprotected Area: Must conform to limits specified in 35.3.4.

· Fire Resistance: Remaining wall should meet fire resistance levels stated in Table 23 or Table 24, including 15 minutes of insulation.

· Sprinkler Systems: Buildings with sprinklers have reduced radiation risk; non-sprinklered buildings must limit non-fire-resisting wall areas based on distance to boundary and fire intensity.

35.3.4 Calculation methods

Methods to Determine Maximum Unprotected Area:

Small Residential (Occupancy Characteristic C): Use Table 35 for buildings ≤3 storeys and ≤24 m long.
Enclosing Rectangles: Refer to Part 1 of BRE Report 187 for details.
Aggregate Notional Area: Refer to Part 1 of BRE Report 187 for details.
Fire Engineering Calculation: Use BS 7974 for detailed calculations.

Table 35 Small residential unprotected area limits and boundary distances

Minimum distance between façade and relevant boundary	Maximum total unprotected area per compartment
m	m2
1	5.6
2	12
3	18
4	24
5	30
6	No limit

35.4 Roofs

COMMENTARY ON 35.4

Roof Fire Performance Recommendations:

Scope: Focuses on roofs exposed to external fire, excluding the overall roof structure.
Standards: Relevant tests are BS 476-3 and BS EN 13501-5.
Relevance:
If the roof is near a boundary and at risk of ignition from neighbouring fires.
To prevent fire spread between compartments via roof coverings.

Figure 46 Roof covering adjoining line of compartmentation

NOTE 1 The circumstances when a roof is subject to the provisions for space separation are explained in Clause 35, Note 1.

NOTE 2 Recommendations and guidance concerning the fire properties of roofs are given in:

a) subclause 30.1.1 for roofs that are part of a means of escape and for roofs that are used as a floor;

b) subclauses 34.1 and 35.4.2 for the internal surfaces of roof lights as part of the internal lining of a room or circulation space;

c) Clause 31 for roofs that pass over the top of a compartment wall;

d) subclause 34.1.6 for the construction of roof coverings in roofs incorporating roof lights.

NOTE 3 Recommendations relating to roofing materials are given in 34.2.

35.4.1 Separation distances

The separation distance is the minimum distance from the roof (or part of the roof) to the relevant boundary, which may be a notional boundary. Separation distances should be as recommended in Table 36 for the appropriate type of roof covering and building use.

NOTE Advice on fire protection of thatched buildings is available from the Dorset Building Control Technical Committee at www.dorsetforyou.com/building-control/ help/technical-committee/thatched-roof 9).

35.4.2 Roof lights

The separation distance for plastics roof lights should be as recommended in 34.1.2 for the appropriate classification. Roof lights should be at least 1.5 m from a compartment wall. Plastics roof lights should not be used in protected stairs.

Table 36 Separation distances for roof coverings

Designation of covering of roof or part of roof A)		Distance of roof from any point on relevant boundary
National class	European class	Less than 6 m	At least 6 m	At least 12 m	At least 20 m
AA, AB or AC	BROOF(t4) CROOF(t4) DROOF(t4) EROOF(t4) FROOF(t4)	Acceptable	Acceptable	Acceptable	Acceptable
BA, BB or BC		Not acceptable	Acceptable	Acceptable	Acceptable
CA, CB or CC		Not acceptable	Acceptable B), C)	Acceptable B)	Acceptable
AD, BD (or CD B))		Not acceptable	Acceptable C)	Acceptable	Acceptable
DA, DB, DC (or DD B))		Not acceptable	Not acceptable	Not acceptable	Acceptable C)

NOTE 1 Unwired glass at least 4 mm in thickness has an AA designation.

NOTE 2 See Table 34 for limitations on plastics roof lights.

A) The performance of roof coverings is designated by reference to the test and classification standards given in BS 476-3 (national class) and BS EN 13501-5 (European class).

B) Not acceptable on any of the following buildings:

· occupancy characteristic A;

· buildings with a volume of more than 1 500 m3.

C) Acceptable on buildings not listed in footnote B, if part of the roof is no more than 3 m2 in area and is at least 1.5 m from any similar part, with the roof between the parts covered with a material of limited combustibility.

NOTE 1 When used in roof lights, a rigid thermoplastic sheet product made from polycarbonate or from unplasticized PVC, which achieves a Class 1 rating for surface spread of flame when tested in accordance with BS 476-7 or classified as C-s3, d2 in accordance with BS EN 13501-1:2007+A1, is deemed to have an AA designation.

The roof covering material surrounding a plastics roof light should be of limited combustibility for at least 3 m distance.

NOTE 2 The designation of external roof surfaces is defined in BS 476-3 or BS EN 13501-5.

Products may have upper and lower surfaces with different properties if they have double skins or are laminates of different materials, in which case the more onerous distance should be assumed.

NOTE 3 The method of classifying thermoplastic materials is given in 34.1.2.

35.5 External fire spread over the external faces of buildings

External Wall Construction:

a) Fire Spread Control: Must prevent rapid fire spread that could threaten people inside or around the building.

Flame Spread: Should be controlled to avoid creating routes for fire to bypass compartment floors or walls.
Ignitability: External surfaces should not be easily ignitable to prevent fire spread between buildings.
Performance Criteria: Walls should either:
Meet BR 135 standards with full-scale tests (BS 8414-1 or BS 8414-2).
Alternatively, comply with specific recommendations for fire performance.

NOTE 1 The total amount of combustible material might also be limited in practice by the provisions for space separation.

a) The external surfaces of walls should meet the provisions in Figure 47.

Figure 47 Provisions for external surfaces of walls

NOTE 1 The national classifications do not automatically equate to the equivalent European classifications, therefore products cannot typically assume a European class unless they have been tested accordingly.

NOTE 2 When a classification includes “s3, d2”, this means that there is no limit set for smoke production and/or flaming droplets/particles.

b) In a building with a storey 18 m or more above ground level, any insulation product, filler material (not including gaskets, sealants and similar), etc. used in the external wall construction should be of limited combustibility.

NOTE 2 This restriction does not apply to masonry cavity wall construction that conforms to Figure 36.

c) Cavity barriers should be provided in accordance with 33.1.

d) In the case of an external wall construction, of a building which, by virtue of the recommendations in 33.2d), is not subject to the provisions of Table 32, the surfaces which face into cavities should also meet the provisions of Figure 47.

35 External fires spread and building separation

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