Building standards technical handbook 2020: non-domestic

In order to reduce the danger to the occupants of other buildings, one building should be isolated from another by either construction or distance. The distance between a building and its relevant boundary is dictated by the amount of heat that is likely to be generated in the event of fire. Generally, this will be influenced by how the building is used, compartmented (see guidance to Standard 2.1), separated (see guidance to Standard 2.2) and the extent of any openings or other unprotected areas in the external wall of the building.

The guidance for fire spread to neighbouring buildings should not be assessed in isolation and reference should be made to the guidance to Standard 2.4 for fire spread in cavities and the guidance to Standard 2.7 for fire spread on external walls.

Scottish Advice Note: External Wall Systems – draft guidance on the risk assessment and remediation of cladding on existing buildings is available online and will be published in summer 2021. Following consultation on the Scottish Advice Note, the document recommends that:

“any building extensively clad with EWS incorporating Category 3 MCM should have the MCM removed without delay.  Interim measures may need to be put in place until this is done.  Buildings that are partially clad in Category 3 MCM may also require remediation.  Any decision not to remove Category 3 MCM from a partially clad building must be robustly justified in the fire safety risk assessment and any accompanying appraisal report”.

To align with the advice above, guidance to this standard is amended, recommending that Category 3 Metal Composite Material (MCM)  cladding should not be used in any situation. For this purpose, Category 3 MCM cladding is defined as two thin metal skins bonded together using a core material, typically 3 to 7 mm thick, with a calorific value of more than 35 MJ/Kg.

As a further interim measure, citation of BS 8414/BR135 tests is removed as ‘Alternative Guidance’. Verifiers are requested to notify Building Standards Division of any building warrant applications made citing BS 8414 as a route to compliance.

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

External walls should have at least the fire resistance duration as shown in the table below, apart from the amount of unprotected area set out in clause 2.6.2. For guidance on automatic fire suppression systems, see clause 2.1.2.

The installation of an automatic fire suppression system greatly reduces the amount of radiant heat flux from a fire through an unprotected opening and this is recognised in the table to clause 2.6.1 and in the guidance to clause 2.6.2.

 

Additional information:

 

 

An unprotected area means any part of an external wall (including a door or window opening) which does not attain the appropriate fire resistance duration recommended in the table to clause 2.6.1.

An unprotected area does not include a fixed unopenable window where the frame and glazing has the appropriate fire resistance duration. Any wallhead fascia, soffit or barge board, or any cavity vents or solum vents may also be excluded from the unprotected area calculation.

Up to 500mm from boundary - where the external wall of a building is not more than 500mm from the boundary there should be no unprotected area, other than any wallhead fascia, soffit or barge board, or any cavity vents or solum vents.

500m to 1m from boundary - where the external wall of a building is more than 500mm but not more than 1m from the boundary, the level of unprotected area is limited to:

More than 1m from boundary - where the external wall of a building is more than 1m from the boundary, the minimum distance to the boundary may vary with the amount of unprotected area. This is derived from the use of the building and can be calculated by:

 

 

The Enclosing Rectangle Method can be applied to a building or compartment more than 1m from the boundary using the Step by Step Guide below. The minimum distance from the boundary may be arrived at by interpolation between the percentages shown in the tables.

Table A should be used for storage buildings, factories (Class 1), shops, and entertainment buildings.

Table B should be used for residential buildings, offices, assembly buildings, and factories (Class 2). Table B should also be used for domestic buildings which exceed the limitations described in clause 2.6.3 of the Technical Handbook for domestic buildings.

Automatic Fire Suppression system - where the building or compartment is a shop, entertainment building, factory (Class 1) or a storage building, Table B may be used instead of Table A provided the building:

Open sided car parks - there is no limitation on the amount of unprotected area for an open sided car park provided the external wall is more than 1m from the boundary.

Step by step guide to Enclosing Rectangle Method

Step 1 - establish a plane of reference which is the most favourable for the side of the building under consideration and which:

Normally it will be best for the plane of reference to be roughly parallel to the relevant boundary. Where the boundary distance has not been set, an assumed relationship with the relevant boundary should be made.

 

Step 2 - on the plane of reference, and at right angles to it, project lines marking those proposed unprotected areas which are 80o or less to the plane of reference. Any external side of a building which makes an angle of more than 80° with the plane of reference can be ignored in the calculation of unprotected area.

Step 3 - construct a rectangle which encloses all the unprotected areas projected onto the plane of reference following the diagrams below. See also clause 2.6.3 for combustible cladding.

 

Step 4 - from the relevant table according to the use of the building or compartment, select an enclosing rectangle which is equal to, or next greater in both height and width to, the rectangle constructed under step 3. Compartmentation of the building can reduce the size of the enclosing rectangle in accordance with the diagrams below. The tables provide for enclosing rectangles of 3, 6, 9, 12, or 15m high, with widths in steps from 3m to no limit.

 

Step 5 - from the unprotected areas projected under step 2:

Step 6 - proceed to steps 7 to 9 where the boundary distance has been set, or steps 10 to 12 where it has not.

To find the maximum unprotected area for a given boundary position.

Step 7 - from the tables below find the unprotected percentage allowed for the distance from the relevant boundary. This distance relates to the plane of reference and not the side of the building, unless they coincide.

Step 8 - where the proposed total unprotected area relative to the size of the enclosing rectangle exceeds the allowable unprotected percentage for that distance, the design should be modified until an allowable unprotected percentage is established, and

Step 9 - repeat the process for all sides of the building situated more than 1m from any point on the relevant boundary.

To find the nearest position of the boundary for a given building:

Step 10 - from the tables below, find the minimum allowable distance for the unprotected percentage. This distance relates to the plane of reference and not the side of the building, unless they coincide.

Step 11 - repeat the process for all sides of the building, and

Step 12 - where these minimum distances are superimposed upon a plan of the building, a zone around the building is established upon which a boundary should not encroach.

 

Additional information:

 

 

 

Additional information:

 

 

External wall cladding includes all non load-bearing external wall cladding systems attached to the structure, for example, composite panels, clay or concrete tiles, slates, pre-cast concrete panels, stone panels, masonry, profiled metal sheeting including sandwich panels, timber panels, weather boarding, thermally insulated external wall rendered systems, glazing systems and other ventilated cladding systems. For the purposes of compliance with Standard 2.6, external wall cladding includes spandrel panels and infill panels.

All situations – Category 3 Metal Composite Material (MCM) should not be used as a component of external wall cladding (see note in 2.6.0).

Not more than 1m from a boundary - External wall cladding not more than 1m from a boundary should have achieve a European Classification A1 or A2.

More than 1m from a boundary - Fire within the building may break-out through a window or door opening and as a consequence, the cladding, once ignited, would contribute to the heat generated from the fire. Therefore where the cladding is more than 1m from a boundary and is constructed from products more than 1mm thick with a European Classification B, C, D or E (as described in annex 2.E), the cladding should be included in the calculation of unprotected area.

See also guidance to Standards 2.4 and 2.7 for additional guidance on external wall cladding to buildings with a storey height more than 11m above ground and to entertainment and assembly buildings, residential care buildings and hospital buildings, regardless of storey height.

However combustible cladding need not be included in the calculation of unprotected area where:

In addition, a cavity formed by external wall cladding should be protected in accordance with the guidance to Standard 2.4 and fire spread on external walls in accordance with the guidance to Standard 2.7.

Notional Boundary - there may be a risk of fire spread between buildings even when on land in the same occupation. A notional boundary therefore is used to determine the safe distance between buildings or compartments in this situation. In order to establish whether a notional boundary calculation is necessary, the size of the opposing buildings or compartments should be established first (see diagram below).

For example, in the diagram, where the combined area of building A plus building B exceeds the maximum allowable area of any compartment as set in the tables to clause 2.1.1, then a notional boundary calculation is necessary. The designer is free to set this notional boundary at any point between the 2 buildings under consideration and this distance will determine the amount of unprotected area. The buildings should be separated by a distance not less than the sum of the distances calculated in respect of each building relative to a notional boundary in accordance with clauses 2.6.1 and 2.6.2, as if the notional boundary were a boundary (see diagram below).

Opposing compartments - a notional boundary is also necessary in the case of a single building having opposing compartments externally.

The roofs of the buildings should be separated by a distance not less than the sum of the distances provided in relation to the notional boundary in accordance with the guidance to Standard 2.8 in respect of each roof.

New and existing buildings - where a notional boundary, described above, is to be established between 2 buildings, one of which is an existing building, for the purposes of this guidance, the existing building should be regarded as if it were a new building.

Every part of an external wall not more than 1m from a boundary should be constructed of products which achieve European Classification A1 or A2. This does not apply to insulation exposed in a cavity that is between 2 leaves of masonry or concrete at least 75mm thick, and which has a cavity barrier around all openings in the wall and at the top of the wall-head.

Also, in shared residential accommodation, a structural frame and any thermal insulation material in the frame not more than 1m from a boundary may be constructed from combustible products provided:

See the guidance to Standards 2.4 and 2.7 for additional guidance in relation to cavities and external wall cladding systems.

Where an element of structure provides support to an external wall (including external wall cladding) which has a fire resistance duration (as recommended in the guidance to clauses 2.6.1 and 2.6.2) the supporting element of structure should also have at least the same fire resistance duration.