External wall systems in existing multi-storey residential buildings - fire risk: advice note

This Scottish Advice Note (SAN) provides advice for those responsible for fire safety in residential buildings including building owners/managers/residents groups responsible for determining the fire risk posed by external wall systems on existing multi-storey residential buildings. It will also be of interest to fire risk assessors and specialist external wall appraisal experts.

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Annex 4: BS 8414 fire tests and BR135

Following an outbreak of fire inside the building and assuming no firefighting intervention occurs, the fire may develop to flashover and break out from the room of origin through a window opening or doorway. Flames breaking out of a building from a post-flashover fire will typically extend 2 m above the top of the opening. The BS 8414 large scale façade tests are based on this principle i.e. a post flashover compartment fire emanating from a window or door opening or from an external fire source. The test has been designed to allow the fire performance of both applied and supported non-loadbearing external wall cladding systems to be determined. BS 8414-1 is for external cladding systems fixed and supported by a masonry substrate. BS 8414-2 are for those fixed and supported by a structural steel frame.

Although representative of the cladding system, the test rig configuration is not a replica of the building. Its design is intended to achieve consistency across tests including a maximum air velocity in any direction of 2 m/s as specified in the test. The test rig consist of a vertical main test face, into which the combustion chamber is located, and a vertical return wall or wing, set at 90° to the main test face. The test specimen should be installed with all the relevant components, and should be assembled in accordance with the manufacturer's instructions. The main test face is at least 8 m high and 2.6 m wide, with the return wing being 8 m high and 1.5 m wide.

A wooden crib is used as the heat source which gives a total nominal heat output of 4500 MJ over a 30 minute period at a peak heat release rate of 3 ± 0.5 MW. The test is terminated early if, for example, sustained flaming extends above the height of the test rig. It is only possible to classify a cladding if the test lasts the duration (30 minutes burn period and up to 30 minutes thereafter).The cladding system fails the performance criteria in BR 135 where the any of the thermocouples at level 2 (5 m above the combustion chamber) exceeds 600 ºC for a period of at least 30 seconds, within 15 minutes of the start time of the test. The fire test and reporting of the test results require a competent professional to assess the extent to which the system tested is applicable to the real building.

BS 9414: 2019 - Fire performance of external cladding systems – The application of results from BS 8414-1 and BS 8414-2 tests

There are a number of practical limitations on the size and design of construction elements that can be evaluated by a standard method of test. For example, if these elements are larger or of a modified design or type, it is necessary to be able to confirm that the integrity of the test result would be maintained if such changes to the system were implemented. The number of changes can vary according to the complexity of the external cladding and the type of construction being considered. Every component of the external cladding could have an impact on the fire spread performance of the system and each change needs to be the subject of a separate evaluation. This can be achieved through the use of standardized rules.

BS 9414 provides direct application rules to be applied when assessing the implications of such modifications in relation to systems which have been subjected to tests in accordance with BS 8414. The following systems are covered by the standard:

  • rain-screens;
  • external thermal insulated composite systems (ETICS);
  • self-supporting double skin metal faced insulating (sandwich) panels.

Extended application enables a prediction to be made on the expected fire performance of a system under specified fire conditions if one or more of the components or parameters evaluated as part of the test specimen were to be changed. In the specific case of external cladding systems, such predictions are based upon the need to abide strictly with the fundamental principle that the fire performance of the modified system would be equal or better if it were to be subjected to a BS 8414-1 or BS 8414-2 test.


Email: Buildingstandards@gov.scot

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