Building standards - managing fire risks associated with use of external wall systems: research

5. Case Studies 1-5

28. The reviews of the Case Studies were intended to gain a better understanding of the existing verification process to inform any potential solutions to provide appropriate, effective, and robust verification and compliance processes with regards to the design and construction of external wall systems. The observations below therefore focus on identifying potential gaps in the verification processes applied to the Case Studies to improve future verification and compliance processes.

Case Study 1

Building Profile:

• Number of storeys: 4
• Building type: private residential
• Top storey height: 9m (initially assumed to be more than 11m)
• Building Warrant application submission date: post October 2019

29. Drawing information was provided for the building’s external wall systems, as well as correspondence between the local authority verifier and the design team.

30. Three external wall types were identified by the applicant design team: facing brick external leaf (general), zinc cladding, and facing brick external leaf (closes). These walls were specified to contain a combination of PIR insulation and glass mineral wool insulation.

31. The buildings are largely constructed of timber structure.

32. The top storey heights are less than 11m and the external walls are more than 1m to the relevant boundary for external fire spread. The external walls can be constructed from combustible products more than 1mm thick which is European Classification B, C, D or E to meet the Domestic Technical Handbook guidance applicable to this building.

33. The approved fire strategy and the external fire spread calculations relied on the external walls not forming the windows to achieve 60 minutes fire resistance from inside.

34. To demonstrate that the external walls achieve at least 60 minutes fire resistance from inside, BS/476: Part 21: 1987 fire testing had been provided for part of the external wall system, i.e. the inner face fire rated plasterboards, PIR insulation, an air/vapour control layer, another layer of plasterboards, glass mineral wool insulation, sheathing board and breather membrane with timber battens in between. The outer part of the external wall construction such as brick and zinc were not tested as part of the fire test.

35. It was noted that there were some component differences in product selection and manufacturer between the tested construction and the outline specification document produced by the applicant design team for the proposed external wall systems.

36. Fire performance data sheets for the replacement products were not available in the information provided.

37. No data sheets related to fire performance were contained in the information provided for the cavity barrier products proposed. The cavity barrier products were proposed in line with 60 minute separating walls and floors. The specification provided by the applicant design team did not specify the fire rating of the cavity barriers.

38. No data sheets related to fire performance were contained in the information provided for the intumescent cavity barrier products proposed. The intumescent cavity barrier products were proposed in line with 60 minute separating floors. The specification provided by the applicant design team did not specify the fire rating of the intumescent cavity barriers.

39. The outline specification did not identify the location of the HPL/ cladding.

40. Parts of the external wall system need to achieve 60 minutes fire resistance from the inside, and there are boiler flue and ventilation penetrations through the 60 minute external wall. This does not appear to be provided with 60 minute fire stopping, and no evidence is provided to support this proposal from the test house to confirm that the construction achieves 60 minutes fire resistance from the inside.

41. The record provided showed the local authority verifier carried out one site inspection for the project.

Case Study 2

Building Profile:

• Number of storeys: 6
• Building type: student residential
• Top storey height: 15.45m
• Building Warrant application submission date: post October 2019

42. Drawing information was provided for only one external wall type, although others were noted, such as curtain walling and other unidentified materials. The external wall type provided was “External Wall (Typical)” and comprises:

• 102.5mm facing brick Dunbar bond
• 50mm cavity zone
• 100mm non combustible insulation Knauf Earthwool
• 12mm external grade sheathing board (non combustible)
• 100mm Stud Framing System to specialist design at 600mm centres
• 100mm Full fill multipurpose quilt insulation
• 2 layers of 15mm Gyproc SoundBloc (60 mins FR)

43. No correspondence was provided to the authors for review between the local authority verifier and the design team, although the authors assume that some dialogue must have taken place.

44. Data sheets related to fire performance for products were not provided for any of the products shown on the external wall system detail drawings.

45. Notes on the drawings suggest that the external wall type 1 build up will achieve 60 minutes fire resistance, but no evidence to support this was provided.

46. No data sheets related to fire performance were provided for the cavity/fire barrier products proposed, although drawings referred to 30 minutes fire resistance.

47. The fire strategy elevations show 30 minute fire barriers, even at junctions with 60 minute fire rated walls (to compartment walls and protected zone enclosures) and 60 minute compartment floors. There was no documentation provided for this research to accompany the local authority’s decision to accept this, although the authors note that the internal wall arrangement achieves 60 minutes fire resistance from the inside.

48. No detail drawings were provided for vertical fire barriers at the junctions between external walls and 60 minute fire rated walls/walls enclosing protected zones.

49. The fire strategy elevation drawings omitted ductwork penetrations, and this is noted on the drawing.

50. No site inspection reports were provided as part of this research.

Case Study 3

Building Profile:

• Number of storeys: 8
• Building type: private residential
• Top storey height: 18.9m
• Building Warrant application submission date: pre-October 2019 (pre-June 2017)

51. Drawing information was provided for the building’s external wall systems, as well as contractor’s construction specification and correspondence between the local authority verifier and the project architect.

52. Please note that the building warrant application has two stage amendments. Given the age of the applications, they are predominantly paper based where the drawings and supporting information were held separately with outgoing communications being in electronic format. During the warrant process, the local authority changed IT/ providers. Whilst every effort was made to capture all data during the change, there may be some gaps in the data provided for this research due to the search process for data.

53. Three external wall types each with two different types of internal masonry walls of at least 100mm thick were noted on the drawings and contractor’s construction specification document: 18mm thick render on 100mm concrete outer block; 90mm granite stonework and Rockpanel achieving Class A2-s1,d0. These external wall systems are designed with combustible materials within their cavities, such as timber framing and combustible insulation.

54. To meet the Technical Handbook applicable to the Building Warrant application submission date, combustible materials are permitted within the cavity of 2 leaves of masonry or concrete construction that are at least 75mm thick and provided with 30 minute cavity barriers around all openings and at the top of the wall-head.

55. The local authority verifier provided 36 comments in one of the building warrant stages that is applicable to the external wall design. Out of these, there were 2 comments related to Section 2.4, 2.6 and 2.7 of the Domestic Technical Handbook guidance. These comments were in relation to the detailing of cavity barriers and the requirement of the cladding to be non-combustible. These comments were subsequently addressed by the design team applicant by changing the initially proposed combustible cladding to Rockpanel cladding and by providing large scale detailed drawings showing the external wall details with junctions between wall finishes and window jamb detail, door jamb detail, and eaves detail.

56. With the exception of Rockpanel cladding, no other datasheets related to the build up of the external walls were provided in this research.

57. The contractor’s construction specification document did not mention the product specification for cavity barriers/fire barriers.

58. Cavity closers were noted on various drawings, but without reference to product/fire rating performance.

59. On one of the drawings, it is not clear whether cavity barriers/closers are provided to window jambs. It wasn’t clear how the cavity barrier requirement is met.

60. It appears that the Rockpanels on the external walls span between floors. To meet the Technical Handbook guidance, fire barriers achieving at least 120 minutes fire resistance should be provided on every floor. There are no drawings detailing this provided as part of this research. The only reference to this is the design team applicant’s response to the local authority verifier’s comments with the following statement “We would also confirm that cavity barriers will be fitted at all floor level lines as well as required horizontal centres throughout.”

61. It appears that flues penetrate the external walls. To meet the Technical Handbook guidance, cavity barriers around the flue penetrations should be provided to these penetrations. There are no drawings detailing this provided as part of this research.

62. No elevation drawings noting the cavity/fire barrier provision were provided as part of this research.

63. No site inspection reports were provided as part of this research.

Case Study 4

Building Profile:

• Number of storeys: 9
• Building type: private residential
• Top storey height: 22.2m
• Building Warrant application submission date: pre-October 2019 (pre-June 2017)

64. Drawing information was provided for the building’s external wall systems, as well as the architect’s and manufacturer’s specification and correspondence from the project architect to the local authority verifier.

65. Three external wall types were noted on the drawings, details were provided for two external wall systems.

66. The project architect’s responses to the local authority verifier’s comments included 28 responses in total relating to the building warrant stage that is applicable to the external wall design. Out of these, there were 9 comments related to Section 2.4, 2.6, and 2.7 of the Domestic Technical Handbook guidance.

67. Datasheets related to fire performance were provided for most key external wall systems products. The BBA/ certificate provided for one of the external wall systems did not match the trading name of the product, so the applicability of the BBA/ certificate is not clear.

68. It is not clear from the drawing information how the edges of the SFS/ systems at windows/doors are closed. Where fire rated separating walls meet the external system, a fire barrier is shown in the cavity between the SFS/ system and the external cladding panel; however cavities within the SFS/ and the external cladding panel were not also provided with fire barriers. No record was observed showing how the local authority verifier decided that this arrangement meets the Technical Handbook’s guidance.

69. The datasheets related to fire performance for the fire barriers noted that it could be applied in Timber Frame or Masonry/Concrete construction types. As the external wall systems on the building are not timber frame or masonry/concrete, the proposed products do not appear to be appropriate in these locations. No record is provided of a decision making process, for example including liaison with the fire barrier manufacturer.

70. Cavity barriers do not appear to be shown around vent penetrations. The information provided for review for this research was a hard copy greyscale, and it is possible that these were shown on the drawings but not visible from the information provided.

71. A fire barrier has been missed on the elevation drawing in line with a separating wall between two flats.

72. Timber decking is proposed on external terraces serving some of the flats. It is not clear the basis for this being considered appropriate.

73. A rigid insulation board is present as part of the external wall systems, on the inside of the SFS/ zone. No datasheet related to fire performance is provided for the rigid insulation board and there has been no record made of the decision making process for approving this.

74. No site inspection reports were provided as part of this research.

Case Study 5

Building Profile:

• Number of storeys: Multiple blocks, 4, 6, and 8 storeys
• Building type: private residential
• Top storey height: 8.1m, 13.5m, and 18.9m tall blocks
• Building Warrant application submission date: pre-October 2019 (pre-June 2017)

75. Typical detail drawings were provided for the building’s external wall systems. Local authority verifier reports were provided and one response from the project architect to the local authority verifier.

76. Three external wall types with different cladding/render materials on the outside were noted on the drawings, details were provided for one external wall system.

77. The local authority verifier’s comments regarding external wall systems comprised a request for manufacturer’s information for one product and requesting typical detail drawings including external wall details, window head details, and cill details.

78. Two of the cladding materials were untreated timber cladding on treated softwood framing. The outline specification provided by the project architect referred to the BRE/ Fire Note 9 (BRE/ 1999). It is not clear how the local authority verifier decided that the proposed untreated timber cladding met the information in the BRE/ fire note.

79. Datasheets related to fire performance were not provided for any external wall systems products.

80. Cavity closers are not noted on the typical drawing information, including penetrations through the external wall systems by windows, doors, and services. A typical detail drawing shows blockwork that appears to return to close the cavity but a gap was left on the drawing.

81. The elevation drawings do not show any fire barriers or cavity barriers.

82. No site inspection reports were provided as part of this research.