November 2009. Location: Cannock, Staffordshire

Building description

The main building consisted of a steel frame with the lower part of the external walls and interior walls constructed of blockwork.

The production area of the RA Wood Adhesive Tapes tenancy was separated from the two storey office and welfare facilities accommodation by nominally 30 minute fire resisting construction; and from the adjoining tenancy by a compartment party wall of two leaves of 200mm blockwork encapsulating the vertical portal frame stanchions for the full height of the building.

The upper parts of the external walls and roof comprised Insulated Panels with a core of PIR (Polyisocyanurate) approved to LPS 1181 Part 1 Grade EXT-B.

The head of the party compartment wall was fire stopped to the underside of the Insulated Roof Panels over-sailing the wall using sand cement mortar.

It is understood that the Insulated Roof Panels did not incorporate a band of limited combustibility material providing a break in the PIR core of the panel at the point where the panels over-sailed the party compartment wall. At the time that the building was constructed, this was a recommendation in clause 9.29 of Approved Document B (2000 edition) and in the current 2006 edition of Approved Document B the recommendation (now in clause 8.30) refers to the width of this band being 300mm*.

*Advisory guidance note: Building Regulations Fire Safety (Part B) – Buildings other than Dwelling Houses FAQs).

“However, an alternative approach might be to use a panel system which has been shown in a large scale test to resist internal and external surface flaming and concealed burning.”

Fire

From statements made it is understood that the fire started with a suspected ignition event involved a high level light fitting towards the northeast corner of the production area. This was a large fire requiring attendance by 60 fire service personnel. The intensity of the fire resulted in the penetration of fire through the roof and flaming well above the height of the building. The report at the time stated that the Fire Service took the decision to allow the fire to burn out and to carry out appropriate procedures to prevent spread of fire to adjacent buildings. It is understood that the fire burned through into the following morning before reducing to an intensity at which fire-fighters were able to extinguish the fire.

Photograph 1 taken from an aerial platform position to the south side of the building and looking along the line of the compartment wall separating the RA Wood Adhesive Tapes (to the west) and Joinery company (to the east) illustrates how the fire completely destroyed the roof structure to the left whilst the roof over the adjacent joinery company is still in place.

The complete collapse of steel supporting structure shown in the photograph, and the presence of melted aluminium from door controls indicates that global temperatures attained in the production area compartment were significantly in excess of 650°C. The reported fire duration and evidence of the temperatures achieved indicate that that the overall fire exposure of the wall separating the production areas of the two tenancies was at least equivalent to a 60 minute fire resistance test.

Photograph 2 shows shows the charred PIR core of the roof cladding where the PIR core remained in place to the centre of the blockwork leaf on the RA Wood Adhesive Tapes side of the party wall.

Photograph 3 was taken from inside the adjacent joinery company looking up at the underside of the roof cladding along the party wall line.

There was smoke leakage into their premises and this was also evidenced by the striations of smoke staining on the roof soffit and local loss of the plastisol coating to the steel skin close to the apex. However, it was clear from the inspection that there was no loss of fire compartmentation provided by the party wall and the fire did not spread to the adjoining tenancy.

The roof cladding over the adjacent joinery company remained sufficiently intact to provide continued weather protection and to allow that business to resume operations shortly after the fire was extinguished by the fire service.

Comments

The following conclusions can be drawn from the site inspection:

  • The fire was sufficiently intense to have subjected the party wall between the adjacent tenancies to a level of exposure equivalent to at least 60 minutes in a standard fire resistance test.
  • The fire compartmentation provided by the party wall prevented fire spread to the adjacent premises.
  • The charring exhibited by the PIR core material indicated the formation of a sufficiently stable char within the panel to provide an effective fire stop between the steel skins of the cladding at the head of the compartment party wall.
  • The omission of a band of material of limited combustibility in the composite roof panel at the point of intersection with the head of the party wall (as recommended by Approved Document B guidance) did not result in a break-down of fire compartmentation and clearly shows that PIR insulated Panels may be considered as ‘an alternative approach’ under the requirements of the AD-B.
  • The findings of the site inspection provides evidence that the use of a LPS 1181 Grade EXT-B certificated Insulated Panel with PIR core can provide sufficient resistance to fire propagation and erosion such that the functional requirement of the Building Regulations (Regulation B3) can be satisfied without providing a 300mm wide band of limited combustibility material to replace the PIR core where Insulated Panel passes over a compartment wall – see advisory note above. (i.e. it is not necessary to adopt the recommendation of clause 8.30 of Approved Document B).