Memorandum by Sheffield City Council (ROF
24)
Please see the following internal memo from
Sheffield City Council's Architects Division. I apologise for
simply passing on the information you require in this way but
your enquiry did not reach me until very late and it seemed like
a reasonable expedient in order to meet your deadline.
S. Jenkinson,
Technical Services Manager
POTENTIAL RISK
OF FIRE
SPREAD IN
BUILDINGS VIA
EXTERNAL CLADDING
SYSTEMS
There are only two major multi-storey housing
developments within your stock that have received overcladding
in recent years, Hyde Park Block C and the various Hillside and
Netherthorpe Tower Blocks. An explanation of the situation regarding
fire spread for both developments is as follows:
Hyde Park Flats Block C
Any fire involving an overclad building appears
to invoke widespread concern regarding other buildings with claddings
of this type. This was also the case during the design of Hyde
Park Block C (Harold Lambert Court), as two significant fires
occurred which informed the design process.
Sheffield Design & Property engaged foremost
fire consultants Arup Research and Development to resolve the
fire issues relating to the overcladding with the consultant specialist
over cladding Architects Peter Bell and Partners. The Sheffield
Building Control Office in 1990 had little experience of dealing
with construction of this type and the approval of the overcladding
with respect to section B2—Internal fire spread (linings),
B3 (2)—Internal fire spread (structure) and B4 (1)—External
fire spread was referred to the Secretary of State for the Environment
for determination. In his letter of 28 February 1991 and after
due consideration the Secretary of State approved the details
put forward for meeting the requirements saying. . . "In
all the circumstances the Secretary of State determines that the
proposals comply with the requirements of Regulations B2/3/4 of
Schedule 1 to the Building Regulations 1985".
The building is constructed with a brick plinth
to separate the cladding system from fires that may be started
at the base of the building. The cladding itself has a system
of fire barriers within it which serve to prevent fire spread
between the dwellings. The cavity behind the cladding contains
some timber members and the insulation, this has been carefully
detailed to the satisfaction of the Secretary of State to meet
the requirements of the regulations in respect of fire spread.
The overroof contains no fire barriers as this was not required.
Since the installation of the overcladding there
has been a serious fire on Block A (the cream and red block) at
deck access level. This fire whilst breaking out onto the facade
through the glazing was successfully restricted to a small area
of the facade and failed to move beyond the installed fire barriers.
Hillside and Netherthorpe Tower Blocks
Fire risk due to external spread via cladding
systems was an emotive issue at the time of the development of
the Hillside (Phase 1) design. There had recently been a fatal
incident due to a gas explosion at Solihull, and a serious fire
via external overcladding (ventilated airspace) had occurred at
Knowsley Heights, Liverpool. This meant that South Yorkshire Fire
Officers took a keen interest in what was being proposed, including
a visit to their colleagues at Knowsley, and Building Control
took a similar interest.
It is correct that until that time there had
been blocks refurbished elsewhere where a board overcladding system
had been used which was full-height ventilated. This is in fact
a principle of rainscreen overcladding as explained in the CIRIA
guide. Some blocks had fire separation at intermediate points
in the elevation. The solution agreed in Sheffield took on board
the lessons learnt in these incidents.
In response to the Knowsley incident board cladding
manufacturers devised a fire barrier solution which consisted
of a perforated steel member coated with intumescent paint which
would expand in a fire incident. This was rejected in Sheffield
as we felt that the active life of the paint would be less than
that of the refurbished building, and this would be an impossible
item to maintain.
The main principles of fire safety in the Hillside
and Netherthorpe blocks are as follows:
1. The new window is set within the opening
of the original window, such that the curtilage of the dwelling
for Building Control purposes remains the brick external wall.
The integrity of the dwelling is maintained by the dwelling door
and habitable room doors being self-closing fire standard, apart
from additional precautions within the circulation areas of the
block.
2. The periphery of each dwelling horizontally
and vertically is protected by mineral wool insulation bridging
the cavity between the original external wall and the new cladding.
This is repeated around every window perimeter.
For this reason we had to use aluminium cassette
panels to allow movement of air within the cavity across the face
of each dwelling. Ongoing research with Hallam University has
confirmed that this achieves acceptable air movement conditions
to prevent detrimental action within the cavity (eg steel reinforcement
corrosion and concrete spalling). These panels are colour-coated
and not paint finished.
3. All supporting members to the overcladding
are steel or aluminium with separation to prevent electrolytic
action.
4. The ground floor in every block is brickwork
which is fire-stopped at first floor level to prevent fire at
the ground (eg rubbish, car vandalism) ingressing behind cladding.
5. All new metal overcladding is incorporated
in the lightning protection of the block.
6. New windows are aluminium, not UPVC.
It may be of interest that there was a deliberate
arson incident to a first floor flat in Martin Block before the
conclusion of Hillside (Phase 1) contract. Due to its nature this
fire burnt unattended for longer than the design precautions had
anticipated. There was smoke-logging of the circulation area outside
but the fire did not spread beyond the flat involved.
I hope the foregoing assures you that the Sheffield
stock which has been refurbished using rainscreen overcladding
has been carried out with full involvement of Fire Officers and
Building Control Officers, and that no known risk has been accepted.
In both cases the panels are colour coated and redecoration should
not be required.
JD Breakey
Practice Manager
Architects Practice
July 1999
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