| In the UK the hazard that
exists of fire spreading to the roof of a building from a nearby fire
outside the building itself is assessed by testing representative
specimens of the roof construction to BS 476: Part 3. This test is not
concerned with the behaviour of roofs when subjected to the effects of
fire from its underside, i.e. from within the building. |
|
BS 476: Part 3 - 'External Fire Exposure
Roof Test' was first introduced in 1958. It was revised in 1975 when the
test procedures were modified and the results expressed in a new way and a
new version of the test, BS 476: Part 3: 1975, was issued. The 1958
version of the test was withdrawn at the same time. However, the 1975
version proved unsuitable and, despite having been withdrawn, the 1958
version continued to be used and to be referred to in the Building
Regulations for England and Wales, Scotland and Northern Ireland. |
| BS 476: Part 3: 1958 has
recently been rewritten to refer to metric rather than imperial units and
to conform to the current format used for British Standards. The
instrument used to measure the heat radiation to which the test specimens
are exposed has also been changed because the original one contained an
asbestos disc. These changes have not significantly altered the test
conditions but merely updated the test. This updated test has recently
been issued as BS 476-3: 2004 - Fire test on building materials and
structures - Part 3: Classification and methods of test for external fire
exposure to roofs. It supersedes BS 476: Part 3: 1975 which has been
withdrawn. |
| The Building Regulations for
England and Wales, Scotland and Northern Ireland require roofs exposed to
external fires to provide protection against fire penetration into the
space beneath the roof and against the spread of flames across the surface
of the roof. The ability of roofs to provide such protection is tested is
tested in BS 476-3. The current Building Regulations refer to BS 476: Part
3: 1958, but in time will be amended to refer to BS 476-3: 2004. These two
versions of the Standard are essentially the same. |
| BS 476-3: 2004 Fire tests on
building materials and structures - Part 3 Classification and method of
test for external fire exposure to roofs. ( BS 476: Part 3: 1958 -
External Fire Exposure Roof Test.) |
| The tests described in this
standard are designed to give information concerning the ability of a roof
to resist fire from a nearby fire outside the building itself from
penetrating the roof and spreading across its surface. A preliminary test
is made in which a specimen section of the roof is subjected to the
effects of flame in the absence of radiation. Unless the specimen is badly
affected by this test further fire penetration and spread of flame tests
are carried out with the surface of the test specimens exposed to radiant
heat. |
| The tests are normally
conducted with test specimens mounted either at an angle of 45o
or horizontally, to represent sloping and flat roofs respectively. |
| The test flame used in the
tests is a simulated town gas flame (230 +/- 10)mm long from a (9.5 +/-
0.1)mm orifice. |
| The effect of a wind speed of
6.7 m/s is simulated by applying suction to the underside of the roof
specimen to reduce the pressure on the underside by (1.5+/- 0.1)mm of
water below that on the upper side, for the duration of the test. |
| The heat radiation is provided
by a radiant panel (915 +/- 5)mm square comprised of four (300 +/- 5)mm
square combustion panels with their centres arranged at the corners of a
square of (480 +/-5) mm side. |
| Test specimens are (840 +/-
10)mm square and must be representative of the complete "end
use" roof construction including at least one specimen of any joints
used in each of the materials to be tested. |
| Preliminary ignition test |
| One specimen is tested. The
test flame is applied for 60s at the centre of the upper surface of the
specimen and directed up the slope with the nozzle on the centre of the
specimen and inclined at an angle of nominally 5o to it. In the
case of specimens tested horizontally the direction of the flame shall
correspond to that indicated in the test method. No external radiant heat
is applied to the test specimen nor is the pressure on the underside of
the specimen reduced. |
| The duration of flaming after
the removal of the test flame, the extent of any spread of flame along the
roof surface and any penetration of fire to the underside of the specimen
are noted. |
| Fire penetration test. |
| Three specimens are tested.
Within five seconds of being mounted in the test equipment the upper
surface of the test specimen of the roof construction is exposed to
radiant heat to achieve a radiation intensity of (12 +/- 1.5) kW/m2
uniformly over the surface of the specimen. This simulates exposure to an
adjacent burning building. Suction is applied to the underside of the
specimens to simulate the effect of wind and to facilitate the observance
of fire penetrating the specimen. |
| Five minutes after the start of
the test the test flame is applied to the surface for 60s in the direction
indicated in the method, moving once up and once down the centre of the
test specimen at a rate of 0.3m/10 sec., to simulate the effect of a
burning brand falling onto the roof. |
| The time at which glowing or
flaming appears on the underside of the specimen is recorded. Any
droplets, mechanical failure or holes are also noted. The test is
continued for 60 minutes, unless fire penetration occurs earlier. |
| Spread of flame test. |
| Three specimens are tested. The
upper surface of the test specimen of the roof construction is exposed to
radiant heat within 5 sec of being mounted in the test equipment, to
simulate exposure to an adjacent burning building. The radiated heat is
adjusted so that the intensity diminishes at a specified rate in the plane
of the specimen down its centre line. The test flame is immediately
applied to the upper edge of the specimen for three minutes with its
orifice on the upper edge of the specimen (115 +/- 5)mm from the centre
line with the flame directed along the edge, the angle between the nozzle
and the specimen being nominally 5o. |
| Presentation of
the results. |
| The performance of
the roof construction in this test is measured by the time for fire to
penetrate to the underside of the test specimen and the length of flame
spread along the surface. These are designated by two letters, the first
concerning fire penetration and the second fire spread. |
First Letter - Fire
Penetration classification
A : Those specimens which have not been penetrated within 1 hour.
B : Those specimens which are penetrated in not less than 30 minutes
C : Those specimens which are penetrated in less than 30 minutes
D : Those specimens which are penetrated in the preliminary flame test |
Second Letter - Spread of Flame classification
A : Those specimens on which there is no spread of flame
B : Those specimens on which there is not more than 533mm, (21 inches),
spread of flame
C : Those specimens on which there is more than 533mm, (21 inches), spread
of flame
D : Those specimens which continue to burn for 5 minutes after the removal
of the test flame or with spread of flame more than 381mm, (15 inches), in
the preliminary test.
|
| Attention is drawn
to any flaming droplets from the edge of the specimen, mechanical failure
or any development of holes by adding a suffix 'X' to the designation.
However, this suffix does not lead to any restrictions in the Building
Regulations. |
| Examples of typical
roof designations are EXT. F.AA or EXT.S.ABX, where EXT = external., F =
flat, S = Slope, AA and AB or the classifications achieved in the test for
fire penetration and flame spread respectively. X indicates flaming
droplets or some other observation recorded in the test report. |
| BS 476 : Part 3 was
revised in 1975 and the 1958 version was withdrawn. The test procedures
were slightly modified and the results expressed in a new way. However,
the 1975 version proved to be unsuitable and the 1958 version continued to
be used and referred to in the Building Regulations despite being having
been withdrawn and not available for purchase. This situation still
continues. |
| BS 476: Part 3:
1958 has recently been rewritten to refer to metric rather than imperial
units and to conform to the current format used for British Standards. The
instrument used to measure the incident radiation has also been changed
because the original one contained an asbestos disc. This updated version
has been circulated for public comment and will be amended as necessary to
accommodate any valid comments early in 2004. It will then be submitted to
BSI for issue as BS 476: Part 3: 2004. The 1975 version will be withdrawn
at the same time. |
| Building
Regulations |
| The fire
performance classification achieved in BS 476 : Part 3 :1958 is used in
the Building Regulations to limit the use, near a boundary, of roof
coverings which will not give adequate protection against the spread of
fire over or through them, as shown in the following table: |
 |
|
The Building Regulations therefore regulate
the location of roof constructions by their resistance to fire penetration
rather than by their resistance to flame spread, in accordance with the
main concern of the Regulations to preserve life rather than property.
Provided it achieves an 'A' classification for fire penetration, a roof
construction can be used without restriction under the Building
Regulations even if it achieves 'B' or 'C' rather than 'A' classification
for flame spread. However an 'A' classification for flame spread is
desirable and may be required by the owner or designer of the building.
|
| Approved Document
B of the Building Regulations, England and Wales and Northern Ireland, and
Schedule 9 of the Scottish Building Standard Regulations give notional
classifications for certain roof coverings to BS 476; Part 3: 1958 on flat
or pitched roofs. For example, a flat roof comprising of bitumen felt
should (irrespective of the Felt specification) be deemed to be of
designation AA if the felt is laid on a deck constructed from 6mm plywood,
12.5mm wood chipboard, 16mm (finished) plain edge timber boarding,
compressed straw slab, screeded wood wool slab, profiled fibre reinforced
cement or steel deck or aluminium deck with or without fibre insulating
board overlay, or concrete or clay pot slab, and has a surface finish of: |
a. bitumen bedded
stone chippings covering the whole area of a depth of at least 12.5mm;
b. bitumen bedded tiles of a non-combustible material;
c. sand or cement screed; or
d. macadam. |
| The Future - A
European Standard. |
| Test Methods |
| Most countries
have test methods for the resistance of roofs to external fire. These
involve measuring the spread of flame along the surface of the roof
specimen, and, in some tests, the time for the fire to penetrate to the
underside of the specimen. Although all these methods have a common
purpose they have significant differences: they use different sources of
ignition; some involve blowing air over the test specimen to simulate
wind; others use radiation to simulate the effect of an adjacent burning
building; and some do both. Not surprisingly the results obtained in the
various tests cannot readily be compared and even similar roofing
constructions perform differently in the tests. |
|
After many years work European Standards
Committee TC127 developed a new European Standard for the external fire
resistance of roofs to replace the existing National Tests, including BS
476; Part3. The result of their work was a draft European Standard,
(prEN1187), consisting of three parts based, respectively, on the current
German, Scandinavian and French external roof fire tests. It was the
subject of considerable controversy and was rejected as a full European
Standard on two occasions.
|
| The delay in the
availability of a full EN for the external fire resistance of roofs
impeded the effort to co-ordinate the European approach for the fire
safety of roofs. It also delayed the development of harmonised European
Product Standards for roof waterproofing products, which must refer to it.
To avoid further delay the CEN Technical Board published the draft
standard as a CEN Report, (CR 1187), which made it available for reference
in the Product Standards for roofing sheets. CEN TC127 also decided to ask
CEN to issue the draft standard as a European Pre-Standard, which was done
as ENV 1187:2001. This also made it available for reference but also
enables further development of the Standard and ensures that it must be
reviewed within two years of being issued. Because CR 1187 and ENV 1187
are not full European Standards their publication did not require the
withdrawal of conflicting National Standards, such as BS 476: Part 3,
which can still be used and referred to in National Building Regulations. |
| Classification
System. |
| A classification
system for use with ENV 1187 has been developed, ( prEN 13501-5 : Fire
classification of construction products as building elements - Part 5:
Classification using data from external fire exposure to roof test ). At
present it is a draft standard and has not yet been adopted as a full
European Standard. The classifications included in this Standard are given
in the following table: |
 |
| The classes
referred to in the table correspond to those currently used in Germany,
Scandinavia, and France with their National tests for the resistance of
roofs to external fire, on which ENV 1187 methods 1,2 and 3 are based. |
| Further Changes. |
| An extensive
collaborative work programme (RADAR 3) involving the fire testing of 25
different roofing systems has been undertaken in the UK. The programme was
managed, and the tests carried out, by Warrington Fire Research Centre and
Fire Research Station of BRE. Organisations from all sectors of the UK
roofing industry were also involved and provided the roofing products and
constructed the test specimens selected for testing. As far as possible
the same roofing systems were tested in accordance with BS 476: Part 3:
1958 and all parts of ENV1187. The results from the European tests were
classified using prEN 13501-5:2002 and the Commission Decision of 21st
August 2001. |
| The RADAR 3 work
programme showed that there is poor correlation between the results
obtained by testing roofing systems to BS 476: Part3: 1958 and to the
three methods of ENV 1187:2001. In particular the ENV 1187:1002 tests do
not provide the same differentiation between classes as achieved with the
BS 476: Part 3: 1958 test, especially in relation to the penetration of
fire through the roof. This is very significant for the UK where fire
penetration through the roof system is the key parameter used to regulate
the location of roofing systems. Adoption of ENV 1187:2002 in place of BS
476: Part 3:1958 in the UK Regulations will effectively necessitate
acceptance of roofing systems in places where they are currently not
permitted and may result in a reduction in the current safety level. |
| As this situation is
unacceptable, the chief Fire Regulator for England and Wales has suggested
three alternatives for consideration by the European Fire Regulators
Group. The preferred one involves including the BS476: Part 3:1958 test
method within the current European system for testing and classifying the
external fire resistance of roofs. This can be done by adding BS 476: Part
3: 1958 to ENV 1187 as a fourth test method and amending prEN13501-5 to
include new classes referring to ENV 1187, method 4. The European Fire
Regulators Group supported this change and it has now been accepted by the
European Commission and by CEN, the European Standard Organisation. CEN
TC127 has now issued a modified version of ENV 1187 with an additional
method 4 based on BS 476-3:2004 for consideration. Although the new method
4 is based on BS476-3: 2004 the test for surface spread of flame has been
omitted because the UK Building Regulations do not require better than a
'C' classification for spread of flame and this can be achieved using the
Preliminary test. Pr EN13501-5 is also being amended to include
classifications based on the results achieved using the proposed method 4,
and which will be suitable for use in UK Building Regulations. These
standards will be adopted later in 2004 provided they are accepted by a
majority of the EU Member States. |
| Because ENV 1187 is not an
approved full European Standard there is no obligation for any country in
the EU to use it in place of their National Standards for the resistance
of roofs to external fire, nor to remove their own standards for the
external fire resistance of roofs.. However it is likely that most
countries will do so and amend their Building Regulations accordingly. In
reality however little will change. For example, in the UK it is likely
that BS 476-3: 2004 will continue to be used, possibly in conjunction with
ENV 1187 method 4, the latter being used as part of the procedure to CE
mark roofing products and the former to obtain more comprehensive
information about the resistance of the roofing system to fire spread. |
| Longer term
developments |
| CEN TC 127 has been
commissioned to develop a new single test for the resistance of roofs to
external fire which can be adopted as a European Standard to replace all
the conflicting national tests and ENV 1187. However, work has not yet
begun and it will be many years before a new test method is available. |
| Conclusion |
| Existing National Standards for
the resistance of roofs to external fire will remain in place and continue
to be used for some years. In the UK BS 476: Part 3: 1958 will continue to
be used and referred to in UK Building Regulations in its updated form as
BS 476-3:2004. When an amended version of ENV 1187 with a method 4 based
on BS476-3:2004, and of prEN 13501-5 is available, the UK Fire Regulatory
Authority will have to decide how use them in the UK Building Regulations.
This is unlikely to happen in the near future. In the longer term BS 476;
Part 3 and ENV1187 will be replaced by a new harmonised European Standard,
but this is many years off. |
