Guidelines on Fire Resisting Construction for Roof Structures

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Structural Engineering Branch, ArchSD Page 1 of 8 File code : SEBGL-OTH1Guidelines on the Fire Resisting Construction for RoofStructures

KTL/CTW/MKLFirst Edition: January 2008

Edition No./Revision No. : 2/- Second Edition: July 2013

SEB GUIDELINES

SEBGL-OTH1

Guidelines on the Fire Resisting Construction for Roof

Structures

(2nd

Edition)

STRUCTURAL ENGINEERING BRANCH

ARCHITECTURAL SERVICES DEPARTMENTJuly 2013


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Contents

1. Introduction ..................................................................................................................3

2. Roof not an Element of Construction .........................................................................4

3. Protection of Adjoining Buildings...............................................................................5

4. Single-storey Building ..................................................................................................6

5. Other Cases Requiring Fire Resisting Construction ...........................................6

6. Discussion on Non-combustible Roof.........................................................................6

References

Copyright and Disclaimer of Liability

This Guideline or any part of it shall not be reproduced, copied or transmitted in any form or

by any means, electronic or mechanical, including photocopying, recording, or any

information storage and retrieval system, without the written permission from ArchitecturalServices Department. Moreover, this Guideline is intended for the internal use of the staff in

Architectural Services Department only, and should not be relied on by any third party. No

liability is therefore undertaken to any third party. While every effort has been made to ensure

the accuracy and completeness of the information contained in this Guideline at the time of

publication, no guarantee is given nor responsibility taken by Architectural Services

Department for errors or omissions in it. The information is provided solely on the basis that

readers will be responsible for making their own assessment or interpretation of the

information. Readers are advised to verify all relevant representation, statements and

information with their own professional knowledge. Architectural Services Department accepts

no liability for any use of the said information and data or reliance placed on it (including the

formulae and data). Compliance with this Guideline does not itself confer immunity from legalobligations.


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Figure 1 Failure modes of construction elements during fire(Source: Wang 2002)

2. Roof not an Element of Construction2.1 Element of construction is defined in the FS Code Part A Section 3 as:

(a) any floor, beam, column or hanger;(b) any load-bearing wall or load-bearing member other than a member

forming the roof or part of the roof;(c) any required staircase including the landings and supports thereto.

It is clear from above that the element of construction does not include memberforming the roof or part of the roof. Therefore a roof is not considered as an elementof construction and there is no fire resisting construction requirement for it. Thisinterpretation is applicable irrespective of the compartment volume bounded by theroof.

2.2 It is also noted that the FS Code mainly follows the old UKBuilding Regulations1976: Part Ein which it stipulates:

Element of structure means-(a) any member forming part of the structural frame of a building or any other

beam or column (not being a member forming part of a roof structure only);(b) a floor including a compartment floor, other than the lowest floor of a building;(c) an external wall;(d) a separating wall;(e) a compartment wall;(f) structure enclosing a protected shaft;(g) a loadbearing wall or loadbearing part of a wall; and(h) a gallery.


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2.3 The present UKs Building Regulations 2010 (in fact, the last few editions) is evenclearer. Paragraph 8.4 of theApproved Document B-Fire Safety issued under the UK

Building Regulations 2010 excludes, among other items, structure that onlysupports a roof from the definition ofelement of structure. The difference withthe FS Code is that the condition ONLY is added to the exclusions forelement of

structure so as to differentiate from cases where:

the ( roof ) structure is essential for the stability of an external wall whichneeds to have fire resistance.

The case is particularly relevant to buildings having so called boundary conditions,which means that the buildings are situated relative to the site boundary in such away that the walls are serving as a barrier to prevent the spread of fire toadjacent properties. Where a column is built into or supports a boundary wall, it shallhave the same degree of fire resisting construction as that required for the wall. If theroof beams/rafter is an integral part of the column, as in the case of portal frame, one

may consider that the roof beam is not only forming part of a roof structure but alsoserves to provide portal action for the column, fire resisting construction has then to

be considered. However, if the foundation and its connection to the columns of theportal frame have been designed to transmit the overturning moment caused by thecollapse of roof rafters in a fire, while allowing the external wall to continue to

perform its fire barrier function, only the columns & walls, not the roof rafters,require to be fire protected. Details of the calculation method can be found in Fireand steel construction: the behaviour of steel portal frames in boundary conditions

published by SCI (Newman 1990). This design method is also referred by Clause 8.5ofBS 5950: Part 8 (BSI 2003).

3. Protection of Adjoining Buildings

The FS Code has the following requirements:

(a) Clause C5.2: Buildings on the same site are regarded as adjoining buildings ifthey are less than 1.8 m apart. To ensure protection against spread of fire toadjoining buildings, any part of any such building within 1.8 m of any suchadjoining building should be enclosed by imperforate external walls and roofshaving the same FRR as that of the internal elements of the construction.

(b) Clause C5.3: Any part of a building within a distance of not more than 900 mmof a common boundary with an adjoining site should be enclosed byimperforate external walls and roofs having the same FRR as that of theinternal elements of the construction.

It should be noted that only portion of the roof needs fire resisting construction evenfor protection of adjoining buildings. However, this portion of fire resistingconstruction includes roofing material (e.g. metal sheeting), purlins as well as anytruss/beam elements.


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4. Single-storey Building

For free standing single-storey buildings that setback from site boundary or adjoiningbuildings, Clause C4.3 of the FS Code allows the steelwork construction to be

unprotected if the building does not exceed 7000m3 in volume and 7.5m in height,

and the setback from site boundary or adjoining buildings is by not less than 6mwidth of clear unobstructed open space.

5. Other Cases Requiring Fire Resisting Construction

There are, however, the following cases where the roof members require fireresisting construction:

(a) roof forming part of an exit route as described in Clause C12.4 of the FS Code.(b) roof performing the function of a floor, e.g. vehicle parking or accessible roof

garden.(c) roof used or intended to be used as a refuge floor or part of a refuge floor,

should have an FRR of not less than that of the storey below (Clause C12.3).(d) for single-staircase buildings, if the level of the highest floor is more than 13m

above ground level, the roof, though might not be designed as a refuge floor,should still have flat area for refuge purpose as stipulated in Clause B6.1(d) ofthe FS Code and the roof should have an FRR of not less than that of the storey

below as per Clause C12.2 of the FS Code.

6. Discussion on Non-combustible Roof

6.1 Clause C12.1 of the FS Code stipulates that all roofs, together with the membersforming the roof structure, should be constructed of non-combustible materialscomplying with Part E [of the FS Code]. Part E of the FS Code deems materials to

be non-combustible if they can comply with one of the following the tests on non-combustibility:

a) BS 476: Part 4 (BSI 1970),b) BS EN 13501-1 (BSI 2007) andBS EN ISO 1716(BSI 2010), orc) BS EN ISO 1182 (BSI 2010a).

6.2 Clause C12.1of the FS Code indeed causes much difficulty for timber roof. The

corresponding clause of theApproved Document B-Fire Safety issued under the UKBuilding Regulations 2010 does not have such non-combustibility requirement on theroof structure but rather control the use near a relevant boundary of roof coverings togive adequate protection against the spread of fire over them. It should be noted thatthe roof covering is used to describe constructions which may consist of one or morelayers of material, but does not refer to the roof structure as a whole. For instance,the roof covering with slates or tiles supported by timber rafters can be used withoutsubject to minimum clearance distance from relevant boundary, if such covering hasPerformance Designation AA (National Class) or BROOF(t4) (European Class)The Performance Designation is defined in Appendix A to the Approved Document

B-Fire Safety, which includes BS 476: Part 4 (BSI 1970), BS 476: Part 11 (BSI1982) andBS EN 13501-1 (BSI 2007).


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Similarly, the US NFPA 220: Standard on Types of Building Construction (NFPA2012) only requires non-combustible construction for all structural elements(including roof) for Type I buildings, which are usually high-rise and/or large sized.Most of timber roof structures being found in historical buildings maintained byArchSD are Type V construction (Figure 2), where non-fire rated construction is

allowed. This is a more practical approach especially for those heritage buildingsand yet fulfils the objectives of fire safety.

Figure 2 Typical Type V construction(Source:http://online.missouri.edu/)

6.3 It can be argued that, as long as the requirements of Clause C5 (Prevention of FireSpread between Buildings) of the FS Code are satisfied, there is a control of thespread of fire and the protection of adjoining buildings. Hence, the requirement of

non-combustible materials is not necessary. As the requirements of non-combustible construction has been specified by the FS Code, the Practice Guidebookon Compliance with Building Safety and Health Requirements for Adaptive Re-use of

and Alteration and Addition Works to Heritage under the Buildings Ordinance issuedby Buildings Department (BD 2012) clarifies this requirement by requiring thetimber members for roof (similar to timber staircase used as a means of escapewithin fire resisting walls) to be protected with fire retardant treatment to achieve aClass 1 surface spread of flame when tested in accordance with BS 476: Part 7(BSI1977). Project officer should therefore note that full fire-rated protection in the formof intumescent coating, fire protection boards or sprayed coating are not required fortimber roof. Such fire retardant paint toBS 476: Part 7has been specified in Clause

21.14 ofGeneral Specification for Buildings 2012, and if fire retardant is considerednecessary, project officer should follow this clause. The cost of the fire retardant

paint is about $27.80/m2 on new surfaces and $31.00/m2 for redecoration worksaccording to the Schedule of Rates Schedule of Rates for Term Contracts for BuildingWorks (2010 Edition) published by ArchSD.
http://online.missouri.edu/http://online.missouri.edu/http://online.missouri.edu/http://online.missouri.edu/


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References

BSI (1970), BS 476: Part 4: Fire tests on building materials and structures Non-combustibility test for materials (London: BSI).

BSI (1982),BS 476: Part 11: Fire tests on building materials and structures

Method ofassessing the heat emissions from building materials (London: BSI).

BSI (1987),BS 476: Part 7: Fire tests on building materials and structure - Method of testto determine the classification of the surface spread of flame of products (London: BSI).

BSI (2003),BS 5950: Part 8: Structural use of steelwork in building - Code of practice forfire resistant design (London: BSI).

BSI (2007), BS EN 13501-1: Fire classification of construction products and buildingelements - Classification using data from reaction to fire tests (London: BSI).

BSI (2010),BS EN ISO 1182: Reaction to fire tests for products - Non-combustibility test(London: BSI).

BSI (2010a), BS EN ISO 1716: Reaction to fire tests for products - Determination of thegross heat of combustion (calorific value) (London: BSI).

Buildings Department (2011), Code of Practice for Fire Safety in Buildings 2011 (HongKong: Building Authority).

Buildings Department (2012), Practice Guidebook on Compliance with Building Safetyand Health Requirements for Adaptive Re-use of and Alteration and Addition Works to

Heritage under the Buildings Ordinance (Hong Kong: Buildings Department) (available:www.bd.gov.hk/; accessed: 23 July 2012).

Davis, L. (1992), Guide to the Building Regulations 1991 for England and Wales (Oxford:Butterworth Architecture).

Department for Communities and Local Government (2013),Approved document B - Firesafety (London: NBS/RIBA Publishing).

Elliott, D.A. (1983),An introduction to the fire protection of steel(Croydon: Constrado).

Newman, G.M. (1990),Fire and steel construction: the behaviour of steel portal frames inboundary conditions (Ascot: SCI).

NFPA (2012), NFPA 220: Standard on Types of Building Construction (Quincy, MA:National Fire Protection Association) (available:www.nfpa.org; accessed: 20 July 2012).

Wang, Y.C. (2002), Steel and composite structures, behaviour and design for fire safety (London: Spon Press).

Whyte, W.S. and Powell-Smith, V. (1982), The Building Regulations: Explained andillustrated for residential buildings (London: Crosby Lockwood Staples).
http://www.bd.gov.hk/http://www.bd.gov.hk/http://www.nfpa.org/http://www.nfpa.org/http://www.nfpa.org/http://www.nfpa.org/http://www.bd.gov.hk/

Documents

Guidelines on Fire Resisting Construction for Roof Structures