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Flat roofs - A very basic introduction

• Introduction
• Why do flat roofs leak?
• Terms and definitions
• Other issues

Introduction

What is a "flat" roof?

• From BS 6229 Flat roofs with continuously supported coverings - Code of practice
• Paragraph 3.1. Flat roof: "roof having a pitch not greater than 10 degrees to the horizontal"
• Paragraph 5.2 (Table 1). Lists five metals, and four other generic product types, suitable for flat roof coverings.
• Paragraph 7.2 (Table 6). Lists minimum finished falls for the materials listed in Table 1.
• For the purposes of this Knowledge page we have excluded the metals, which are normally laid well above the minimum falls in BS 6229, to concentrate on the four other generic product types which are generally laid to minimum falls or even, for some proprietary products, completely flat.

Further reading

• BSi : BS 6229 Flat roofs with continuously supported coverings - Code of practice. 2003.
• CIRIA : CIRIA/BFRC Flat Roofing Design and Good Practice. 1993.
• NBS : NBS Shortcut 07 Flat roofing. 2006 (Revised 2008).
• Most other publications refer to specific materials or uses.

Why do flat roofs leak?

Flat roofs have a reputation for being more susceptible to leaks than pitched roofs although thousands of totally watertight flat roofs are constructed every year. There are, in principle, two main reasons why flat roofs leak:

• Mechanical damage : By far the most common reason for flat roof failure is that puncture or cracking of the waterproofing material allows water to penetrate the roof construction and, eventually, to leak into the areas below. This damage can be caused during construction, or by traffic across the roof when the building is in use. Most of the advice on this Knowledge page relates, directly or indirectly, to ways of minimising the potential for mechanical damage.
• Environmental damage : All roofing materials are subject to degradation or stress caused by temperature variations, freeze/thaw cycles, exposure to ultra violet light, and similar environmental factors. Different materials react differently to each of these factors but, in principle, minimising environmental damage will extend a material's life. Some of the advice on this Knowledge page relates to ways of minimising the potential for environmental damage.

The single most important factor in minimising the potential for flat roof failure is to choose a form of roof construction appropriate to the level of access anticipated. Read more on the Flat Roofs Coverings page.

Terms and definitions

Access

From BS 6399:Part 3:1988 Code of practice for imposed roof loads

• Maintenance access : Paragraph 4.3.1. Defined as "no access (other than that necessary for cleaning and maintenance)". This is assumed to be for the cleaning and maintenance of the roof itself and to occur infrequently.
• Limited access : Paragraph 4.2. Defined as "access". This is assumed to be more frequent than the cleaning and maintenance of the roof itself, but by trained personnel only. A typical example would be where access is required to maintain service equipment on the roof.
• Regular access : Paragraph 4.2. Defined as "access for specific usages". Note that the final paragraph of clause 4.2 refers specifically to floor loads, i.e. this is the case where the roof is being used as a floor, however infrequently.

Roof construction

From BS 6229:2003 Flat roofs with continuously supported coverings - Code of practice

• Types of flat roof : BS 6229:2003, Section 4.
• Warm roof (waterproofing system over insulation over deck).
• Inverted roof (insulation over waterproofing system over deck).
• Cold roof (waterproofing system over deck over insulation).
• Materials for flat roof coverings : BS 6229:2003, paragraph 5.2 : Table 1 lists five metals and four other materials. The advice on this Knowledge page is specific to the four other materials (mastic asphalt, liquid systems, flexible bitumen sheets, and single ply membranes) rather than to sheet metal roof coverings. Read more on the Flat Roofs Coverings page.
• Materials for flat roof decks : BS 6229:2003, paragraph 5.3 : Table 2 lists ten structural deck materials. For the purposes of the ArchiMentor Knowledge pages, concrete is considered as a “heavyweight” material and the other nine are all considered as “lightweight” materials.

Other issues

• Rainfall intensity: The rainfall intensities given in BS EN 12956-3 - the key source document to be used for calculating rainfall and sizing drainage systems - are now over 30 years old. There is a view that these figures may be seriously out-of-date and that peak rainfall intensities over the last few years may be much higher - the flash floods, such as that in Boscastle during summer 2004, are used as examples. The implication is that roofs where the drainage has been designed to BS EN 12956-3 are far more likely to suffer water build-up and ponding for short periods of time in the future. This has obvious implications for roofs laid completely flat and consideration must be given to the proper design of wiers or overflows to cope with extreme surge situations. Read more on the Deluge Rainfall page.
• Workmanship - jointing : Specifiers should be aware that all sheet products, both flexible bitumen sheets and single ply membranes, require high quality workmanship, often in adverse site conditions, to achieve a satisfactory result. Monolithic products, such as mastic asphalt and the liquid-applied systems, are more forgiving in adverse site conditions.
• Workmanship - hot working : The desirability, or otherwise, of hot working must be taken into account when specifying roof coverings. Most flat roof coverings, with the exception of cold-applied polyurethane liquids and solvent-jointed single ply membranes, require hot working.

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