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Flat roofs - Flat or to falls?

• Introduction
• Forming falls, ... or not
• Designing a roof without falls
• Selecting the waterproofing membrane

Introduction

BS 6229 Flat roofs with continuously supported coverings - Code of practice is, in Section 7, quite clear that flat roofs should be provided with falls in accordance with Table 6. Generic methods for forming falls are covered in paragraph 7.5 but with little discussion of the relative merits of each method.

Forming falls, ... or not

Forming falls with a sloping deck

• Advantages : Eliminates disadvantages of tapered insulation or screeds. Complies with BS 6229.
• Disadvantages : Sloping soffit. Disliked by some structural engineers, especially for insitu reinforced concrete decks.
• This is the first preference option, for many specifiers, when a sloping soffit is visually acceptable.

Forming falls with a tapered deck

• Advantages : Eliminates disadvantages of tapered insulation or screeds. Complies with BS 6229. Easily achieved with tapered firrings for small-scale (residential) timber roof structures.
• Disadvantages : May cause unacceptable increase in overall depth of roof construction at furthest point from drainage outlets.

Forming falls with tapered insulation

• Advantages : Dry construction. Easier than screeds to lay around obstructions and penetrations.
• Disadvantages : Expensive. May need high density materials (even more expensive) to minimise crushing at the thickest points. May cause unacceptable increase in overall depth of roof construction at furthest point from drainage outlets.

Forming falls with screeds

• Advantages : Cheap and well-understood technique.
• Disadvantages : Wet construction with problems of water entrapment and de-bonding. May cause unacceptable increase in overall depth of roof construction at furthest point from drainage outlets. Can be difficult to lay accurately around obstructions and penetrations.

Laying a roof without falls

• Advantages : Eliminates disadvantages of all methods for forming falls. Minimises overall depth of roof construction. Much quicker, easier, and cheaper to construct. Reduced weight of total roof construction may generate savings in the supporting structure. If the waterproofing membrane is asphalt, 30mm three-coat work is less susceptible to mechanical damage during construction than the 20mm two-coat work used for asphalt laid to falls.
• Disadvantages : Does not comply with BS 6229. Limited range of suitable waterproofing materials. Ponding, and potential for environmental damage, inevitable.
• When designing roofs without falls there are two main requirements:
• The design of the entire roof construction including the structure, membrane, detailing, and drainage, must take account of the implications of there being no falls.
• The waterproofing membrane must be appropriate for this situation.

Designing a roof without falls

• The roof and its supporting structure must be designed on the basis that the roof will hold water, to a depth as discussed below, and the structural design must accommodate this increased loading.
• All roof upstands should be tanked and/or designed, at all perimeters and penetrations, on the assumption that the roof will hold water up to a certain level. Lightweight parapet upstand structures, such as may typically be used where the curtain walling system below is extended to form the roof parapet, may be inadequate to hold a significant build-up of water.
• An increase in the number of rainwater outlets may be necessary and careful consideration should be given to their location to minimise the chance of ponding. Syphonic rainwater systems have the advantage that outlets can be placed with less regard to their distance from downpipes.
• Construction tolerances may need to be tightened to minimise ponding, and consideration should be given to power-floating or power-trowelling concrete surfaces to provide these tighter tolerances.
• Standing water on the roof may reduce the thermal performance of the inverted roof insulation. This is a somewhat contentious issue as many believe that this applies to all inverted roof systems, including those laid to falls.

Selecting the waterproofing membrane

As laying flat roofs completely flat is contrary to the current version of BS 6229, particular care must be taken in the selection of the waterproofing membrane. There are, in principle, only two options:

• Mastic asphalt laid 30mm thick in 3 coats. This is in various British Standards as an accepted tanking specification for asphalt and has been used extensively for completely flat roofs for at least 20 years with satisfactory results.
• Proprietary liquid applied coatings with a current Agrement Certificate for use in the completely flat situation. By far the "best" products are the monolithic systems such as hot-melt rubberised bitumens and cold-applied polyurethanes. All other proprietary products should be specified only after the most thorough research into their track record and warranty provisions - an Agrement Certificate is not enough.

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. Sheet products are also unlikely to have a current Agrement Certificate for use in completely flat situations. Sheet products should not be used for flat roofs without falls.

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