Designing Falls for New Flat Roofs

In today's cost-conscious, fast-track construction industry there seems to be an increasing number of flat roofs being designed at zero degrees (particularly Inverted podium roofs and intensive Green Roofs).
The concern over flat roofs being designed with zero falls is shared throughout the flat roofing industry and acceptance seems to be made based on commercial monetary benefits, instead of technical / product developments.

Correct Design

Listed below are excerpts from various official documents which condemn zero falls. The following points should be highlighted to any parties that suggest that this is acceptable design practice.
Building Regulations Approved Document H
"Rainwater drainage" - Requirement
H3  (1) Adequate provision shall be made for rainwater to be carried from the roof of the building.
(2) Paved areas around the building shall be so constructed as to be adequately drained.
*Requirement H3 does not apply to the gathering of rainwater for reuse.

Design Rainfall intensities - Guidance
1.2  Where the design incorporates valley gutters, parapet gutters or
      Drainage from flat roofs should be carried out in
      Accordance with BS EN 12056 (See below).

BS12056 Gravity Drainage Systems in Buildings - Part 3 Roof drainage, layout and calculation
7 Layout
7.1  General:
       Design of roof drainage systems shall take account of construction tolerances and settlement so as to avoid back falls and ponding, which may adversely affect durability.

Based on the requirements of the Building Regulations Part H and the requirement for tolerances and settlement to be accounted for as indicated in BS12056, it is clearly essential to design adequate falls on all flat roofs in order to shed rainwater effectively. The relevant codes of practice for flat roofs including BS6229 - Flat roofs with continuously supported coverings, BS8217 - Re-inforced bitumen membranes and BS8218 - Mastic asphalt roofing, clearly indicate the requirement to design falls onto flat roofs.

The respective BBA certifications for these systems often state that it is satisfactory for use in 'completely flat' roof applications, although for the purpose of those certifications, a completely flat roof is defined as a roof with a fall of less than 1:80. Therefore, designing a roof deck completely flat (zero degrees) may well invalidate the BBA as this method would not account for settlement of the deck or construction tolerances, thus creating back falls leading to ponding of rainwater. Not complying with Part H of the Building Regulations or with the relevant British Standards for flat roofing or flat roof drainage.

In summary, the following definitions would apply to flat roof designs for rainwater drainage:
  • 'Flat Roof': Defined as a flat roof with a minimum finished fall of 1:80.
  • 'Completely Flat Roof': Defined as a flat roof with a fall of less than 1:80.
  • 'Failed Roof': Defined as a flat roof with a backfall that results in excessive rainwater ponding.

Vapour Permeable Membranes

The use of vapour permeable membranes above the insulation within an inverted roof design allows for the rainwater cooling factor to be reduced, thus reducing the amount of XPS insulation required to achieve the desired U value. Manufacturers recommendations on installation of this type of product states this 'should be loose laid over the insulation, at right angles to the slope with 300mm laps running down the slope'. Designing with a zero fall slope provides no slope and inevitably would result in a backfall, thus allowing rainwater to penetrate the insulation (through the loose laid laps) defeating the overall purpose of the vapour permeable membrane by allowing rainwater to reach the waterproofing.

This approach to designing flat roofs is concerning and the various industry standards are consistent in their need for a minimum finished fall of 1:80, which are also supported by the Building Regulations and further defined within the BBA's own definitions.