| Foreword |
| 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 ...design 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. |
| Conclusion |
| 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. |
| For
further information on Bauder Ltd see
www.roofinfo.co.uk/bauder |
| |
