Improving Flat Roof Drainage

Roof drainage systems should be designed to BS EN 12056-2:2000 which covers wastewater drainage systems which operate under gravity.  

The design will determine the size, number and location of outlets and/or downpipes. The roof drainage can be improved by using high flow-rate outlets, sumps around outlets and scuppers and introducing gutters.


Martin Bidewell - Head of Technical & Product Management at Sika Limited

Ponding or standing water on a roof should be avoided. The effects could be any of all of the following:

  • Increased loads - it  may  cause  progressive  deflection  of  the  deck,  15mm  ponding  on  a  500m2  roof  has  a  dead  load  of  7.5  tonnes!


  • Unsightly / vegetative growth - It  encourages  the  deposition  of  dirt  and  leaves and the growth of moss and vegetation which  can  be  unsightly and  may  obstruct  outlets.


  • Health & Safety - it can become a slip hazard and can promote odours and the development of unwanted insects.

Before: Ponding water on flat roof

  • Contravenes good practice / warranty - it contravenes the BS and roofing bodies guides to good practice and may invalidate the  terms  of  the  warranty  for  the  waterproofing  membrane   


  • Increased risk - in  the  event  of  damage,  risk of water  ingress  to  the  interior  will  be  increased and it  can  lead  to  thermal  stresses which  may  decrease  the  life  expectancy  of  roof  finishes


  • Thermal conductivity / die back of green roofs (inverted roofs) – standing water, particularly under insulation on inverted roofs, will significantly reduce the thermal performance of the roof. waterlogging of green roofs will seriously affect the condition of planting and may lead to failure.

After: Improved drainage

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