Falls

To encourage the removal of rainwater from the roof surface falls need to be formed to direct it towards the gutters or rainwater outlets. BS6229:2003 Code of Practice for Flat Roofs with continuously supported coverings recommends that no part of the finished roof should have a fall of less than 1:80.


The guidance given is that to ensure the finished fall requirements are met then design falls need to be greater in order to allow for deflections caused by dead and live loads and for tolerances in construction. Space in the absence of any accurate information or calculations about the level of those deflections and tolerances etc, the design fall should be double that of the minimum required 1:40.


Where two planes of a roof meet in a mitre, the fall along the junction is less than that of the two planes; this should be taken into account when deciding the design falls.


Creating falls in the existing substrate can be achieved by laying a screed to falls or employing the use of a tapered insulation scheme designed to accommodate the fall requirement whilst considering low upstand details and abutments.


Sika Liquid Plastics coatings are unaffected by slight ponding and this will have no detrimental effect on their waterproofing integrity, life expectancy or guarantee.

Drainage

BS EN 12056 – 3:2003 Gravity drainage systems inside buildings, roof drainage, layout and calculation, and the building regulations approved document Part H, provide guidance on calculating the amount of precipitation to allow for and the number of outlets and downpipes required to remove water from the roof.

Ponding Water

Normal good practice is to remove rainwater from the main roof surface as rapidly as possible, avoiding ponding which is often unacceptable to building owners. Ponding can also cause a number of undesirable aspects including;

 

  • Encourages dirt/moss build-up
  • Impact on the condensation risk performance
  • Too much retained/ponded water on a roof can lead to deflection of the roof structure
  • Standing water in sufficient quantities can form a breeding ground for insects
  • Standing water forms a reservoir that can allow a greater volume of water to enter the building than would have been the case in the event of any accidental damage.

 

Correct falls and drainage provisions are therefore always recommended.