To encourage the removal of water from the roof surface falls need to be formed to direct it towards the gutters or rainwater outlets. BS 6229:2003 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.

Tilting the supporting structure, laying screed to falls or the use of tapered insulation can create the necessary falls. With long runs of roof, care should be taken to ensure that the build-up of thickness does not compromise details at abutments with walls or make the actual construction of the various layers difficult.


Sika-Trocal membranes are not adversely affected by a reasonable amount of retained water, ponding is not a material related issue. Occasionally areas of standing water can occur on the roof surface due to errors in construction. Ponding water on a roof when associated with fallen leaves can sometimes lead to the development of certain bacteria that can cause some staining of the membrane. On exposed membranes this stain is purple and ballasted ones, brown. Whilst unsightly, the stain does not affect the Sika-Trocal materials.


It should be noted that some insurance based design guides and some clients will actually require falls to be constructed at greater angles than the minimum’s specified in BS 6229:2003 Code of Practice for Flat roofs with continuously supported coverings.


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 the water from the roof.

To assist the drainage engineer with these calculations Sika-Trocal provides rainwater outlet capacities for each of its drainage products (with leafguards) in accordance with BS EN 1253-2:2003 Gullies for buildings Test methods. If using alternative outlets this data should be clarified with the manufacturer and passed to the services engineer calculating the drainage requirements.

When using Sika-Trocal roofing membranes it is not generally necessary to create box gutters where two roof planes meet, nor when a single plane meets an abutment, which provides a significant benefit as box gutters are slow to construct and difficult to incorporate into current building construction without accidentally creating a cold bridge.

Where roofs are drained internally, i.e. when there is a parapet all the way around the roof perimeter, they should incorporate some sort of over flow provision to at least give warning and relief in the event of the normal outlets becoming blocked.

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, which can become dangerously slippery
  • Impact on the condensation risk performance; water is an extremely effective vapour control layer
  • Water can freeze in winter and present a safety hazard
  • Standing water forms a reservoir that can allow a greater amount of water to enter the roof than would have been the case in the event of any accidental damage.
  • Standing water in sufficient quantities can also provide suitable breeding areas for insects such as midges.
  • Too much retained water on a roof could lead to deflection of the roof structure, and in some extreme cases, collapse of the roof itself.

Correct falls and drainage provision are therefore recommended