Good practice guidance recommends that flat roofs (defined as <10º) should not be completely flat and should be designed with adequate finished falls to ensure the completed roof drains freely.

 

Sika Limited strongly recommends that in accordance with BS 6229:2018 Flat Roofs with Continuously Supported Flexible Waterproof Coverings - Code of Practice and the SPRA Code of Practice, "a minimum 1:80 finished fall is recommended to both the general area of the roof and to any formed internal gutters. All flat roof surfaces (including gutter beds) should be designed with a fall of 1:40 to ensure finished drainage falls of 1:80 are achieved. This should take account of construction tolerances, permitted deviations and deflection under load, and, unless justified by more detailed structural analysis, to account for deflections/settlement".

 

Whilst Sika recommends that this guidance is followed, we recognise that it is not always possible and in such circumstances even if the minimum fall cannot be achieved, steps should be taken to ensure that the roof has a positive fall and is free draining with no negative or back falls.  Relevant parties should be notified of the additional risks of ponding water on the roof surface, especially where a lesser fall is provided.

 

When a roof is installed at less than the recommended fall, careful consideration should be given to the following;
 

  • Contradiction of all good practice guidance, including BS 6229, SPRA, NFRC and NHBC advice.

  • Loading on the structure (deck deflection) - the additional weight of ponding water must be accommodated.

  • Aesthetics - risk of standing water, algae, moss, mosquitoes, etc.

  • Health & Safety - odour/hygiene issues, dangerous conditions may occur as the water freezes in winter or becomes slippery after rainfall.

  • Increased risk of leakage if the roof waterproofing gets damaged

 

Any ponding water will not be detrimental to the Sarnafil membrane Product Guarantee.

 

Note:

Sarnafil Agrment Certification may be invalidated if no fall is provided as this is in contravention of good practice. This will depend on the circumstances and the interpretation by Local Authority Building Control.