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 strongly recommends that in accordance with BS 6229:2003 Flat Roofs with Continuously Supported Coverings – Code of Practice and the SPRA Code of Practice, “Single ply membrane roofs should have a minimum finished fall of 1:80.

 

To ensure the minimum falls are achieved allowance should be made for deflection of the structural members and decking under dead and imposed loads and for construction tolerances. The falls assumed for design should, therefore, be steeper than the recommended finished falls.  In the absence of a detailed analysis, a fall of twice the finished fall (1:40) should be assumed for design purposes”.

 

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; 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 Sarnafil membrane Product Guarantee.

Note

Sarnafil Agrément 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.