Zero Falls and Thermal Performance – Where Does SPRA Stand?

Jim Newsletter ImageJim’s Jotter by Jim Hooker

The so-called zero falls inverted roof to apartments – a common application – but does it comply with Building Regulations Part L?

The reason why SPRA has long upheld the rules regarding a 1:80 ‘minimum finished fall at any point’ as stated in BS6229:2003 have been reiterated frequently in this column. Technical Committee has also confirmed its opposition to the confusion created by the British Board of Agrément’s attempts to define ‘flat roofs’, ‘zero fall roofs’ and completely flat roofs’. The reasons for falls are well documented in the BS, SPRA Design Guide, NHBC Standards, Premier Guarantee Technical Standards and so on. But inverted roofs add a further set of reasons:

  • Thermal insulation may not perform thermally as intended if subject to standing water.
  • U-value correction factors (ƒx) may be optimistic unless the water control membrane effectively stops drainage through the insulation; a tall order at ‘zero falls’.

Roofs that do not function to their intended U-value may not be big news just now. After all, above-expectation heating input may not be insured and we lack the tools to conduct large-scale quantitative assessment of elemental U-value. But that time will come soon enough and certainly the spotlight is already pointed at the many disappointing comparisons between theoretical whole-envelope carbon output and actual performance.

BBA Information Bulletin No. 4 (2012) available from HERE states that ‘it is essential that roof falls and drainage paths are correctly designed to avoid ponding and subsequent risk of silt build up,…’ and ‘Drainage points need to be located at the lowest point of the roof, to facilitate effective removal of rainwater. Care is needed to identify these locations. For example precast concrete decks will deflect between spans…’ and ‘’Where water control layers are used, drainage must be provided at two levels;’

Let’s get away from the laboratory and into the real world! Given normal construction tolerances ‘zero falls’ will in effect mean perhaps 20mm of ponding; it is impossible to lay a water control membrane such that it will hold water at details and abutments; rainwater outlets are generally very widely spaced and located at columns (see my Jotter comment 7 January); I almost never see double-level rainwater outlets on my frequent visits to see inverted roofs.

The consensus reached at the joint SPRA/NFRC/LRWA flat roof technical committee last year agreed an action that a task group should further into the falls issue and prepare a position that could be adopted in the revised BS6229 and in future Agrément Certification or guidance. Nigel Blacklock (Sika) who chairs SPRA Technical Committee, Andrew Pack (Kingspan Insulation) and Richard Powell (Dow) and I met last week. A draft statement is being prepared.

This post by Jim Hooker originally appeared in the Single Ply Roofing Association website