Nigel Blacklock, Head of Technical at Sika, discusses the current industry debate surrounding minimum falls for inverted roofs.

“In line with the renewed momentum in inverted roof specifications, the subject of falls has been a key subject of debate within the flat roofing industry.


BS 6229:2003 requires a flat roof to be designed with an appropriate fall as well as supporting adequate drainage to prevent risks such as ponding and subsequently preventing or slowing the egress of trapped moisture in the roof build up or the supply of a ready- made reservoir of water above a damaged or poorly installed waterproof layer.


It is not just the case of designing to these standards; the minimum designed falls must actually be achieved by the finished roof, without negative falls or hollows, to ensure that drainage provided is sufficient for the requirements of the roof.  Allowing for deflection of the structure or site inaccuracies, it is recommended that flat roofing systems are designed to a fall of 1:40 with a minimum achieved fall of 1:80.


But in reality things can be somewhat different. Often for cost reasons, occurrences of zero fall design, defined at less than 1:80, are increasingly common. But why is this so? It is made possible by the inclusion of an asterix in Table 7.2 of BS 6229, which stated that inverted roofs could be designed to the manufacturer’s documented advice and the British Board of Agrément certification.


With this, the BBA opened up the potential for inverted roofs to be laid completely flat. As a result what we’re seeing is a rise in inverted roofs that are not draining correctly and are subsequently failing. Designed with zero falls, risks such as deflection and ponding are often overlooked or not deemed important and, as a result, are becoming increasingly common causes of roof failure.


Current industry debate centres around how to accurately calculate the thermal performance of an inverted roof construction where there is extensive ponding. A cooling effect caused by any rainwater that reaches the waterproofing membrane will absorb heat from the underlying structure and thus reduce the thermal performance of the roof. These considerations have to be in place when calculating the U-value (as per the method outlined in Annex D of BS EN ISO 6946) and can be critical in whether the designed value is achieved.


Within the design of an inverted roof, especially laid at zero falls, every effort should be made to prevent ponding water. Even where laid with adequate drainage outlets their position is important as, get them in the wrong place and ponding water can cause progressive deflection of a deck due to increased loading. As this happens, the depth of water will increase, thus increasing the load on the structure and causing further deflection. So often is the case that the rainwater outlet is placed near the supporting structure which ends up being at the high point after deflection.


Ponding also inhibits the effectiveness of a vapour permeable membrane, which is often relied upon to eradicate issues caused by moisture trapped within a concrete deck. In the absence of adequate falls, water cannot drain sufficiently and will pond, rendering the vapour permeability ineffective. This causes the moisture that would otherwise have been driven out to condense on the underside of the membrane, allowing the resultant moisture to find leak routes in the deck construction manifesting themselves through the ceiling.

Why U-values are not being met:

To be effective, the water-flow reducing layer must not allow much of the rainwater to get onto waterproofing layer, otherwise the U-value calculations must be adjusted to reflect its affect. Proved in test situations but often underperforming in real situations, a water-flow reducing layer relies on correct installation and the inclusion of a double outlet to ensure any rainwater that reaches the membrane can still be drained, which is difficult if there is no fall. Improving an insulation value by calculation, the layer becomes an attractive way to cut costs. In reality, if installed incorrectly or on a roof with inadequate falls, the insulation value will not be met. This is a growing occurrence in the industry, and time and time again we’re coming across buildings that are unlikely to meet their target value.


Recognising the potential issues caused by the asterix in Table 7.2, the BBA’s Information Bulletin 4 was published in 2012 to reinstate it’s guidance on correct drainage systems and the need to adjust calculated U-values to account for the rainwater cooling effect. It also defined zero pitch roofs as having a slope not exceeding 0.7 degrees, thus reiterating that ‘zero’ falls doesn’t necessarily mean zero.


Defining zero falls as not absolute zero has gone some way in stressing the importance of designing to positive falls. This has been strengthened by the recommendation that identifying correct locations for drainage points and other design considerations must be tackled at an early stage before insulation is installed.


While not condoning the continuation of zero fall roof design, the BBA’s outlined recommendations aimed to set the correct practice standard for designers who wish to continue to specify zero falls. These outline the various solutions to avoid deflections without the additional cost of screed, but a positive result is very sensitive to the slightest failure of any one of the precautions necessary and this is probably not covered well enough at present.


Recent discussions by a NFRC Joint Flat Roofing Committee work group have made a number of recommendations that we think could go even further to tackling these issues. We believe that falls should always be positive falls – water should always have a chance to drain, even on a low slope and if this is not likely then do not risk it. Also the exemption in Table 7.2 should be modified to show that it is to apply to podium decks and not roofing. These two recommendations will have a great bearing on our advice on inverted flat roofs and, if implemented, will affect the entire construction industry and we believe in the long run, will have positive outcomes for all concerned.”