Guidance Note No. 7 by the Liquid Roofing and Waterproofing Association
Following the publication of Guidance Note No. 7 by the Liquid Roofing and Waterproofing Association (LRWA), there appears to be significant uncertainty as to its content. Simon Poe, Business Operations Director with Alumasc Roofing, explains what implications the Guidance Note has for flat roofing specifications.
A common misconception is that the guidance prevents the use of liquid-applied waterproofing with zero falls. However, whilst the use of some products in certain zero falls applications are prohibited, specific provision is made (in both the Guidance Note and BS 6229:2003) for the use of products that are specifically-designed, and independently-accredited for use in zero falls applications.
The bare facts of the guidance are that:
1. All waterproofing solutions in exposed roof applications should have a minimum fall of 1:80.
2. High performance liquid applied waterproofing solutions can accommodate ponding water without detriment to the waterproofing integrity of the roof. Indeed, certain systems are “specifically designed for permanently submerged applications.”
3. Independently-accredited high performance liquid applied waterproofing can be used at zero falls in buried applications.
4. “Inverted roof decks should … be constructed to the designed falls without back-falls”.
The Guidance Note does not therefore state that inverted roofs should be constructed to positive falls; but rather to designed falls. As acknowledged in the Guidance Note, designed falls can include zero falls – provided that an appropriate waterproofing solution, such as Hydrotech MM6125 monolithic membrane, is used. This solution benefits from:
– BBA approval for both use in zero falls applications and as a damp-proofing membrane
– A proven track record stretching back more than 50 years and with more than 180 million square metres installed worldwide
– NHBC acceptance for use in zero falls applications of inverted or buried roof systems
Clearly, not all waterproofing solutions will be suitable. For example, there are cold-applied liquid waterproofing solutions that are moisture permeable. Their use in an inverted build-up would introduce the risk that a pressure differential (attributable to the mass of insulation, ballast and any ponding water above the membrane) could force moisture downwards through the membrane into the underlying construction.
However, through the specification of an appropriate solution, the waterproofing integrity of the roof can be assured at zero falls – in both buried and inverted applications.
Thermal performance of insulation on an inverted roof
The Guidance Note also raises concerns regarding the performance of thermal insulation within an inverted roof; highlighting that any water ponding below the insulation would be detrimental to the thermal performance of inverted roofs. This is indeed true. However, widely-marketed insulation and water reduction layers are available with independent (BBA) accreditation specifically for use in inverted roof applications with zero falls. Indeed, the accredited water reduction membrane had zero water penetration when tested over an 8 hour test period in accordance with ETAG031 (Annex C).
The Guidance Note states that “as the quantity of ponding water on a roof is usually unknown it cannot be taken into consideration when calculating the U-value.” However, prior reference has been made in the same document to the fact that BS EN ISO 6946 – the European
norm that prescribes U-value calculation conventions – includes a corrective factor (fx) specifically to account for any water penetration. Therefore, water penetration can be accounted for within the U-value calculation at design stage. As a result, where water penetration is reduced or eliminated by incorporating an accredited water reduction membrane, the thermal performance of the system in practice would actually typically exceed the designed performance level.
Buried and inverted roofs can be designed with zero falls using high performance liquid waterproofing (e.g. Hydrotech MM6125) that has the specific BBA-accreditation.
Standard (ISO) U-value calculation methods can account for the thermal impact of water below inverted insulation. Yet, this impact is limited through the specification of inverted roof thermal insulation solutions that are BBA-accredited in zero fall applications.