The Importance of Water Flow Reducing Layer in an Inverted Flat Roof

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Key Learning outcomes

Core principles of an inverted roof
Difference between declared lambda and design lambda
How to calculate the moisture conversion factor
Value of assurance schemes

The Water Flow Reducing Layer is central to the thermal performance of an inverted flat roof, minimising heat loss due to the rainwater cooling effect. When designing an inverted roof it is essential to understand their crucial role and how to properly account for their presence in U-value calculations.

Portions of this article first appeared in RIBAJ. Copyright 2020 Ravago Building Solutions.

1.0 The core principles of an inverted roof

In an inverted flat roof build-up the waterproof roofing membrane is installed directly on top of the structural deck and is protected by the insulation layers without the need for an additional vapour barrier. A water-flow reducing layer (WFRL) is placed on top of the insulation which is then held in place by either paving slabs or gravel ballast. This means the insulation is easy to install and provides convenient access for repairs to the underlying concrete deck.

Inverted roofs offer design advantages given their ease of use for Green Roofs and Blue Roofs, and they comply with the building energy requirement that thermal protection should be unbroken and installed on the outside surface of the building structure. Thermal insulation is an important issue both for winter chill and summer heat, ensuring the efficient reduction of outbound heat flows from a structure when it is being heated and significantly reducing the need for air conditioning.

It is important to consider the suitability of different types of insulation for this type of roof. Extruded polystyrene (XPS) insulation boards are the best solution for inverted roofs due to their unparalleled levels of low water absorption, high compressive strength and exceptional thermal performance.

An inverted roof with paving slabs and gravel ballast.

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Ravago Building Solutions

Ravago Building Solutions
Estuary Road
Kings Lynn
United Kingdom
PE30 2HJ

The information contained in the CPD article web pages is not intended and accordingly shall not be relied upon either as a substitute for professional advice or judgement or to provide legal or other advice with respect to any particular circumstance. RIBA Enterprises accepts no responsibility for loss occasioned to any person acting or refraining from action as a result of the information contained.

The Importance of Water Flow Reducing Layer in an Inverted Flat Roof

1.0 The core principles of an inverted roof

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