Inverted roof
An inverted roof, also referred to as a ‘protected membrane’ or ‘upside down’ roof, is form of flat roof in which the waterproofing layer is beneath the thermal insulation rather than above it.
The waterproofing layer is typically a liquid-applied membrane above a concrete roof slab, with insulation boards laid over the top, weighted down with either paving slabs or gravel ballast to protect against wind uplift and flotation. Rainwater percolates down through and between the insulation boards to the membrane where it is drained away through rainwater outlets.
One of the main benefits of the inverted roof method is that the membrane is protected by the insulation from the expansion and contraction caused by weather fluctuations, such as frost and solar radiation. It is also protected from damage by traffic on the roof, such as people walking, the movement of equipment and so on.
When calculating the U-value of the roof, consideration must be given to the fact that the insulation is likely to be wet a lot of the time, with some moisture trapped beneath the boards. This can create the risk of condensation occurring within the build-up of the roof, as the temperature of the membrane can be below the dew point. However, once the roof deck (and the membrane) warms up, the condensation will be able to evaporate. Problems can occur if the accumulation of condensation persists.
For more information, see Specifying insulation for inverted roofs.
For a case study of an inverted roof defect, see Inverted roof defect - case study.
[edit] Related articles on Designing Buildings
Featured articles and news
Future Homes Standard Essentials launched
Future Homes Hub launches new campaign to help the homebuilding sector prepare for the implementation of new building standards.
Building Safety recap February, 2026
Our regular run-down of key building safety related events of the month.
Planning reform: draft NPPF and industry responses.
Last chance to comment on proposed changes to the NPPF.
A Regency palace of colour and sensation. Book review.
Delayed, derailed and devalued
How the UK’s planning crisis is undermining British manufacturing.
How much does it cost to build a house?
A brief run down of key considerations from a London based practice.
The need for a National construction careers campaign
Highlighted by CIOB to cut unemployment, reduce skills gap and deliver on housing and infrastructure ambitions.
AI-Driven automation; reducing time, enhancing compliance
Sustainability; not just compliance but rethinking design, material selection, and the supply chains to support them.
Climate Resilience and Adaptation In the Built Environment
New CIOB Technical Information Sheet by Colin Booth, Professor of Smart and Sustainable Infrastructure.
Turning Enquiries into Profitable Construction Projects
Founder of Develop Coaching and author of Building Your Future; Greg Wilkes shares his insights.
IHBC Signpost: Poetry from concrete
Scotland’s fascinating historic concrete and brutalist architecture with the Engine Shed.
Demonstrating that apprenticeships work for business, people and Scotland’s economy.
Scottish parents prioritise construction and apprenticeships
CIOB data released for Scottish Apprenticeship Week shows construction as top potential career path.
From a Green to a White Paper and the proposal of a General Safety Requirement for construction products.
Creativity, conservation and craft at Barley Studio. Book review.
The challenge as PFI agreements come to an end
How construction deals with inherited assets built under long-term contracts.
Skills plan for engineering and building services
Comprehensive industry report highlights persistent skills challenges across the sector.
Choosing the right design team for a D&B Contract
An architect explains the nature and needs of working within this common procurement route.
Statement from the Interim Chief Construction Advisor
Thouria Istephan; Architect and inquiry panel member outlines ongoing work, priorities and next steps.