Home Contractor's Corner Overlays: all you need to know
Flat Roofing

Overlays: all you need to know

by Jennie Ward

Sarah Spink, CEO of the Liquid Roofing and Waterproofing Association (LRWA), explains what contractors should be aware of before deciding to overlay a flat roof.

Sarah Spink, CEO of the LRWA.

Many liquid applied systems can be used as overlays to existing waterproofing on flat roofs. Liquid applied membranes are seamless and fully bond to the substrate making them ideal to be used over waterproofing solutions that have reached the end of their design life.

However, it’s important to understand the decision-making process behind whether you can simply choose a liquid applied solution as an overlay to an existing roof, or if it requires a full strip off and a completely new system installation.

Substrate condition
The decision whether to overlay depends on the condition of the substrate and other investigations which should be carried out during an initial site visit. A roof which looks in ‘good condition’ should not provide the automatic go ahead for a roof overlay system, especially if the thermal performance does not meet current Building Regulations.

A site visit must assess the condition of the support structure, the roof deck, the insulation and the existing waterproofing, taking into account moisture, condensation and the possible requirement for ventilation or vapour control measures. A full examination is particularly critical for timber and other degradable or non-durable decks.

Areas such as site storage, access requirements – including scaffolding – plus health and safety procedures should also be discussed at length. And some manufacturers will look at feasibility studies for long term projects.

Core samples should be taken where possible to assess the condition of the existing construction and identify areas of concern such as damp insulation – whether from leaks or interstitial condensation. If a roof deck cannot be inspected from below, core samples should be taken through to the deck to assess its condition. All findings should be thoroughly documented with photographs, which will be used to produce a bespoke condition report.

On larger scale jobs, an initial consultation will ideally include the building owner and liquid manufacturer. But in some cases, the contractor will go directly on site to inspect the works that need to be done.

Can you overlay?
We have seen many instances where new materials have been laid on an old waterproofing system, only to find that the roof deck itself has an inherent problem that requires a complete uplift and re-roof – presenting costly and timely rectification.
One aspect which contractors should be wary of is the thermal performance of the roof, and whether this still meets current Building Regulations if you choose to overlay.

In England and Wales for example, replacement of more than 50% of an existing roof requires the entire area to be thermally upgraded, as long as it is technically, functionally and economically feasible to do so.

Where this work affects at least 50% of the surface area of the individual roof – or where it constitutes more than 25% of the entire building envelope – the roof’s thermal performance should be improved.

This can involve a full strip off – replacing the existing insulated roof system to the structural roof deck – meaning a new insulated roof would have to be re-installed to the entire area to meet the current Building Regulations.

However, a partial strip-off involves replacing the waterproofing membrane of a flat roof, meaning it should be thermally upgraded across the whole roof, again to meet Building Regulations.

The U-value target specified in the Building Regulations’ Approved Documents should be achieved where it is technically practical to do so – otherwise the aim should be the best improvement possible within the constraints.

For dwellings, the thermal performance of a new extension depends on the existing and, where applicable, proposed U-values of the walls and roof of the existing building. Where an existing roof is altered, it should meet a specified target if practical to do. If in any doubt, it’s advisable to refer to the Local Authority Building Control (LABC) for advice.

Product specification
Although there are some good liquid products available through distribution networks or builders’ merchants, contractors should ensure they choose a reputable liquid manufacturer for their project – no matter what size the job is.

An effective way to assess product performance is to look for BBA, BDA and ETA certifications, which independently verify manufacturers’ product claims. BBA and BDA certificates also demonstrate that a liquid waterproofing system will comply with UK Building Regulations.

Furthermore, liquid manufacturers should have a third-party accredited quality management system such as ISO 9001, which ensures the product is manufactured to a consistent specification.


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