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The Need for Careful Examination of a Flat Roof before Overlay

It may be possible to overlay an old flat roof that needs refurbishment but carrying out a thorough and careful examination of the existing build-up and structure is crucial before the decision to overlay is made.

Benefits of Overlaying

There are multiple benefits to overlaying a roof, many of which are linked with saving costs. Overlaying a roof eliminates the labour, material, and end-of-life costs of removing, and disposing of, the existing system entirely. From an environmental perspective, there is less waste material going to landfill and the time taken to complete an overlay refurbishment is a fraction of the time it would take to retrofit an entire roof.Overlaying a roof also minimises the disruption to the building users and the ongoing activities within the building, as well as eliminating the need and cost of a temporary roof covering.If there is insulation in the existing build-up that can be retained, the thickness of insulation within the overlaid layer can often be reduced, contributing to further cost savings.

When Should a Flat Roof be Overlaid?

If the existing design and build-up is achieving the correct U-Values and draining well, it may just be that the remaining waterproofing is coming to the end of its life and can be overlaid simply with one or two layers of new material. The existing buildup, material and structure of a roof will determine which flat roof products will need to be specified to correctly overlay it and achieve the appropriate fixing and adhesion. If a roof is performing well and an overlay is the best refurbishment option, there are lots of benefits to going down that route. But there are also lots of potential issues to be aware of. So, before the decision is made to “go over” the roof with an overlay, a detailed survey needs to be carried out to “go over” the roof to determine whether an overlay is suitable.


Condensation in old roofs is often a common problem compromising the existing deck and structure. Condensation is often found on old flat roofs because of poor design, a lack of ventilation and misplacement of insulation layers. A cold roof with no ventilation will rot the structure from the inside. Old timber structures and decks are particularly susceptible to being compromised by condensation.

Structural Deck

The existing structure and deck must be able to withstand the imposed loads of the new system. The existing deck must also be checked for deflections. Old decks, especially the ones listed as ‘Fragile’ in HSG33, usually cause ponding water. Ponding water causes a host of other issues for a flat roof. Read more about a particular issue caused by standing water in a previous blog.

The roof deck material itself may be reason enough to strip the entire roof. The likes of old Stramit decks are considered ‘Fragile’ under HSG33 Guidance. Read more about the types of fragile deck in this blog.


Asbestos materials are another consideration for both full roof replacement and overlay refurbishments. Asbestos is often present in older buildings, usually pre 21st Century. Before any refurbishment work starts that could disturb these elements, a thorough survey and removal of asbestos should be carried out by a qualified and certificated Asbestos Removal Company, following the latest Health & Safety Regulations. Asbestos is not dangerous if undisturbed, so overlaying a roof may prevent having to remove asbestos and eliminate the associated risks and costs.

Water Ingress

Old roofs are often found to have suffered water ingress, although it may not be immediately obvious. Water can ingress into some of the layers of a failing system without it becoming apparent on the inside of the building. Taking core samples and moisture detection testing to all areas to be overlaid are a great way to determine:

  • The existing build-up layers.
  • Whether there is any moisture trapped.

When overlaying a flat roof, the correct design is critical. It must ensure that no water is in the system and that the integrity of the newly installed system is not compromised. If water has reached the insulation layer of a warm roof build-up, this will be reducing the insulation’s thermal efficiency. Moisture trapped in a waterproofing system can also turn to vapour, causing blistering of the membrane layers. This would be intensified if the roof was overlaid as the existing moisture in the system would be trapped to a larger degree.

Flatness and Deflections

Flat roofs are not completely flat. BS 6229:2018 ‘Flat roofs with continuously supported flexible waterproof coverings – Code of practice’ states that “All flat roof surfaces (including gutter beds) should be designed with a fall of 1:40 to ensure finished drainage falls of minimum 1:80 are achieved.When a flat roof is being upgraded, any existing deflections will be duplicated if the roof is overlaid. Tapered insulation schemes can be utilised to counter this but if the deflections are already there, it is best practice to strip the existing system back to the support structure and renew the deck. If a sagging deck is causing ponding water, it is highly likely that the deck is already compromised and unsafe, meaning that the deck must be stripped and replaced. The cause of such deflections can be poor design. Bailey can help design-out such problems by providing the correct specification.


If a roof is aging, it is recommended that it is maintained frequently, with regular surveys conducted to assess its condition. This can ensure that the roof structure has not degraded to the point where it is compromised, eliminating any possibility of overlaying the roof. This way, the roof can fulfil its useful life whilst enabling an overlaid roof with its associated benefits, when the time comes to refurbish.

The Next Step

For a free of charge site visit and flat roof survey, book today by emailing A Flat Roofing Representative can carry out a thorough survey of your flat roof project at a time to suit, providing you with the correct solutions and a subsequent guarantee of up to 25 years.