Flat Roof Surveys – What To Look For…


Flat roofs on buildings are usually not visible to occupants and passers-by, and are often unfortunately ignored – until they begin to present problems.  These roof problems will usually be in the form of water ingress into the building, appearing down walls or worse, through light mountings in the ceiling.

Understanding Roof Construction

At this point, the building owners or occupants will call for assistance and hopefully, this will come from experienced and knowledgeable consultants, Building Surveyor or roofing contractors – people who understand the construction of a flat roof and the likely points of deterioration and failure.

They will also be able to provide a survey report and discuss with the client the practicalities of repair or replacement and tailor any recommendations to the client’s budget.

So where to begin when surveying a flat roof?  It can be tempting to just look at the waterproofing to try to find defects, but an assessment of the whole roof structure will give a better understanding of how the roof might be failing and may flag up other hidden problems.  We need to look first at what is holding the flat roof up – the structural deck and its supports.

Roof Structure

A sound deck is vital since if without one, or if there is a fragile deck, the roof area may not even be safe to walk on for the purposes of inspection.  And it’s long term future to support the waterproofing may be in doubt.

It is necessary to either view the deck from below if it’s visible or to take core samples through the roof build-up to locate and assess the deck type and condition.  If the deck is decayed, such as rotten timber boards, or a fragile material such as chipboard, the deck should be replaced.  A long term guarantee on the waterproofing is no use if the deck itself is going to collapse.


Next up is the location of the insulation layer.  All habitable and most working-space buildings require to be thermally insulated at roof level.  Insulation may be present either below or above the deck.  A core sample will usually show the type of insulation, its thickness, and an indication of its condition.  Thickness is important to determine the roofs thermal properties, and how these might compare to the required thermal performance under Building Regulations.

Typical Warm Roof System

Image Source – NFRC

Condensation – Not ‘Roof Leaks’

We next need to consider condensation control.  The fact that the roof is insulated thermally, and that moisture passes through construction components, means that there is always the possibility of condensation forming within the roof substrate.

An understanding of the rules and design guidance on this matter is important as condensation control takes different forms according to whether the roof is ‘cold’ or ‘warm’ i.e. whether the insulation is above or below the deck.

Cold roofs require a vapour check at ceiling level and cross-flow ventilation in the ceiling void.  A warm roof requires a vapour barrier membrane on the deck and beneath the thermal insulation layer.

Roof Area Drainage

Next, we want to determine the means of rainwater drainage from the roof.  The term ‘flat roof’ is a bit of a misnomer since flat roofs are required to have a fall of at least 1 in 40 (25mm fall per metre run of the roof).  Older flat roofs tend to have either no falls or minimal falls which aren’t effective.  If the roof falls are not compliant with the Code of Practice (BS6229:2018) then provision should be made to provide them.  This may be done when replacing a deck or can be achieved with tapered insulation boards.  Most insulation suppliers will provide a design scheme of boards for a particular roof, giving adequate drainage falls to the outlet locations.

Outlets are also another aspect of the survey.  Are they in the right locations for the roof falls?  Are there sufficient number for the volume of rainwater (especially taking account of our worsening climate, with more intense rainfall storms), and are they of the right size?

Roof Construction

The configuration of the roof in terms of its perimeters, abutments to other buildings or structures and features on the roof such as extract plant, rooflights and access hatches all need to be assessed with a view to their waterproofing integrity.  Flat roofs do not generally leak within the main field area.  Leaks predominantly occur at abutments and other detail work, especially details which are difficult to effectively waterproof with some types of roofing materials.

The Roof Covering

Last, but not least, for the purposes of this roof survey blog is the waterproofing system itself.  The survey will need to determine the type of material and its condition.  Is it suitable to be retained, or is the correct advice to remove and replace it?  The roof surveyor will be looking for the effects of weathering, natural wear and tear, mechanical damage or defects within the membrane or its attachment to perimeters and abutments.

Thermal Imaging

In addition to a visual inspection and the taking of core samples, other more sophisticated methods of roof survey are available.  Thermographic cameras can see differentials between hot and cold areas on a roof, which in turn can indicate the presence of moisture under the roof covering.  Various forms of electronic leak detection are also available and provided by specialist roofing surveyors.

Typical Cold Roofing System

Source Image – NFRC

Roof Report

Having carried out the survey according to the aforementioned criteria, and obtained all of the relevant data, an assessment of the condition of the roof will be made, together with options for remedial work or refurbishment.  This will generally be done after consultation with the client to determine their requirements for the future of the roof and their budget.  Guarantee requirements may also be discussed at this point.

Where a roof has not deteriorated seriously, it is often feasible to overlay with the new system on top of the existing flat roof covering.  But if the roof has suffered from water ingress, and is wet within the substrate the correct advice will be to remove the roof covering down to the deck, possibly replacing the deck if necessary, and installing a new thermally insulated waterproofing system.

Warm Roof Balcony

Image Source – NFRC

All of the above requires experience and expertise, and AAC Waterproofing has been advising on, and refurbishing, flat roofs for forty years.   We will be happy to take a look at your roofs and offer advice on the most suitable method of repair or renewal, together with proposals for the most suitable roofing membrane system for your building.

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