Surveying A Flat Roof | What To Look Out For

Maguire Brothers - Surveying a Flat Roof-05

Part two of our guide explores what to look out for when surveying a flat roof.


What should I look out for when surveying a flat roof, and how?

A visual survey will tell you a number of things. These will be a factor in determining the system and the design of the system which is to be installed, including the following:

  • What the building is being used for.
  • If there are areas of ponding water or evidence of them (stains etc.) These may suggest the falls on the roof are inadequate and preventing efficient drainage. It may also be evident that the roof decking or insulation has failed/collapsed and is sagging, or it could be occurring as a result of blocked outlets. Further investigation would therefore be necessary to determine the cause/s.
  • Holes, splits, blisters or rippling in the roof surface are all evidence of fatigue and failure. Further investigation should be carried out to determine what has caused them. This may enable you to ensure the new solution does not fail in the same way and lasts longer.
  • Roofs are often used to locate plant, particularly air handling units and compressors. Holes, made through the waterproofing systems for cables and pipework, are often inadequately weather-proofed. Plant, pipework, ducting and cables, if not raised high enough above the roof surface, can obstruct drainage, leading to a build-up of moss and debris on the roof, which can attract plant growth. All of the above can lead to premature failure.
  • Flat roofs usually fail at details, such as penetrations, or where one plane of the roof meets another, such as junctions with parapet walls and buildings on the roof. Careful examination of all the details should be carried out to determine if there are any weak spots which require re-designing.
  • Water-filled blisters are evidence of trapped moisture, usually because the roof has failed.

Core samples when surveying a flat roof

Core samples are one of the most important tools a surveyor has. A core sample can be used to determine the following:

  • The existing roof build up, including the number of layers and the type of materials used to form the waterproofing, insulation and vapour control layer. There could be several roof systems laid on top of each other. Identifying the build-up is not always as easy as it might seem, and a good knowledge of roofing systems, old and new, is needed.
  • What the roof deck is made of? Again, a deal of experience is necessary because of the very wide range of materials and systems which have been used over the years.
  • By taking a number of core samples, one can establish how the falls on the roof are created. These could be formed by the structure, by a screed or by a ‘cut to falls’ or ‘tapered’ insulation system.
  • Core samples can also tell you if there is moisture trapped in the roof build up, and may provide evidence of the roof deck failure. Trapped moisture will almost always require the existing roof system to be stripped off.
  • It can be revealing to take core samples in blistered or deflected areas to see if the cause of the failure can be identified. For example, interlayer blistering can occur within an asphalt membrane, ruling out the possibility of overlaying it.
  • Note of caution: Core samples only expose a small area of the roof, which may not reflect the condition of the roof generally. The more that are taken, the more accurate the information is likely to be.

What else to measure & consider when surveying a flat roof

  • Consider how the work can be carried out safely, how will the roof be accessed and how any risks can be reduced and managed.
  • Can the existing system be retained and overlaid, or will it have to be stripped off and replaced? Retaining and overlaying any existing roofing system offers considerable cost benefits. It also reduces land fill and carbon miles, and it means that the building is less exposed to the risk of water ingress during the re-roofing process.
  • Is there any insulation, and does it comply with current regulations? Can it be incorporated into the new system?
  • If the roof is to be overlaid, what is the surface finish of the waterproofing and in particular, how well are any chippings embedded, and what pre-treatments may be required?
  • Consider the existing buildings use – take particular note where the use is industrial, or where there may be any higher than normal condensation risk (swimming pools for example).
  • If additional insulation is to be provided this will raise the finished roof level. Particular attention will be necessary when surveying details which may have to be re-designed to ensure minimum upstand heights, as required under building regulations, are maintained.
  • Raising the roof level may also affect any plant, roof lights or other structures on the roof and these may also have to be raised, adapted or moved.
  • Check soffits and fascias to see if they are in good condition and if the roof is well ventilated.
  • Some roofs are obscured by plant, and this needs to be surveyed to make sure it will not prevent access to undertake the works. It may have to be moved and reinstated.
  • Pay particular attention to telephone masts, satellite dishes, aerials and the like. Strong signals can be harmful to health. Take details of the providers of the equipment, any serial numbers or site references and investigate. The equipment may have to be temporarily turned off or even moved and reinstated.
  • Check to see if any mechanical plant is connected physically or electronically to internal machinery, ducting or electrical supplies. Consider what the consequences of disconnecting it will be on the operation of the building and what measures may need to be taken.
  • Check any pipes which pass through the roof and make notes as to their use. Take particular notice of any chimney flues. Remember, any works to gas flues have to be carried out by a Gas Safe Engineer. Hot flues will require special shields to prevent them damaging the new roof system.
  • Check and record the condition of any structures on the roof including masonry, doors, windows, etc.
  • Identify the location of outlets and measure their internal diameter.
  • Consider what measures can be taken to improve poor falls. Tapered or cut to falls insulation perhaps?
  • Check and record any defects to the existing building. (e.g. broken windows or skylights).

Know What The Regulations Say

All flat roofing should be specified and undertaken to meet current regulations and codes of practice, including:

“BS 6229:2018 Flat roofs with continuously supported flexible waterproof coverings – Code of practice”.

The NFRC Safe2Torch Campaign

Building Regulations Approved Document Part L 2014 (updated 2022)

The next part of our guide to surveying a flat roof will explore the various flat roofing materials and their respective properties.

This article, as with all other articles we produce, is for guidance purposes only. It does not constitute formal advice and should not be relied upon as such. For bespoke, unbiased advice relating to your commercial roofing project please contact us and we would be pleased to assist.

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