Don't Gamble With Your Deck

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Ever wondered what structural deck sits beneath the flat roof that you have walked on, are standing on now, or will be climbing onto in the future?

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You may be as sure-footed as a mountain goat but if the flat roof that you find yourself standing on has a fragile deck, you could be taking a gamble on an accident that is waiting to happen.

According to the HSE publication ‘Health and safety in roof work HSG33’: “Falling through a fragile roof is one of the main causes of accidents during roof work, occurring in both the construction of new roofs and maintenance of old ones…”. HSG33 also states that: “118 On flat roofs, falls most frequently occur…through surfaces that are, or have become, fragile”. The HSE publication goes on to identify woodwool slab decks, strawboard, and chipboard as fragile, particularly if compromised by moisture.

The structural deck supporting a flat roof needs to be able to withstand the weight of the roof system, the operatives and any tools or machinery required on the roof during installation. However, the integrity of the deck does not end there. To maintain the guarantee on a flat roof waterproofing system, the roof will need periodic maintenance. The deck must retain its structural performance throughout the system’s life, to allow the roof to be maintained safely. 

The inspection and refurbishment of flat roofs often uncovers cold roof build-ups that have been installed incorrectly and not vented. These suffer from condensation build-up and rot the deck from the inside-out. It is therefore critical that, when refurbishing a flat roof and considering overlaying, the roof deck and structure are inspected by a qualified and competent Structural Engineer and signed off as suitable to take the weight loadings of the new system before any flat roofing works commence.

Core sampling as part of an initial Site Condition Survey is a vital part of assessing the existing roof build-up. This will establish whether the system has suffered any water ingress and to what extent; or if dry, the suitability of the existing roof construction to be overlaid. 

Some key elements of a flat roof construction that are likely to be fragile are listed below, although this is not an exhaustive list:

  • Old roof lights
  • Old liner panels on built-up sheeted roofs
  • Non-reinforced fibre cement sheets
  • Corroded metal sheets, either as the primary waterproofing system or as the structural deck supporting a membrane roofing system
  • Glass (including wired glass)
  • Rotted chipboard or similar
  • Woodwool slabs

Some slates and tiles Woodwool (slabs) may fracture beneath someone’s weight. The following should be treated as fragile:

  • Woodwool slabs less than 75 mm thick without a nylon net reinforcement
  • Wood wool slabs with a nylon net reinforcement but less than 50 mm thick
  • Wood wool slabs that are wet or previously damaged
  • All straw boards, but especially those affected by water
  • All slabs where it is not possible to determine either their condition or the specific type that has been used. 

If you are considering a flat roof refurbishment and are concerned about the integrity of the original structural deck, we offer an initial Site Condition Survey*, which includes core sampling to establish the condition of the existing flat roof. From there we can create a bespoke flat roof specification. Get in touch today to find out more. 


*The Site Condition Survey is not a structural survey. Any comments on roof structure or other building associated matters in these reports should not be taken to mean that its integrity has been evaluated or regarded adequate. It is necessary for a qualified party to substantiate any issues regarding the suitability and/or capabilities of any element of the existing roof structure.