Best practice for fragile deck replacement – ADC

Ian Dryden, National Specification Manager at SIG Design & Technology, discusses best practice for replacing fragile roof decks with Architecture Today’s Technical Editor John Ramshaw.

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Fragile roof decks occur for a variety of reasons, including unsuitable materials choices, poor detailing, unplanned alterations or additions over time, and lack of regular maintenance. Not only can they blunt building performance in terms of airtightness, watertightness and thermal efficiency, they can, more seriously, pose a significant health and safety risk to building occupants and maintenance staff.

So what are the key dos and don’ts when it comes to identifying and replacing fragile roof decks? How should specifiers approach the design and specification process? And what can SIG provide in terms of knowledge, expertise and warranties? Ian Dryden, National Specification Manager at SIG Design & Technology, discusses these questions and more, with Architecture Today’s Technical Editor John Ramshaw.

What are the main issues that compromise the performance of flat roofs making them fragile and potentially dangerous?

There are a number of reasons why flat roofs become fragile and dangerous – not least the use of non-robust or weaker materials that have a limited design life, such as wood wool slabs, and/or the use of materials that are not inherently suitable for roofing applications, such as RAAC (reinforced autoclaved aerated concrete). Roofs that have not been maintained properly – whatever they are made from – may be subject to decay and water ingress, which can in turn weaken the deck over time leading to failure and associated safety issues. Problems can also arise when later additions are made to roofs without proper care and consideration. A good example is removing metal reinforcement bars when installing rooflights.


Core sample revealing a disintegrating timber deck beneath an asphalt roof covering.

What are the main dangers associated with fragile decks?

Falls through weak or fragile decks are without doubt the number one danger. I’ve actually been up on a roof taking core samples – having first carried out checks from inside and other due diligence – only to discover that the structure was not safe enough and would require another approach to collecting the data.

What is the best way to investigate an existing deck?

There are three main options. The first and perhaps safest method is to remove the suspended ceiling tiles and take a look from inside. Saying that, it’s important to first check that the roof build-up doesn’t include dangerous materials, such as asbestos. This approach can also be problematic if plasterboard ceilings are used in place of suspended ceiling tiles. The second method is to take core samples of the roof from outside. Last but not least is to check the roof build-up from the ‘as built drawings’. However, these do not always reflect what was actually constructed. The wide-spread adoption of the golden thread would be of particular benefit in this regard.


Cracked RAAC roof deck.

How should specifiers approach the design process with regard to renewing or repairing fragile decks?

Having correctly investigated the roof deck, it is then a question of analysing the extent of the damage. Typically, you are looking for RAAC with concrete decks, rot on timber decks, and corrosion on metal decks. Specifiers should ascertain if the roof is imminent danger of collapse and take appropriate action. Hopefully this is not the case, and they can start to implement a plan of action to remove and replace the damaged elements, such as substituting RAAC panels for a metal deck, or patch-repairing corroded areas of an existing metal deck. Rotten timber decks are commonly replaced with marine-grade plywood.

The key to successful deck replacement is to take sufficient time and care throughout the whole process, from investigation and analysis to design and implementation. This should eliminate nasty surprises further down the line. Specifers should also ensure that they have the right weathering solution(s) in place prior to renewal works starting on site. It is critical to protect the building fabric from the elements throughout the construction period.


The safest method of inspecting the existing roof deck is from inside the building.

How does SIG Design & Technology work with architects on roof renewal projects and what services does it provide?

SIG is in a unique position where it can help architects through the whole process from investigation and analysis of the existing deck to advising on the replacement strategy, assisting with the roof design, and preparing technical documentation. The only area of responsibility that we can’t take on is structural design, so the architect or specifier will need to partner with an engineer for this aspect of the work.

What are the key considerations when renewing a fragile deck?

Speed of installation is critical (to minimise the period when the building is not watertight), alongside cost effectiveness and robustness of the roofing solution. Upgrading the thermal performance of the roof is commonplace, but this can have a major knock-on effect on inter-related items, such as existing upstand and perimeter heights, high-level windows, and doors to terraced areas. These items, together with facias and rainwater goods, all need careful consideration at the design and implementation stage. It’s not just a question of ‘putting a new roof on’.

Is it ever possible or acceptable to reuse certain elements of an existing roof build up when carrying our renewal works?

Not really, due to thermal performance requirements of Part L of the Building Regulations. However, specifiers and their clients should consider roof renewal projects as an ideal opportunity to implement renewable technologies, such as solar-thermal panels or photovoltaic arrays. This makes sense when the roof is being renewed (and frequently strengthened), and is a potentially easier proposition than retrofitting in the future.


Care should be taken to protect the building fabric from the elements while remedial works are carried out.

What should be done to ensure the remedial works are carried out safely, particularly with regards to buildings that remain in use?

It may be possible to partition off specific areas, but If we are talking full deck replacement, then the building’s occupants will need to be decamped while the work takes place. It’s a question of health and safety; you don’t want to be removing a concrete deck, for example, with the building occupants directly below. Deck replacement is a very disruptive business and it’s important for all parties to accept this at the outset of the project.

Can SIG Design & Technology provide warranties and if so what do these cover and for how long?

SIG can provide a full system warranty for up to 25 years, covering design, workmanship, and contractor insolvency among other aspects.

Contact Details

if you require design assistance on a roof replacement project, please call 01509 505 714, email, or visit the SIG Design & Technology website.

Source: Architecture Today