Deflection, back falls and the vision of BS6229:2018

Deflection - An interesting word to describe a common amusement relating to in situ poured roof decks within concrete framed buildings and any resultant back falls that can mean the game is lost under the updated rules of BS6229:2018.

The real challenge is how to maintain a level playing field, so to speak, so that everyone wins!

So, what do we know? Well, we know that deflection is going to occur. At some point there will have been a predictive survey carried out to show where it will be, for all floors including the roof. Everyone in the project team now knows where the issues will be. Not a conundrum if deflection is present at internal floors, there are numerous solutions to level things up, but when it comes to the roof…not so many ways to resolve the challenge and a reluctance to employ any of them, for some reason. The roof also has the added issue of being exposed to the external elements and any water collecting in the deflected areas, and insulation sitting in it, then becomes an issue with more than one regulatory body.

In situ concrete is poured with tolerances for level, both positive and negative. If it was poured with mainly positive tolerance everything would be fine, unfortunately that’s a rarity. The deck support structure and layout contribute to minor or major deflection.

The easy choice for specifiers that are progressing the design of a concrete frame building is to keep everything flat, including the roof slab. Keeping the roof deck flat reduces costs and reduces construction time. Take falls out of the equation, especially if you have a building with numerous balconies and terraces, and the developer could save enough height per floor to gain an extra floor at the top. What developer is not going to want to achieve that? It’s at this stage that deflection needs to be eliminated and ideally by the concrete installer, repeatedly though this doesn’t happen. The concrete installer has given the main contractor a flat deck, with areas of deflection, all of which come within his tolerances and was totally foreseeable.

So, keeping the roof flat is basically creating zero falls… isn’t it? Or so you would think. However, the interpretation of zero falls really depends on to whom you are talking.
And so, the next turn passes to the waterproofing company to work on the roof deck.
Most roof decks of this type will utilise an inverted roof waterproofing system, and generally one that comes with BBA certification stating it is suitable for zero falls so that it links back to the current Code of Practice and/or insurer requirements.
Concrete slab with deflection
A typical question that we get asked pretty much every day, and even more since the publication of BS6229:2018, is this:

Your BBA certificate states that your Inverted Roof Waterproofing System (Hot Melt) is certified for zero falls, however it makes reference to ‘decks to which the system applies must comply with the relevant requirements of BS 6229:2018 – however, it does not make reference to the issue of ponding or back falls in zero fall roof situations.

Well it does, but not in those exact words. By relating to compliance of BS 6229:2018 where zero falls are specifically defined as ‘a roof with a fall between nil and 1:80; and within section 4.4 of the standards it states that ‘back falls on a roof deck are not acceptable and need correction through either an additional recessed rainwater outlet or localised screed to falls’, so the BBA certification indicates that any deflection, back-falls etc has either been designed out or dealt with prior to the installation of the waterproofing.

To get over the challenge of deflection and back falls, BS6229:2018 recommends that all flat roofs should be designed with a 1:40 fall to ensure a 1:80 fall is achieved on the completed roof deck. Although not a simple task, it is possible using in situ poured concrete. One method which concrete installers could deploy is to lay the concrete slab to falls. Unfortunately, this practice seems to be something of a dying art and is rarely used. Although it is a more skilled technique and subsequently more time consuming, laying the concrete to falls would reduce corrective action needed further down the line by following trades.

Meanwhile, on site, the roof deck has now dried out sufficiently and is ready for the waterproofing. Everyone knows where the deflections are. The roofing installer needs to be given a deck that is zero falls to comply with the waterproofing system BBA certificate and again to abide by Codes of Practice and Building Insurers requirements, but the deck does not comply with BS6229:2018.
Stalemate?

At the design stage the two elements haven’t been joined up and so are in conflict and the waterproofing installer is now pressured into providing a solution to make up for the deficiencies in falls caused by builders or designers not allowing for the challenges.

Deflection in concrete could be rectified by adding a screed to falls, but this puts pressure on upstand heights and lengthens the project due to drying out times, not to mention the additional costs. Localised self-levelling screed could be included to take the pressure of upstand heights, but whose responsibility is that - the concrete installer or the roofing contractor? Another option could be to place outlets in deflection areas rather than where it is convenient to place them.

So, who holds the ace card to resolving the problem? The answer is… Everyone. Because a workable solution is best achieved at the design stage, which fundamentally requires dialogue between all concerned.

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