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.