The meaning of a flat or zero falls inverted roof deck

BS 6229:2018 Flat roofs with continuously supported flexible waterproof coverings code of practice incorporates zero falls applications, however there is some debate about the requirements, achievability and responsibilities. 

Responsibility sits clearly with the main contractor to ensure the deck complies, not the roofing contractor.  This is being made very clear by NHBC, Premier Guarantee and LABC Warranty.

The requirements are also clear, a zero falls roof should:

• have a slope from 0 to 1 in 80, and the concrete deck should be designed to 1 in 80 to achieve a minimum finished fall of 0.

• not have negative falls or back falls.

• be subject to a detailed structural/deflection analysis to identify potential negative falls, which will need to be addressed on site if they exist.

• achieve effective drainage without any back falls or ponding.

• have outlets positioned relative to the effective drainage points, based on the deflection analysis.

• have outlet flanges recessed relative to the SSL of the waterproofing by 10mm.

• be waterproofed with a product/system that has third-party certification, such as a BBA Certificate, for use in inverted roof or ballasted warm roof designs.

• Zero falls are not accepted for roofs with exposed waterproofing layers i.e. the roof must be ballasted, paved or greened.

QI would contend that the insulation board should also hold third-party certification for use in inverted roof or ballasted warm roof designs.

In the real world how easy is this to achieve?

Casting a concrete deck to billiard table tolerances is not going to happen, so how can the zero falls designs be achieved?

Currently a project engineer will specify the concrete surface regularity, the surface finish and the design tolerance: typically +/-15mm from datum Surface Slab Level (SSL) with a concrete surface regularity of SR3, which allows for 10mm deviation from a 2m straight edge.  This suits roofs designed to 1 in 40 falls to achieve a 1 in 80 finished falls, but for a zero falls roof there needs to be a change in specification.

Moving to a minimum specification that requires a +/-10mm or +/-15mm from datum from planned SSL with a concrete surface regularity of a minimum SR2 to BS8204-2:2003, would reduce deviation from a 2m straight edge to 5mm. We would also suggest that the concrete surface finish is installed to an “Ordinary” or “Plain” to BS EN 13670: 2009.

For more information or help with flat or zero falls inverted roof decks