Here at QI HQ we spend a lot of time talking to roofing contractors, main contractors and specifiers about threshold heights; sometimes at design stage and others at construction stage, with the latter generally being due to there being a problem with achieving a level threshold or preventing water draining into the building.
In new build the discussion has often been around the conflict between the British Standard recommendations and the NHBC Chapter 7.1 requirements. Fortunately the conflict between BS 6229:2003 requiring waterproofing upstands to be terminated a minimum of 150mm above the surface finish of the roof has now been cleared up with the publication of BS 6229:2018 which now provides the same guidance as NHBC Chapter 7.1, namely that at thresholds you only need the waterproofing to achieve a 75mm height before termination. But remember, that is only at the threshold, all other upstands should terminate a minimum of 150mm above the surface of the roof.
The other area of confusion we discuss is where exactly the surface finish is. In the case of a warm roof, where the waterproofing membrane is on top of the insulation, the waterproofing membrane is the surface finish. But in an inverted roof the waterproofing membrane is at the bottom, below the insulation, WFRL and finishes. In this case the surface finish of the roof is whatever the walkable top layer is: ballast, paving, decking etc. So, in an inverted roof the upstand has to be 150mm above the walkable top layer and the threshold 75mm above the walkable top layer. The same is true if applying a walkable layer above a warm roof.
Green roofs are the same, the surface finish is the top of the green roof, so the upstand has to be 150mm above the top of the green roof and the threshold 75mm above the same point. A big part of our job at QI HQ is to help our customers ensure they are achieving the U-value requirement of the roof whilst meeting the threshold height requirements; not always as easy as it may sound, especially in refurbishment or when things don’t go to plan on site such as when a slab is cast to thick or a screed is added due to a poor surface finish.
Fortunately, this is where our range of market leading insulation boards and our understanding of British Standard and insurers requirements come together to enable us to propose solutions that achieve the desired U-value, 75mm threshold height and support proper drainage.