On a new build with an elevated deck over habitable space, the door threshold is where the drawings and the actual framing most often disagree. The architect specs a slider sitting flush with the internal floor for a clean look. The builder frames to that. Nobody checks whether the finished deck surface, once the membrane and any tiles or decking go on, still leaves enough upstand between the outside and the inside. By the time a waterproofer gets on site, raising it can mean re-framing a door opening that’s already installed.
This is one of the most common ways a new build deck fails a pre-line or CCC inspection, and it’s almost always avoidable if it gets checked before the frame goes up rather than after the membrane does.
Why the threshold is the failure point, not the field of the deck
A waterproofing membrane on the flat part of a deck rarely fails on its own. It fails at the junctions, and the door threshold is the junction under the most pressure: it has to shed water away from the building, stay low enough for step-free or near-flush access, and still clear the minimum height the membrane needs to terminate safely above the finished deck surface. Those three requirements pull in different directions, which is exactly why this detail gets value-engineered down when nobody’s watching for it.
Once the slider is ordered and the rough opening is framed, there’s very little room left to fix the number. The upstand height has to be right at design stage, not discovered at waterproofing stage.
What does Clause E2 require at a door threshold?
NZ Building Code Clause E2 sets a minimum upstand height for the membrane to terminate above the finished deck surface at a door threshold, commonly 100mm above finished floor level where the deck sits over habitable space below. Away from doors, at wall junctions, parapets and other terminations, membrane manufacturers frequently call for more, often 150mm or higher, because those junctions don’t have the same fall-and-drainage advantage a threshold gets from the door sill design.
E2 is a minimum, not a target. It’s the number that keeps a design compliant, not necessarily the number that keeps a deck dry in a hard southerly with the wind driving water uphill against the glass. A specification that details to exactly 100mm and nothing more is leaving no margin for a blocked outlet, a heavier-than-expected rain event, or a slight fall error during construction.
What changes when the deck sits over habitable space
A deck over a garage or an unoccupied void is a maintenance problem if the detailing fails. A deck over a bedroom, a living room or a kitchen is a different category of risk entirely. Water that gets past a low threshold doesn’t pool harmlessly, it tracks along the top plate, down into the wall cavity, and shows up as a stain on a ceiling weeks or months later, long after the trades have moved on to the next job.
That’s also why council scrutiny is heavier here. A restricted building work waterproofing package over habitable space typically needs a written specification, not a verbal agreement on site, and the Code Compliance Certificate won’t issue without the documentation to back the detail. If the threshold height wasn’t resolved at design stage, it becomes a problem the whole build has to wait on.
Why fixing it after the fact costs so much more
Correcting an upstand height once the frame, the joinery and the flooring either side of the threshold are already in is not a waterproofing job, it’s a demolition and re-build job that happens to end with waterproofing. The door has to come out. The framing around the opening often has to be cut back and re-set. Whatever finish is already down on the inside, tile, timber or carpet, usually has to come up to get the floor level right again. None of that is membrane work, and none of it is cheap, but all of it happens because a height that should have been settled on paper got settled on site instead.
Designing the detail in costs the price of a conversation between the architect, the builder and the waterproofer before the frame goes up. Correcting it afterward costs materials, labour, and time on a program that’s already running.
Where Your Subbie fits
We get called in most often after this detail has already gone wrong, when a threshold is sitting 30 or 40mm short and everyone’s trying to work out who pays to fix it. We’d rather be in the conversation before the frame is finalised. As an LBP (Roofing Classes R4 & R5, #BP181755), we can confirm the upstand height a given membrane system needs before the joinery is ordered, and we register a Record of Work with council on every job so the documentation is there when the CCC application goes in.
If you’re specifying or building a deck over habitable space and want the threshold height checked before it’s locked in, get in touch and we’ll look at the drawings before the frame does.