28/04/2026
Revisiting the NCC
I have recently spoken with tradies that are waterproofing under and over the screed. I had a puddle fl**ge manufacturer call and said his puddle fl**ges were made to accommodate the “over and under” system. When the NCC first came out I read a lot, watched many videos and attended an information seminar.
The consensus of opinion was that the NCC stated you could waterproof under or over the screed, not both. After revisiting the question of “over or under” I am finding conflicting information. If you simply google the question, 4 out of 5 times Google AI stated that it was one or the other and quoted the relevant paragraph in the NCC. The fifth time I tried it the AI answer was the same, however, it added that there were exceptions, although it did not reference which part of the NCC was relevant.
After a bit more research I found an old video “Under vs. Over Screed Waterproofing: Considerations and Solutions 2023” featuring David Previte - Waterproofing Expert. I think he was of the opinion that you could do either or both. He went on to state the pitfalls of either under or over or both. He had a diagram to show that if you did both and the top membrane failed, the water would be trapped in the screed layer. He also had the bottom membrane on a slope yet elsewhere in the video he seemed to indicate if you waterproofed the original substrate it was a secondary membrane and did not require a fall and only the top membrane was required to be on a fall. I fail to see the point of putting a membrane on a substrate that does not fall towards the drain.
Experience and common sense would tell me that you should apply the membrane above the screed. If you place a screed directly on top of the membrane there is no release area for the membrane to ”float” and stretch. The screed will compress the release area. I have seen very few waterproofers apply closed cell backing rod and tape to the release area before applying the screed over the top, so as to not comprimise the bond breaking. I also have little faith in concreters providing a fall. I have experienced too many failures from concreters. Call the Waterproofing Warehouse for all your waterproofing and concrete repair needs. We are here from 6:30 am to 3:30 pm, Monday to Friday.
