underslab plastic extending past slab edges
I just want to highlight how important it is to have the black plastic (200um polyethlyene generally) extending out past all edges of the slab, by at least 600mm (it can be trimmed back later on to size).
This is one of the biggest issues I see with many project builders. They are not doing this.
I'm not making this up either. Here is the link from the NCC:
https://ncc.abcb.gov.au/editions/ncc-2022/adopted/housing-provisions/4-footings-and-slabs/part-42-footings-slabs-and-associated-elements
Figure 4.2.8(a) shows the typical edge beam detail for brick veneer houses.
4.2.8 Vapour barriers
(1)A vapour barrier must be installed under slab-on-ground construction for a Class 1 building and for a Class 10 building where the slab is continuous with the slab of a Class 1 building in accordance with (2), (3), (4) and (5).
(2)Materials: A vapour barrier must be—
- 0.2 mm nominal thickness polyethylene film; and
- medium impact resistant,
- determined in accordance with criteria specified in clause 5.3.3.3 of AS 2870.
(3)A vapour barrier must be branded continuously “AS 2870 Concrete underlay, 0.2 mm Medium impact resistance”.
(4)Installation: A vapour barrier must be installed as follows:
Lap not less than 200 mm at all joints.
Tape or seal with a close-fitting sleeve around all service penetrations.
Fully seal where punctured (unless for service penetrations) with additional polyethylene film and tape.
(5)The vapour barrier must be placed beneath the slab so that the bottom surface of the slab is entirely underlaid and must extend under internal and edge beams to finish at ground level in accordance with Figure 4.2.8.
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So why is this important? Well, we built a house once in North Lakes QLD and the owner reporting rising damp issues, efflorescence.
After removing their pathways around the house and digging exploratory holes it was found that the house was built on top of a naturally spring, as in there was high water table in an area of the site.
This did not come up in the original soil test either (in case you were thinking this question).
The builder i worked for was instructed that they would be liable for the repairs because the vapour barrier did not comply with Figure 4.2.8(a).
So, they best way to mitigate future issues, and comply with the subsidence policy (QLD) is to ensure your vapour barrier meets the requirements of NCC 4.2.8.
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