Is the internal lining of an external wall a part of the wall?

Is the internal lining of an external wall a part of the wall?

So, if a wall was a part of the external wall, is the plasterboard on the inside of that wall a part of the wall? Why would anyone ask this question? Well, this question comes up when someone is applying C1.9(a)(i) to Type A or Type B building. C1.9(a) requires a number of elements in Type A or Type B building to be non-combustible and the first of these is (i) external walls and common walls, including all components incorporated in them including the facade covering, framing, and insulation.

There's some defined terms in there which are worth remembering. The first one we'll look at is non-combustible. As you can here, we talk about a material or a form of construction. When you have a material, it must be material that can withstand an AS 1530.1 test. When it's a form of construction, it must be made up of materials which are non-combustible. Then next definitions are things that C1.9(a)(i) requires to be non-combustible, external walls and common walls.

External walls are the outer wall of the building which isn't a common wall, and a common wall is a wall which is common to adjoining buildings. And common walls and external walls need to be non-combustible, as well.

So as the internal wall lining of an external wall or even the internal lining of a common wall a part of that wall? Well, we say yes. If you want that in writing, here it is. The Fire performance of external walls and cladding Advisory Note. Part 1.2 of that document talks about what an external wall is and specifically points out that the internal lining of an external wall is considered a part of that wall. This Advisory Note is available from our website, "abcb.gov.au".

This answer may seem obvious but it's likely this question comes up on account of C1.10 which is about fire hazard properties. C1.10 makes the requirements of certain internal building elements, including wall linings and that includes the internal lining of an external wall.

So it has been suggested that C1.9 doesn't apply because C1.10 applies instead. But, that's not the case. C1.9 applies and C1.10 applies, as well. They are concurrent requirements, you have to meet your fire hazard properties under C1.10, which means a group number and other things spelled out in Clause 4 of Spec C1.10.

And in addition to that, you also need to meet your combustibility requirements of C1.9 if your building is of Type A or Type B construction. Someone might be thinking, most internal linings are plasterboard and plasterboard wouldn't withstand that AS 1530.1 test.

Perhaps not, but C1.9(e) contains a concession and lists a number of materials that can be used whenever a non-combustible material is required and plasterboard is the first material on that list. So, the internal lining is a part of the wall.