Getting closure

Building code acceptable solution E2/AS1 does not have a detail how to construct the base of a cavity for timber or concrete piles, so where do we start?

Firstly, two aspects need to be considered:

• The clearance distance between the bottom of the cladding and the finished ground level.
• What is required to ensure the base of the cavity is not open to the subfloor.

Although both issues are addressed in E2/AS1, different sections need to be applied when the foundation system is timber or concrete piles.

Minimum clearances

E2/AS1 provides minimum clearance distances between the bottom of the external cladding and ground level (paragraph 9.1.3, Figure 65 and Table 18). Table 18 gives minimum clearance distances depending on floor type – either concrete or timber. Paragraph 9.1.3.5 refers specifically to suspended timber floors and stipulates that the base of the cladding must overlap the timber floor structure by at least 50mm. This is also included in Table 18, note (2). See Table 1 for a summary.

Paragraph 9.1.3.5 also states that care must be taken to prevent air from the subfloor space, where moisture levels may be high, entering the drained cavity.

Stop vermin!

E2/AS1 paragraphs 9.1.8.2 and 9.1.8.3 cover cavity closer requirements. Paragraph 9.1.8.2 states that there should be no air movement between the drained cavity and a subfloor space, but also that the bottom of the cavity should be drained and vermin-proofed.

Paragraph 9.1.8.3 describes requirements for vermin-proofing, including that cavity closers should:

• Have 3–5mm drainage slots
• Provide a total opening area of 1,000mm² lineal metre of wall
• Be located so that there is a drip edge of at least 10mm at the base of the cladding.

Figure 1 shows how both these requirements can be met with timber pile construction.

Remember the subfloor ventilation

The subfloor space in fully piled foundations must have 3,500-mm² ventilation per square metre of floor space, to prevent subfloor dampness.

NZS 3604:2011 section 6.14 gives methods of providing ventilation including:

• For sheet material – provide ventilation openings at no more than 750mm from each corner in both directions evenly distributed at not more than 1.8m spacing for the perimeter of the foundation.
• For baseboards – provide continuous 20mm slots between baseboards.