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Wall Sarking Installation: A Practical Guide to AS 4200.2

Wall sarking — the pliable building membrane, or "building wrap", fixed to the outside of the frame behind the cladding — is the concealed second line of defence in an external wall. No cladding is perfectly watertight, and wind-driven rain will eventually find a joint or fixing. Installed well, the membrane collects that water and drains it safely to the outside. Installed poorly, it traps moisture against the frame and drives the very condensation and decay it was meant to prevent.

The difference is almost always workmanship, not product. The failures we see on remediation jobs are laps run the wrong way, torn membrane left open, wrap fixed behind the internal lining, and untaped reveals — not defective materials. This guide sets out how to select and install wall sarking to AS 4200.2, and the common installation defects to design and build out.

Sarking, Building Wrap or Wall Wrap?

The three terms describe the same layer. AS 4200.2 (the installation Standard) and AS 4200.1 (the materials Standard) refer to a pliable building membrane. Strictly, "sarking" is reserved for a membrane classified as a water barrier; "building wrap" and "wall wrap" are the everyday trade names. Whichever term is on the roll, the installation rules are the same, and they come from AS 4200.2.

What the Membrane Has to Do

In a wall the membrane performs up to four control functions, and it must be classified for the ones the wall actually needs:

• Water control — a classified water barrier that collects water passing the cladding and drains it to a flashing and out.
• Vapour control — allows or restricts water-vapour transfer. AS 4200.1 sorts membranes into four vapour-permeance classes, from Class 1 (vapour barrier) to Class 4 (vapour permeable, or "breathable").
• Air control — restricts air movement through the wall. It only works if every lap, joint and penetration is sealed.
• Thermal control — a reflective foil face reflects heat across an adjoining air gap.

Getting the class right for the climate, and installing it correctly, is what keeps the frame dry. Getting either wrong is a leading hidden cause of wall failures. For a fuller explanation of the classes, see our note on waterproofing membrane classes.

Choosing the Right Class for the Climate

Since NCC 2022, the vapour permeance of the membrane in an external wall has been regulated by climate zone (NCC 2025 Volume One F8D3, and ABCB Housing Provisions Table 10.8.1). The principle is simple: in cooler climates the wall must be able to dry outward, so the membrane must be vapour permeable; in hot, humid climates the greater risk is outdoor moisture driven inward, so a less permeable membrane is preferred.

The NCC's own explanatory table expresses the requirement as AS 4200.1 classes, and it differs depending on whether the cladding is direct-fixed (no cavity) or fixed over a drained and ventilated cavity:

Climate zone	Direct-fix (no cavity)	Drained & ventilated cavity
Zone 1 (tropical)	Class 2 or 3	Class 1, 2 or 3
Zone 3 (hot dry)	Class 3 or 4	Class 2, 3 or 4
Zones 4 & 5	Class 4	Class 3 or 4
Zones 6, 7 & 8 (cool / cold / alpine)	Not permitted — cavity required	Class 4

Based on NCC 2025 (Housing Provisions Table 10.8.1 / NCC Volume One F8D3). Always confirm the edition adopted in your State or Territory, and note there is no separate row for climate zone 2.

The key line is the bottom row: in climate zones 6, 7 and 8 — Melbourne, Canberra, the cold and alpine areas — cladding cannot be direct-fixed. It must be cavity-fixed (battened out to a drained and ventilated cavity) with a vapour-permeable Class 4 wrap behind it. Whether your cladding is direct-fixed or cavity-fixed is a separate question driven by the cladding type and the weatherproofing risk score; we cover that in direct-fix vs drained cavity cladding and the NCC F3V1a risk assessment.

Which Way Does the Reflective Side Face?

This one trips up a lot of installers. On a reflective (foil) wrap, the printed / anti-glare face goes outward and the reflective face faces the cavity. The foil only performs where an air gap adjoins it — in direct contact with cladding or lining it does nothing. Always follow the manufacturer's printed instructions, and keep reflective foil clear of live wiring, as it conducts electricity.

Installation Essentials

Orientation and laps

• Fix the membrane to the outside face of the frame, behind the cladding.
• Work in horizontal courses, bottom-up, so each upper course laps over the top of the one below on the weather face — like shingles. Water is shed down and out, never into a joint.
• Lap by at least 150 mm, or at least 50 mm where the lap is taped on the exterior face. Land end laps on a stud.

Fixing

• Fix to timber at roughly 150 mm centres with broad-headed, corrosion-resistant fasteners; to steel, use the membrane manufacturer's adhesive or fixings.
• Do not over-drive fasteners or use plain, non-washered screws — both tear the membrane around the fixing.
• Tape is a seal, not a fixing. Support taped joints so wind load goes into the fixing, not the tape bond, and use only the membrane manufacturer's specified, compatible tape.

Openings — the cut and fold

Openings are the most common leak path. At windows and doors the membrane must divert water away from the opening, never toward it. Cut a diagonal cross over the opening, fold each flap into the reveal, tape the exposed corners, then tape all four sides — dressing the membrane into every side of the reveal and lapping it over the flashings so water drains out.

The Common Installation Defects

Most sarking defects are workmanship at laps, corners, openings and penetrations. These are the failures we see most often on site — and the ones to build out:

• Reverse laps. An upper sheet tucked behind the one below channels water in behind the membrane. Always lap over, shingle fashion.
• Reflective face installed outward. The foil must face the cavity or it does nothing.
• Tears and punctures left open. Every unrepaired tear is a direct path for water and air. Patch before cladding.
• Wrap fixed behind the internal (fire-rated) lining. The membrane belongs on the outside face of the frame; behind the internal lining it can trap moisture and does nothing to shed water.
• Untaped reveal corners. Folded-in flaps with open corners let wind-driven water and air track straight into the wall.
• Membrane cut back at the base. Cutting the membrane to fit a sub-sill or flashing lets water behind it onto the frame. Dress it over the flashing and tape the junction.
• Poorly sealed penetrations. Pipes, cables and ducts sealed with loose scraps leave the cut edges open. Cut a neat, close-fitting hole and seal the membrane continuously all the way around.
• Unspecified tape. A generic tape may not bond to the membrane surface; the adhesive fails, the seal opens, and the warranty is voided.

Frequently Asked Questions

Which way does the reflective side of wall sarking face?

Printed / anti-glare face outward, reflective face toward the cavity. The foil only reflects heat where an air gap adjoins it.

How much should wall sarking overlap?

At least 150 mm, or at least 50 mm where the lap is taped on the exterior face. Work bottom-up so laps shed water down the outside face, and land end laps on a stud.

What is the minimum drained cavity depth?

The NCC requires at least 12 mm (F8D3(4) / Housing Provisions 10.8.1(4)), commonly detailed at around 20 mm. In climate zones 6, 7 and 8 the cavity is mandatory and cladding must not be direct-fixed.

Is wall sarking the same as building wrap?

In practice, yes — both name the pliable building membrane behind the cladding. Strictly, AS 4200.2 reserves "sarking" for a membrane classified as a water barrier.