Materials compatibility: order, direction and junctions

Most material failures are not caused by a bad product. They are caused by a good product put next to the wrong neighbour, or in the wrong position in the sequence. A copper pipe is fine. A Colorbond roof is fine. Copper above Colorbond, with rain running from one to the other, is a slow failure that appears two or three years later, long after the trades have gone.

This article is about three questions that apply to almost every junction in a building: what sits above what, which way does water travel, and what touches what. We cover the chemistry of cleaners in cleaning chemicals and building materials and the choice of sealant in choosing silicone sealant. Here the focus is position and connection.

Rule 1: water carries its history downhill

Rain does not only wet a surface. It picks up whatever the surface sheds, then deposits it on whatever is below. That makes the order of materials from top to bottom a design decision, not a detail.

The NCC makes this explicit for metal roofing. Clause 7.2.2 of the Housing Provisions, Part 7.2 requires that where different metals are used in a roofing system, including flashings, fasteners, guttering and downpipes, they must be compatible with each other. It adds two specific restrictions:

• no lead materials upstream from aluminium/zinc coated materials; and
• no copper materials upstream from galvanised coated materials.

Tables 7.2.2b to 7.2.2e then set out compatibility for stainless steel, zinc-coated steel, zinc/aluminium coated steel and lead accessories and fasteners, split by atmospheric conditions.

The word that matters is upstream. Copper and zinc do not have to touch. Copper ions dissolved in rainwater are enough to attack a zinc or zinc/aluminium coating further down the flow path. In practice, the same logic applies to:

• a copper pipe, hot water unit or air-conditioning line draining onto a steel roof or gutter;
• a lead flashing above a Colorbond or Zincalume sheet;
• copper-based treated timber sitting against or above coated steel, which roofing manufacturers advise against;
• a new feature in one material added during a renovation above an existing roof or facade in another.

A useful working rule: the material further downstream should be the same as, or more noble than, the material above it. Where that is not possible, break the flow path with a separate drainage route.

Rule 2: overlap in the direction water travels

Every weather layer in a building (roofing, flashings, wall wrap, damp-proof courses) works on the same principle as roof tiles: the upper piece laps over the lower piece, so water falling down the face runs onto the next layer and not behind it.

Getting this backwards is the most common installation error, and it is invisible once the cladding is on:

• Wall wrap and sarking lapped upper-under-lower funnels water behind the layer. The installation standard is AS 4200.2. For walls, clause 3.7.1 requires joints to be lapped not less than 150 mm, or lapped not less than 50 mm and taped (butt-jointing with tape is allowed only where the combined material thickness exceeds 5 mm and overlapping would compromise the cladding), with end laps fixed at a stud. Clause 3.7.3 requires the upper sheets to lap over the lower sheets where the membrane acts as the water control layer. Roofs have their own clause (3.3), so check you are reading the right one.
• Flashings sit over the layer below them. For sheet roofing, NCC Housing Provisions clause 7.2.7 requires joints in flashings and cappings to be not less than 75 mm and lapped in the direction of fall. That clause sits in the sheet roofing part, so it does not set the lap for wall cladding flashings. Flashings also have a top edge that turns in or tucks behind the layer above. Head flashings over windows must discharge outward over the cladding, not into the wall. Masonry cavity flashings need weepholes above them so collected water can leave; see waterproofing the building envelope.
• Membranes on balconies and podiums turn up behind wall cladding and are covered by flashings, not the other way around.

If a sequence needs to be changed on site (for example, a window is installed late), the fix is to re-plan the lap, not to seal the gap with sealant and hope.

Rule 3: put layers on the correct side of the insulation

Position also controls moisture inside the wall. Under NCC 2022, Part 10.8 of the Housing Provisions says a pliable building membrane in an external wall must comply with AS 4200.1, be installed in accordance with AS 4200.2, and be located on the exterior side of the primary insulation layer. It must also meet minimum vapour permeance: 0.143 µg/N.s in climate zones 4 and 5, and 1.14 µg/N.s in zones 6, 7 and 8.

In plain terms, the outer layer should let vapour escape. A low-permeance layer on the cold side of the insulation, such as foil-faced insulation used as a wall wrap, can trap moisture inside the wall, where it feeds mould and timber decay. Moving the same product to the other side, or choosing a vapour-permeable version, changes the outcome completely. If there is no membrane, Part 10.8 requires a drained cavity between the water control layer and water-sensitive materials (with an exception for single-skin masonry and concrete).

The same idea applies to floors. A timber floor laid over a slab or subfloor with the wrong moisture treatment cups and fails; see timber flooring, moisture and the subfloor.

Rule 4: check what touches what

Contact problems are the second half of compatibility. Common pairings to check:

1. Dissimilar metals in contact. Fasteners, clips, straps and flashings should be compatible with the sheet they hold. Tables 7.2.2b to 7.2.2e cover roofing; the same thinking applies to balustrades, fixings through cladding and plumbing brackets.
2. Metals and wet cement. Fresh mortar, render and concrete are strongly alkaline and can attack aluminium and zinc coatings. Sleeves, wraps or coatings are used where metal is embedded or runs through masonry.
3. Metals and treated timber. Some timber preservatives contain copper. Manufacturers advise keeping coated steel away from copper-treated timber, and specifying fasteners and flashings rated for contact with treated timber.
4. Sealants and the surface they bond to. Some silicones cure by releasing acetic acid, which can attack metals such as copper, brass and zinc, so a neutral-cure product is specified for them. Sealants can also stain when they touch bituminous materials. Cured silicone generally cannot be painted, so a paint-ready joint needs a different product.
5. Membranes, adhesives and finishes. Under-tile membranes, tile adhesives, sealants and primers are tested as systems. AS 3740 requires products used with a wet-area membrane to be compatible with it, which in practice means using the system the manufacturer has tested. See wet area waterproofing.
6. Solvents and foam. Some solvent-based adhesives, bituminous products and sealants soften expanded polystyrene and some plastics. Separation layers and compatible adhesives are specified for this reason.

Where it goes wrong: the junctions

Most compatibility problems sit at the joins, where two trades meet and each assumes the other has checked.

• Roof to gutter to downpipe. Three materials, often supplied by different people. The gutter's finish, the fixings and any flashing above it all need to agree. Our article on roof plumbing covers the details.
• Cladding to window to flashing. The flashing, sealant, frame finish and wall wrap must all work together and be lapped in the right order. See where buildings leak.
• Plumbing penetrations. Pipes through membranes and cladding must match the sealing method; a collar designed for one pipe material may not suit another.
• Retrofits and additions. A new air conditioner, solar array or balustrade installed years after construction brings new materials and runoff paths onto an old building. Nobody re-checks the sequence.

Questions worth asking before work starts

1. What materials are in the flow path above and below this component, and are they compatible under NCC clause 7.2.2 and the manufacturer's data?
2. Which direction does water travel across this junction, and which layer laps over which?
3. Are fasteners, flashings and clips the same material as, or approved for use with, the sheet they hold?
4. Is the membrane on the correct side of the insulation, and does it meet the permeance requirements for our climate zone?
5. Which products are being used as a tested system, and has anyone substituted one?
6. Who owns this junction in the program, and when is it inspected before it is covered?

How we approach it

We list the materials at each junction before ordering, check the flow path above and below, and treat every substitution as a design change. Junctions are inspected before the next trade covers them and recorded in the handover file, which is the idea behind hold points and our stage-by-stage envelope inspections. If you are planning a build, a renovation or a repair and want a second set of eyes on the materials, get in touch.

Sources

• NCC 2022 Housing Provisions, Part 7.2 Sheet roofing, Australian Building Codes Board (clauses 7.2.2 and 7.2.7, Tables 7.2.2a to 7.2.2e)
• NCC 2022 Housing Provisions, Part 10.8 Condensation management, Australian Building Codes Board (clause 10.8.1)
• AS 4200.2:2017, Pliable building membranes and underlays, Part 2: Installation, Standards Australia (clauses 3.3, 3.7.1 and 3.7.3)
• NCC 2022 Housing Provisions, Part 5.7 Weatherproofing of masonry, Australian Building Codes Board
• NCC Navigator: Waterproofing in houses, Australian Building Codes Board (AS 3740:2021)

This article is general information, not technical or legal advice for your project. Compatibility depends on the products, the exposure and the manufacturer's current guidance, so confirm it with the product data sheets, your certifier and the relevant standards. Full website disclaimer