NZ Building Code is primarily based on gravity drainage

A good example of what you should normally not be doing with a New Zealand building: waterblasting it. Why? Because the NZ Building Code is primarily based on gravity drainage and deflection of water.

Consumer NZ warns

In that regard an extract from consumer.org.nz:

Don’t waterblast stucco-type houses Waterblasting claddings made from fibre-cement sheet or stucco is a big no-no. That’s because the high-pressure water can penetrate cladding (or any cracks in its surface). And once it’s in, the water can’t get out. This warning especially applies to “monolithic clad” houses built from the early 1990s up till around 2003. Be careful with other claddings too Waterblasting weatherboard, brick, concrete block or steel (“galvanised iron”) claddings is acceptable – as long as the nozzle is not brought too close to the surface (less than 500mm) and the spray is not directed closely around door openings, window frames and other openings in the cladding.

A water blaster however injects water into cavities, underneath head and sill flashings and forces it onto the building wrap or further. In small quantities this may be okay, in larger quantities damage can occur and if the water cannot drain out in case it entered too deep into the building envelope, decay will also occur over time.

Most cladding system are appraised for 2.5 kPa pressure

Most building constructions and façade systems in New Zealand are designed for a wind load (pressure) of 2.5kPa. That equates to a pressure of 0.362594 psi. Most home water blasters have a pressure of 2100 psi, commercial ones are higher. That is almost 10,000x the pressure your cladding system was designed for. Now that pressure from a water blaster decreases exponentially with distance, so the more distance you have, the less the pressure will be. Still, you see the issue.

The latter one makes sense when looking at NZ building code pictures, taken from NZBC E2:

NZBC E2, Figure 7
NZBC E2, Figure 7

As we can see from this example, once your water has been blasted past the 75mm flashing overlap and the hem, it is straight into your framing.

NZBC E2, Figure 86
NZBC E2, Figure 86

Similarly for a typical window head flashing, that has usually no seal between the window and the flashing, the water gets funneled directly behind the window, dropping down on the building wrap with a chance of getting onto the jamb liner or running around the lintel and ending up on the sill, going either to the inside or outside.

Residential water blasters use about 8l per minute, which is 130ml a second, half a glass of water every second.

Half a Glass

Just to throw another number at you, most domestic water supply pressure is around 40 tp 70 psi, so even when washing your house with a garden hose from the underside, you are likely to have issues. Hence the good old advice: Always wash from the top down or avoid washing those flashings.

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