Forces acting on water

A number of forces act on rainwater, and these can add to the potential for water to leak through a building exterior.

Gravity

Gravity acts on water by forcing it to run downwards. Gravity leaks in buildings are common, particularly if water is allowed to pond on flat surfaces.

Gravity can also cause water to leak through claddings along leakage paths. Once water has penetrated the building exterior, gravity can carry it further through the wall assembly.

However, gravity is also useful in managing water by draining water off building surfaces, such as roofs or walls, and from within wall assemblies.

Drainage paths

Drainage paths can use gravity to move water that has leaked into the wall assembly back out to the building exterior. Drainage paths are formed on the back surface of the wall cladding, particularly within a drained and vented cavity assembly or in the space between the back face of the cladding and the face of the wall underlay in a direct-fixed cladding assembly.  

The building exterior also forms a drainage path where water runs off it. This water also needs to be deflected away from critical junctions.

Direct-fixed cladding systems have less potential for drainage to occur, as the back of the cladding has a high percentage of contact area with the wall underlay, which limits the space for drainage down the back of the cladding. These systems are less effective at draining water than a drained cavity system.

Other design aspects

Aspects of design and construction such as falls or slopes, flashings and drip edges can all be used to help gravity move water off building surfaces and from within building assemblies.

Designers and builders need to understand gravity drainage on building exteriors and within wall assemblies and ensure that the construction allows drainage to occur.

Back to top

Momentum

Wind can drive rain against a building with varying intensity and from different angles and directions. 

Wind can also force rain to bounce off one surface and onto another. It may cause water to move upwards behind a cladding (for example, rain bouncing off a horizontal surface, like a paved courtyard or a deck, and being deflected up behind the bottom edge of the cladding).

Building exteriors must be designed and built to deflect water impacting at varying speeds, angles and directions and to allow water to drain off surfaces. Designers and builders should be aware of the potential for wind to drive rainwater onto areas of the building that might appear to be sheltered.

Back to top

Differences in air pressure

Wind impacting on a building increases the exterior positive pressure on the windward face of the building. This pressure varies around the building depending on wind direction and can be suction on the leeward sides. The windward pressure is generally higher than the interior pressure of the building. This creates a pressure differential, with the higher-pressure external air trying to move towards the lower-pressure building interior through gaps or breaches in the building’s exterior envelope. The paths this higher-pressure air follows are known as air leakage paths.

If air leakage paths are present, rainwater can be carried along them and into the wall assembly. When water moves along air leakage paths, they become known as water leakage paths. Water can move into wall assemblies at quite low and quite common pressure differentials.

Details that allow pressure moderation to occur need to be incorporated into the design and construction of the exterior walls to reduce the potential for water penetration. See air movement and pressure moderation for details.

Pressure variation around the building exterior

When the wind impacts on a building from a particular direction, it is diverted around and over the building. This means that the exterior is subject to different levels of wind pressure, with the strongest positive pressure (push) on the windward side and a negative pressure (suction) on the leeward side. 

This pressure variation also occurs across the roof (dependent upon the roof pitch) with generally a positive pressure on the windward side and a negative pressure on the leeward side (which can cause a suction effect that has the force to lift off the roof).

Pressure variation around a building also causes wind-driven rain to eddy and change in direction, with the rain quite often being blown upwards. This can make it difficult to predict exactly how wind-driven water will behave, so building design and construction needs to consider all eventualities to ensure that the building is weathertight.

For buildings on exposed sites, the actual wind pressure and pressure differentials can be substantial, so buildings in these types of locations have a higher risk of weathertightness failure than buildings in sheltered locations. 

Pressure variation can also occur as the wind speed and direction is altered by aspects of the building’s surroundings, such as adjacent structures, trees and the contour of the surrounding land. Wind can funnel up valleys and hillsides, over ridges and between and around adjacent buildings, turning what seems like a sheltered site into an exposed one.

Never underestimate the potential for the wind to drive rain into and onto areas of the building that may be sheltered in most situations.

Updated: 9 September 2014