Leaky buildings often have features in common. The more of these features a home has the higher the risk of it being a leaky building.

When assessing a potentially ‘leaking building’ our consultants look at the weathertightness risks and ways to manage them, or recommend on how to reduce the risk. The risk factors identified in the Acceptable Solution are:

• Wind zone
• Number of storeys
• Roof/wall intersection design
• Eaves width
• Envelope complexity
• Deck design

Using a risk matrix in E2/AS1 these risk factors are given a risk score. This determines the type of claddings that can be used and if any specific design features need to be used.

Existing homes with a high risk score require careful examination to establish if leaks are likely to occur. Maintenance has to be far more rigorous for these buildings than those with a low risk score.

The ability of the building envelope (or shell) to keep water out is seriously affected by the speed of the wind driving rainwater against it. The higher the wind speed the more likely it is for water to be driven into the structure. This can occur when there is air-flow through gaps in the cladding that allows water to flow into the building or when the impact of the rain splashes or bounces behind laps or over flashings.

New Zealand is subject to a lot of strong winds. These sea winds are particularly strong in the far north, through Cook Strait and in the south. The country is therefore divided into wind zones. These zones determine how strong any building needs to be, what materials should be used, and how it should be maintained.

The detailed calculations involved in making these decisions will be done by your architect or designer. Wind speeds are categorized as LOW, MEDIUM, HIGH or VERY HIGH.

The zones are from New Zealand Standard 3604:1999 Timber Framed Buildings. This standard is an Acceptable Solution to the Building Code and guides much of the construction of timber-framed buildings in New Zealand. For more information on these and other zones used in deciding the design needs of your home see Zones in New Zealand.

The more storeys a house has, the higher the risk. Single storey homes will score lower on the risk matrix. Two storey homes have more wall exposed to the weather and have a junction between storeys. Three storey buildings are most likely to have a combination of claddings with junctions between the storeys, and the uppermost storey is likely to be fully exposed to the weather. Tall walls also catch a lot of water increasing the flow of water down the claddings and onto any junctions or openings, such as windows or doors.

Eaves provide shelter to the cladding surface, reducing the amount of water that will collect on the surface from rain. They also provide shelter to windows, doors and other openings in the cladding, greatly reducing the risk of water entry.

Wide eaves, minimum 600mm, on a single storey building will provide the highest level of protection. Narrow or no eaves on a three storey home will obviously provide the least and will have a higher score on the risk matrix.

Simple buildings will have fewer cladding junctions, or junctions between the cladding and roof planes or other materials. Windows are also likely to be less complex shapes and sizes. Simple shaped buildings are rectangular, T or boomerang shape. With a single cladding type these buildings will have a lower risk score.

Medium risk buildings are described as moderately complex, incorporating angular or curved shapes and with no more than two cladding types, e.g. brick veneer with fibre-cement.

Complex angular or curved buildings with multiple cladding types are high risk. Those with additional complex intersections, box windows and pergolas attached to the cladding are very high risk in the matrix.

Decks are a major cause of leaks. For timber slatted decks problems have often occurred where the deck is attached to the house or where cantilevered joists have been used. Cantilevered joists are continuous from the inside of the building, out through the cladding to the outside of the building and are very difficult to seal where they penetrate the cladding.

Decks that form the roof to a room below have caused problems because of:

• Insufficient step-down at the door way or interior.
• Lack of fall to the deck surface.
• Poorly designed water outlets to the exterior of the deck.
• Claddings too close to the deck surface.
• Blocked outlets with no overflow.
• Solid balustrades with penetrations through their tops.
• Solid balustrades with no flashing on the top surface.
• Solid balustrades with flat tops.
• Cracking at the junction between the balustrade and the wall surface.
• Poor maintenance of the deck surface.
• Tiles covering the deck and hiding the waterproof surface from maintenance.
• This is why decks increase the risk of a leaky building. Decks are a risk to the house structure if leaks occur, but a leaking deck also threatens those who use them because the structure may be seriously weakened by rotting timbers.

The lowest risk is obviously where there is no deck, or where the deck is supported from the ground at ground floor level.

Of medium risk are waterproof decks completely covered by a roof, or a timber slatted deck at first floor level or higher.

Of high risk are waterproof decks, fully exposed to the weather or any deck using cantilevered joists at first floor level or higher.

Very high risk, are waterproof decks exposed to the weather or any cantilevered at second floor level or higher.