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. The Acceptable
Solution for weathertightness design (E2/AS1) became effective in
February 2005 and now includes a section on assessing potential
weathertightness risks and ways to manage them, or to reduce the risk.
Knowing and eliminating risk factors Knowing what to look for and eliminating as many risk factors as possible is
a good way of minimising the risk of buying a leaker. Because this
assessment of weathertightness risk was carried out in July 2004, homes built
before then are at greater risk than those built since.
The
risk factors identified in the Acceptable Solution are:
Wind zone 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.
Number of storeys 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 width 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.
Envelope complexity 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.
Deck design 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 thre atens 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.(CB)