This is the eighth article in a series based on a number of workshops on weathertight remediation for builders which the Ministry of Business, Innovation and Employment – Building and Housing group has been running at centres all over New Zealand. In this article we will be going back to basics.
Our industry has borrowed from leaky building veterans Canada and adopted the 4 “D”s first principles of design.
These principles can be applied to all types of building projects, but have particular relevance to an already failed building.
The first of these is deflection. A house with eaves, for example, can create a weather shelter to areas of the cladding. If less of the cladding is being exposed to moisture there will be less of the building with potential to let moisture in.
This is also supported by the analysis carried out on WHRS assessment reports which shows that houses with no eaves feature at a disproportionately higher percentage than they appear in the overall building stock.
Two of the other “D”s, drainage and drying, are equally fundamental. A majority of the costly damage to a “leaker” is from trapped moisture where any moisture that has breached the cladding can’t escape, and insufficient ventilating air is present to permit drying.
The last of the “D”s is arguably the most important: durability. Making durability a primary concern when designing and constructing a building will often dictate the use of the other basic principles.
We have many standard detailing solutions, including E2/AS1 which, if used, are enough to demonstrate design compliance in certain situations.
Is using these always the best way to enhance durability of a building in all situations, or is there sometimes a better way? In many cases it may not make a material difference to the overall cost.
Differential air pressures and their effect on a building and how they are dealt with can really affect the durability of a building.
High air pressure always wants to go to lower pressure, as does positive pressure to negative. Positive pressure on a building (e.g. wind pushing on an elevation) will create negative pressure (uplift) on both sides of a 25° pitch roof.
Similarly, on the other three elevations of the building, air is effectively sucked off the building, creating negative pressure. As positive pressure travels to negative pressure, what does that mean for the building wrap at external wall corners or wall to roof junctions?
Should we be choking them to avoid moisture-driven air being potentially sucked through the corners of a cavity system, under parapets or into roof spaces via barges? Similarly, air seals around windows effectively choke the high pressure outside, with possibly moist air being sucked through to the low pressure interior.
Properly installed cladding cavities are excellent at assisting with all of the 4 “D”s, as well as providing redundancy in the system and simplifying detailing solutions. With the 2011 changes to E2/AS1 there are now very few situations where an Acceptable Solution design will not require a cavity.
The “Dry” side of a cavity is the framing side of a cavity, with the “Wet” side being directly behind the cladding. Ideally, all moisture should be kept to the outside of a cladding system. However, if a design detail does not make this possible then moisture should be contained on the “Wet” side.
If moisture is allowed to easily and regularly get to the “Dry” side where the structural framing is, it potentially compromises durability. BRANZ research has found that the size of the vents in a wall cavity is the single biggest factor in the system’s drying rates.
If building wrap is baggy (especially if pushed out by insulation) or the cavity batten installation hinders the air flow, how does that affect the drying capabilities of the cladding system?
Changes to the original design of a building, such as a different cladding type, installation of eaves, or a pitched roof could also be considered by a client for better adherence to the 4 “D”s, resale value or just aesthetic reasons.
In consideration of all of this as builders, we need to ask the question: Is there a better way of achieving a durable weathertight building?
If you believe the specified detail may fail or could be improved, talk to the designer and request a change to the specifications.
In serious cases it may even be advisable to flag and document your concerns to the home owner and consenting authority. By doing so it may assist in showing you have done your best to fulfil your obligations in case the detail fails later on.
The final article in the series will summarise our past articles and avenues for further information.
Suggested follow up areas for more information
Building and Housing web site: www.dbh.govt.nz/ws-info-for-building-professionals
The Building and Housing publications
Guide to Remediation Design
Guide to the Diagnosis of Leaky Buildings
Dealing with Timber in Leaky Buildings
Code Watch Issue 1: October 2011
• The author: Harry Dillon has been involved with the repair of more than 300 homes as a builder over the past 10 years. This article represents Harry’s views which may not necessarily be the same as the Department’s.