BRANZ weathertightness poster

, Build 78

In the August/September 2002 edition of Build you would have received the BRANZ weathertightness poster. This column takes a detail from the poster and provides a ‘good practice’ solution to it.

Risk factor 1 on the weathertightness poster ‘complex roofs’ highlighted the following detail.

The roof is one of the most important elements of a house for keeping out water. It is the first line of defence against most of the weather. A roof is like an umbrella — it should largely keep the water off the building’s walls and never leak.

Generally, roofs work well when they are simple shapes with good slopes and overhangs, but are more difficult to detail and construct when they are complex. Every lap in the material and every junction is a potential leak point and because roofs are exposed directly to the weather, it makes sense to reduce the number of junctions as much as possible. Parapets, skylights, dormer windows, low-pitch slopes, valleys, internal gutters, penetrations and junctions between roofing and wall cladding materials are all risk areas for leaking. They need to be carefully designed and constructed.

Simplify, simplify, simplify

To reduce the risk of weathertightness failure, the roof form should be carefully designed and simplified where possible. Low-pitch roofs should be designed and constructed very carefully, ensuring that the minimum pitch recommendations are adhered to and that all flashings provide enough cover for the roof slope and the wind zone. Apron flashing upstands should have a hook on the top edge as a final barrier to water ingress. Parapet tops should be fully flashed and sloped to shed water. Internal gutters should be avoided where possible and all penetrations need to be under- and over-flashed.

Eliminate, minimise or isolate

Complexity in roof form should be approached with a similar philosophy to that applied to safety hazards, namely we should ‘Eliminate, Minimise or Isolate’. This mantra is worth adhering to because it encourages a layered and considered approach.

To not have a complex roof is the first choice (eliminate).

The second choice, if you must have complexity, is to have as little of it as possible (minimise).

If that is not an option, then at least segregate the risky details from where they may cause problems with other building elements (isolate).

For more

To obtain a copy of the BRANZ weathertightness poster phone (04) 237 1170.

Download the PDF

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