Keeping Homes Weathertight | What are the 4Ds?

Author: Martin Kovacs

When it comes to keeping your home weathertight, the 4Ds are the four key design standards to ensure that your home stays warm and dry. Canstar explains everything you need to know about the 4Ds.

Leaky buildings have been an ongoing issue for the property sector in NZ stretching back to the 1990s. A range of factors can lead to a leaky home. And, unfortunately, substandard construction can see homeowners facing significant issues that can impact both their health and finances.

It’s against this background of poor construction practices that the 4Ds of water management have gained prominence. Weathertightness is a key consideration at the design and construction phase, and the 4Ds approach lays out clear principles for water management.

In the following guide, we take a look at  the 4Ds, and how each component contributes to keeping buildings weathertight. Plus we explore what classifies as a leaky building, features that can potentially cause issues, and some of the warning signs to look out for when house hunting.

Keeping Homes Weathertight | What are the 4Ds? In this article we cover:

What are the 4Ds?

From the mid-1990s, many houses in NZ were built that couldn’t withstand normal weather conditions. Blame was laid on shoddy builders, greedy developers, a change in building materials, and even the Building Code itself.

But regardless of the causes of the leaky home problem, as a result, weathertightness and water management were thrust into the spotlight. And government and industry set about addressing a range of challenges related to combating leaky buildings.

The Building Code now features plenty of guidance about compliance in regards to external moisture. For more information, External Moisture – An Introduction to Weathertightness Design Principles is essential bedtime reading. Much of the advice is based on Canadian research into the 4Ds, which, in order of importance, covers:

  • Deflection – shedding water by a cladding system, including deflecting devices such as eaves and weathering deflectors
  • Drainage – a back-up system directing water that may bypass the cladding back to the outside
  • Drying – removing remaining moisture by ventilation or diffusion
  • Durability – providing materials with appropriate durability

Weathertightness: 4Ds definition

Each element of the 4Ds contributes to water management, cumulatively helping to keep buildings weathertight:


Deflecting water away from cladding junctions can significantly reduce water load on junctions. This can be achieved by using an effective cladding and reducing the amount of water able to reach a potentially vulnerable entry point.

Types of deflection:

  • A cladding system
  • Overhangs and shelters, such as eaves, porches and verandahs
  • Projections or drip edges over horizontal junctions
  • Overlaps of both vertical and horizontal junctions


Providing paths for any water that does make it past the cladding, allowing for quick removal back outside before it can damage wall components.

Types of drainage:

  • Cladding overlaps
  • Flashings
  • Wall wraps and roof underlays
  • Cavities behind masonry veneer
  • Drained cavities


Removing water from the face of the cladding and any that does not drain out from behind, occurring primarily by ventilation. Diffusion of water vapour also contributes where ventilation rates are very low.

Without drying (or with very slow drying), building materials such as timber will eventually reach moisture contents that allow decay to start.


Materials need to have an appropriate level of durability for the anticipated environment. Prematurely failing components mean walls and roofs can no longer be considered weathertight.

While this is not a leak prevention mechanism, durability determines threshold leakage levels that can be accommodated without causing damage.

What classifies as a leaky building?

Leaky buildings are exposed to outside water finding its way in and causing damage. The website provides the following overview of what classifies as a leaky building:

“A leaky building is one where moisture gets between the exterior cladding of the house and the inside walls. If there’s no way for the water to drain out because of a lack of drainage and ventilation between the cladding and the framework, the water becomes trapped.”

In this situation walls can potentially rot and dangerous fungus can grow. This results in structural problems for the building and health problems for its occupants.

Points to consider, include:

  • Trapped water or moisture – behind certain cladding types, with no drainage and ventilation between the cladding and the framework, rapidly increases the potential for fungal growth and rotting
  • Impacts – among the many issues with leaky homes are the effects it can have on health or finances if the damage worsens
  • Internal leaking – from sources such as plumbing does not classify as a leaky home. Water needs to come from outside a house and make its way in, causing damage, to meet this classification

Features that can cause problems and signs to look out for

There are a number of common building features associated with weathertightness issues. So when house hunting, it’s worthwhile being aware of potential warning signs.

These features include:

  • Flat roofs, or roofs with parapets
  • Roof-to-wall junctions
  • Pergola fixings
  • Handrail fixings
  • Lack of flashings to windows and penetrations

  • Decks over living areas
  • Balustrade-to-deck or balustrade-to-wall junctions,
  • Clearances at the bottom of claddings
  • Level of ground outside above the interior floor level

Most weathertightness issues aren’t obvious. Of course, dripping or pooling of water inside when it rains should be of immediate concern. But other signs to look out for include:

  • Sagging of ceiling linings
  • Corrosion of fixings, such as screws and nails
  • Uneven floor surfaces, such as the lifting of vinyl
  • Mould or fungi formation on surfaces (although often due to poor ventilation)
  • Musty smells
  • Swollen materials, such as skirtings and architraves
  • Staining or discolouration of materials or surfaces
  • Stained or rotting carpet, or rusting of carpet fixings

While you won’t be able to claim for a leaky home on your insurance, it’s still important to ensure that your most valuable asset and possessions are covered in the event of an accident. And if you’re searching for a better deal on your insurance, Canstar can help. We rate and compare a variety of insurers on our database across home and contents (see below), pet, health, and car policies, so you can find a policy that works for you! 

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