Investigations of failed buildings have identified that the majority of leaks occur through wall claddings, and a number of high-risk details and design features have been identified. While roofs have not tended to feature in failure statistics, they still need to be detailed and constructed accurately. The most common areas where water has been found to penetrate the cladding are at:
· joints and junctions at cladding penetrations (particularly around windows and doors)
· junctions between different cladding materials
· joints in the cladding
· parapet and solid balcony walls
· service penetrations (pipes and meter boxes)
· structural penetrations
· movement cracks in the cladding (particularly at joints and in monolithic finishes)
· roof-to-wall junctions
· absorption through the cladding.
While many leaks are a result of rain being driven against a building exterior at variable pressures, angles and directions (by wind), many buildings have leaked in calm conditions where water has entered the building through the effects of gravity (particularly when water has been allowed to pond on flat surfaces).
HOW WATER BEHAVES:
Water has a simple molecular structure made up of two atoms of hydrogen and one atom of oxygen. In any single water molecule, the hydrogen atoms will have 'spare' electrons that can bond to the oxygen atom of other water molecules (this is known as hydrogen bonding). This bonding allows water molecules to form a liquid and influences the properties and behaviour of water, such as surface tension and capillary action.
· Surface tension
· Capillary action
· Absorption and wicking
· Water from cleaning
When water molecules bond, those on the surface are pulled inwards by the hydrogen bond. This creates a kind of skin effect, called surface tension, which can be strong enough to resist gravity and allows droplets to cling to building surfaces.
In general, when a water droplet comes into contact with a hydrophobic material (such as gloss-painted weatherboards or glass), where there is no hydrogen bond between the water and the material, the water will tend to run off. But when a droplet comes into contact with a hydrophilic material (such as uncoated concrete or unpainted timber), which absorbs water, surface tension will cause the droplet to flatten against and hold on to the material surface. The more the droplet flattens against the material, the greater its chance of resisting gravity and being held on the surface.
Water that is being held on building surfaces by surface tension will still drain down drainage paths on the vertical face of the cladding but may also drain from the vertical surface and cling onto an adjacent horizontal surfaces. Once it is clinging to a horizontal surfaces, it can be blown into a junction where it may penetrate and cause damage. Creating a sharp transition to an upward slope or surface will make the water drip off at the junction. Surface tension therefore needs to be broken at all vertical to horizontal junctions. This is done with a drip edge, weathergroove, flashing or drip moulding. Surface tension allows water droplets to cling to building surfaces, even downwards facing ones.
Capillary action is where water bonding to two adjacent surfaces is drawn upwards against the force of gravity between the two surfaces. How far the water can be drawn upwards depends on the size of the gap between the surfaces and how hydrophobic or hydrophilic they are. Wind pressure can also act on the water and drive it upwards even further.
Incorporating a capillary break by detailing a gap of 6 mm between surfaces will stop capillary action occurring, as the surfaces will be too far apart for water to bond between them. The incorporation of weathergrooves, seals or hooks/seams on a flashing can also assist, as these will break the contact between the adjacent surfaces.
ABSORPTION AND WICKING
Absorbent or porous materials and surfaces (such as raw fibre-cement, uncoated concrete, weathered coatings and unpainted timber) will absorb moisture.
They can also wick moisture off an adjacent surface, where it can be absorbed. Once water has been absorbed, it will migrate or wick through the material from a warm area to a cold area. It may also be absorbed by other adjacent materials – for example, water may be absorbed by a poorly coated cladding and migrate through to be absorbed by a dry absorbent wall underlay and ultimately by the dry timber framing.
The use of non-absorbent materials or finishes will limit absorption, and the use of capillary breaks or a separation between surfaces (such as a gap at the bottom of the cladding above a waterproof deck) will restrict wicking. Rapid heating by the sun of surfaces containing moisture can also drive water vapour from absorbed moisture through materials – a process known as solar-driven moisture transfer.
Air contains water vapour, with the amount of vapour present increasing with temperature. As air cools, its ability to hold water vapour is reduced, and the vapour is released and condenses as water. When air is cooled by contact with a cold surface, the released vapour forms as condensation on that surface (for example, the steam created from a hot shower will condense when it comes into contact with the cooler glass of an exterior window).
If condensation occurs, it can be absorbed by materials and can cause material deterioration, so it needs to be managed within wall assemblies. This can be done by ventilation and by incorporating absorbent wall underlays or using cladding materials that have some degree of absorbency on the back of the cladding that will hold the condensation water until it dries again as a result of ventilation.
WATER FROM CLEANING
Building exteriors should only be cleaned with very low-pressure water, as high-pressure water (such as that from water blasting or a high-pressure hose) has the potential to be directed at and driven through gaps in the exterior cladding and up under flashings, where it can enter the roof or wall assembly and be absorbed by components.
High-pressure water can also damage softer cladding materials and damage or remove protective exterior coatings.
GLOSSARY 10-YEAR ELIGIBILITY REQUIREMENT
To be eligible to repair your home through the Weathertight Home Resolution Services Act, the house must have been built (or alterations giving rise to the claim made to it) before 1 January 2012 and within 10 years of lodging the claim. If a Code Compliance Certificate was issued, this may be taken as the date the house was built.
The key mechanisms by which a house remains weathertight and in sound condition — deflection, drainage, drying and durability.
A prescriptive design/construction solution published by the Ministry of Building, Innovation and Employment (MBIE). Where proposed construction follows an Acceptable Solution exactly, it must be accepted as being code compliant for that specific Building Code clause.)
AIR LEAKY CLADDINGS
Claddings such as weatherboards that allow air (and also water) to readily pass through the cladding joints. The airleakyness also assists in drying any water that gets behind the cladding
A building product or method of construction that is partly or completely different from the products or methods described in an Acceptable Solution or a Verification Method.
A means of compliance with the Building Code not wholly in accordance with an Acceptable Solution or a Verification Method. An Alternative Method becomes an Alternative Solution when accepted by a BCA and the building consent is issued
A useful or desirable feature. Building another bedroom or adding an en-suite bathroom to a bedroom increases the amenity of a property.
Building consent authority – usually a territorial authority, such as a city or district council, that has authority to issue building consents.
A technical opinion of a product or system’s fitness for purpose based on specific Building Code requirements, issued by BRANZ.
A synthetic rubber typically used as a fully adhered membrane for roofing, decking and other applications where weatherproofing is required.
A break or gap of at least 6 mm between two materials (or within one material) designed to stop the movement of water between the material(s).
A building element with slots or holes that allows water to drain from a cavity and air to circulate, while keeping vermin out.
Copper, chrome and arsenate timber preservative.
A code compliance certificate. This issued by a BCA after final inspections have been carried out. A CCC can only be issued if the building work complies with the building consent.
A compliance path is a way chosen to demonstrate compliance of intended work with the Building Code. Compliance paths include Acceptable Solutions, Product Certification, and Verification Methods.
A compliance path is a way chosen to demonstrate compliance of intended work with the Building Code. Compliance paths include Acceptable Solutions, Product Certification, and Verification Methods.A compliance path is a way chosen to demonstrate compliance of intended work with the Building Code. Compliance paths include Acceptable Solutions, Product Certification, and Verification Methods.
Damage that occurs as a consequence of a particular action or event. For example, a leak around a window may lead to such consequential damage as rotting timber framing or carpets.
A sum of money available to cover unexpected building or renovation costs.
Copper azole timber preservative.
Sometimes called safety data sheets, these documents protect the health and safety of people in the workplace by giving information on the hazards of materials and how the materials should be used, stored, transported and disposed of.
A decision by MBIE on whether a set of documents for a proposed building, a construction element or specific detail, or a recently-built building or specific building component, complies or does not comply with the Building Code.
The exterior cladding of the building is fixed over a wall underlay and directly to the framing.
Damp-proof membrane, often polythene sheeting, typically used under a concrete slab to prevent ground moisture entering the slab, or used as a ground cover under suspended timber floors.
The path water that leaks through a cladding will travel down the back of the cladding.
DRAINED AND VENTED CAVITY
Described in E2/AS1 as a 20 mm bottom-vented cavity behind lightweight wall claddings design to dry and/or drain any water that might penetrate the cladding.
The Acceptable Solution for the weathertightness of timber-framed buildings up to 10 m high and located within a low, medium, high, very high or extra high wind zone.
Water-soluble salts that crystallise on a masonry surface (brick, concrete block and concrete) as moisture evaporates from it. Efflorescence usually has a whitish appearance.
Exterior Insulation and Finish Systems – a type of wall cladding where polystyrene sheets are typically plastered with a reinforced polymer modified cement-based plaster and then painted.
A synthetic rubber membrane typically used as a fully adhered membrane to waterproof roofs and decks.
Referred to as EPS, light because it is 98% air. Often used as insulation under suspended timber floors. EPS can be recycled.
Referred to as XPS, a closed-cell, rigid type of insulation with a continuous skin surface. It has a greater density and compressive strength than expanded polystyrene (EPS).
Referred to as XPS, a closed-cell, rigid type of insulation with a continuous skin surface. It has a greater density and compressive strength than expanded polystyrene (EPS).Referred to as XPS, a closed-cell, rigid type of insulation with a continuous skin surface. It has a greater density and compressive strength than expanded polystyrene (EPS).
The board that runs along the edge of the roof at the eaves. Guttering is usually attached to the fascia.
Thin strips or areas of impervious material, often sheet metal, installed to stop water moving through a joint and into a structure. Flashings are often shaped to fit a particular location.
FLEXIBLE FLASHING TAPE
A tape installed into and around framed joinery openings (typically for windows and doors). It goes over the underlay and exposed framing. Also used at joinery heads to seal flashing upstands to the underlay.
Steel sheet or element that has a thin layer of zinc added to help protect it from corrosion.
Something that absorbs water or is easily wetted. Unsealed plaster, plasterboard, timber and fibre-cement are examples of hydrophilic materials.
A surface finish or material that does not allow water or water vapour to pass through.
INSULATING GLASS UNIT (IGU)
Double-glazing or triple-glazing.
The vertical sides of a door or window opening.
Horizontal framing that supports a floor or ceiling. Floor joists sit above the bearers.
LEAKY HOMES FINANCIAL ASSISTANCE PACKAGE
A package where the government and local council each paid 25% of the repair cost and the homeowner paid 50%. (If the council did not approve the original work or did not participate in the FAP, the homeowner paid 75% of the costs.) The scheme ran for five years from 23 July 2011 to 23 July 2016.
A fair and reasonable estimate of the actual financial damage suffered as the result of a breach of contract.
Light Organic Solvent Preservatives – insecticides and fungicides in a spirit-based carrier for treating timber
Medium density fibreboard, a compressed engineered wood product.
Monolithic cladding is sheet-cladding material (such as fibre-cement) that is plastered and coated to give a seamless finish.
A type of synthetic rubber that can be used to aid in weathertightness. For example, neoprene washers used with screws in steel roofing are slightly compressed when installed to provide a watertight seal around the screw.
NOTICE TO FIX/NOTICES TO FIX
A statutory notice issued by a building consent authority or territorial authority requiring a person to remedy a breach of the Building Act 2004 or regulations under that Act. A notice to fix can be issued where work is done without a building consent, building work does not comply with the Building Code, and so on.
The New Zealand Institute of Building Surveyors (NZIBS), the main professional body for building surveyors.
A natural or organic compound with a particular type of molecular structure. Many plastics and resins are polymers.
An expert opinion often provided in building consent applications to support alternative methods of demonstrating Building Code compliance. Manufacturers, engineers, and others with specialist knowledge may complete producer statements. Producer statements are accepted at a building consent authority’s discretion.
Products or systems that are certified through the voluntary CodeMark scheme must be accepted by a building consent authority when used as specified. The scheme, established by the Building Act 2004, is a way to show that a product or system meets the requirements of the Building Code.
A product manufactured or sold by a particular company or companies, which typically have copyright or trademark ownership. Contrast with ‘generic’ products (such as ordinary black PVC plastic sheeting) where none of the manufacturers have intellectual property rights.
Plasticised polyvinyl chloride, a flexible type of plastic typically used for flooring, flexible pipes and so on. PVC is one of the most commonly used plastics. It is made from common salt (sodium chloride) and petroleum products.
Quality assurance – maintaining a particular level of quality.
Rigid air barrier – typically a proprietary sheet system (fibre-cement or plywood together with accessories such as joint flashings) applied as a rigid wall underlay to the outside face of wall framing. RAB must be submitted for consent as an alternative method.
Restricted building work is work that must be carried out or supervised by a licensed building practitioner (LBP). RBW includes work on foundations and structure, wall and roof cladding and fire safety systems for houses.
RELATIVE HUMIDITY (RH)
the percentage of water vapour in the air at a specific temperature compared to the maximum amount that the air could hold at that temperature.
Fixing faults, deficiencies and damage. A term typically used with the repair of leaky buildings.
A tool within E2/AS1 that allows designers to calculate the weathertightness risk score for each face of a particular building design. See paragraph 3.1, Figure 1 and Tables 1 and 2 in E2/AS1.
Resource Management Act 1991
A variety of blackish mould that grows on materials containing cellulose and which can be harmful to health
The detailing applied to the end of a window head or apron flashing to prevent water getting behind the cladding at the ends of the flashing.
A type of wall cladding where a reinforced sand/cement plaster is applied in two or three applications, often with a textured finish, to a non-rigid (flexible wall underlay) or rigid (plywood or fibre cement) backing. It must be painted to remain weathertight.
An underlying layer. For example, H3 CCA-treated plywood is the substrate commonly used under membrane roofs.
Where aluminium windows have a very strong spacer with a higher level of thermal performance between the inner and outer parts of the aluminium frame. BRANZ testing has shown that frames with this feature can be almost 60% more thermally efficient than those without it.
Features on the surface of the land, including hills, trees, and buildings.
Thermoplastic-sheathed cable. The type of electric cabling used in houses today
A profile where the top and bottom of the profile are parallel, but the sides are not parallel. On trapezoidal steel roofing, each side of the profile typically slopes outward.
Unplasticised polyvinyl chloride, a hard plastic typically used for window frames, pipework, cladding, guttering and downpipes.
A heavy cladding supported by a structural base that is separated from the supporting framing by a ventilated cavity. For example, brick houses today are built with brick veneer laid onto a concrete foundation, while the structural support of the house is provided by timber or steel framing behind the bricks and not by the bricks themselves.
According to the Building Act 2004, a verification method is a method by which compliance with the Building Code may be verified using an identified testing regime or method of calculation.
A flexible or rigid sheet material applied to the outside of framing to provide a second line of defence against water getting into the framing.
Grooves in the back of timber weatherboards, designed to stop the capillary movement of water across the back of the board.
WEATHERTIGHT HOMES RESOLUTION SERVICE
A service established by the government to help owners of leaky buildings.
Weathertight Homes Resolution Service
Wind regions are given in Figure 5.1 of NZS 3604:2011 Timber-framed buildings.
Wind zones can be calculated from Table 5.1 of NZS 3604:2011 Timber-framed buildings. They are classified as a low, medium, high, very high or extra high wind. Buildings outside of the extra high zone must have the structure specifically designed.
Dean Norrie, Director/Building Inspector - LBP Qualified Builder, with 20+ years residential building experience, three generations of builder in the family.