Many modern homes have decks. While open-slatted decks attached to buildings at ground level don't cause many weathertight problems, open-slatted decks at upper levels do cause issues, and so do waterproof decks (decks that have a membrane installed to intercept and drain water). Waterproof decks and solid balustrades are heavily represented in weathertight failure statistics and require significant skill to design and construct accurately.

Timber-slatted decks – attached

Timber-slatted decks consist of a decking surface of timber planks installed over timber joists that are supported by a stringer attached to the building exterior. Water must be able to drain between the planks, so they need to be installed with a gap between them.

While slatted decks can be installed to the level of the internal floor that they are accessed from (particularly where disabled access is required), it is good practice to set the deck level at a minimum of 50 mm below floor level. A 12 mm gap between the first board and the face of the cladding is required to ensure that water does not pond against the cladding, as this could compromise the cladding durability. A gap is also required behind any timber fixed to the face of the cladding.

The key concern is the connection to the building, where the connection bolts or brackets penetrate the wall cladding and create a potential water leakage path into the wall assembly.

Where a slatted deck is connected to the building at ground floor level, the connections penetrate below floor level. There is no way that water can leak through the connections into the exterior wall assembly, so they are a lower weathertightness risk.

Where a slatted deck is connected at first floor level or above, the deck connections penetrate the wall assembly and create a potential water leakage path and higher risk.

Deck connections 

The deck joists need to be supported by a stringer attached to the building. The cladding needs to be finished (including coating) prior to stringer installation. The stringer should be packed out 12 mm from the face of the cladding to allow water to drain between the two. 

The stringer is connected to the building structure through the wall cladding with bolts. The bolt penetrations need to be sealed with an external neoprene washer that compacts when the bolts are tightened, forming a watertight seal against the exterior face of the cladding.

Where a stringer is connected to a compressible cladding or over a cavity-fixed cladding, blocks must be installed behind the cladding at each bolt location to ensure the cladding is not crushed when the bolts are tightened. Some compressible claddings (such as EIFS) have specific details for these locations.

Note that E2/AS1 does not permit the connecting of stringers to stucco, EIFS or profiled metal claddings.

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Timber-slatted decks – cantilevered

Timber-slatted decks can also be formed by cantilevering joists out from the floor structure of the building. These types of decks have a higher weathertightness risk as each joist that penetrates the exterior cladding creates a potential water leakage path.

Cantilevered decks must be set down a minimum of 50 mm from the interior floor level.

Each joist penetration through the cladding needs to incorporate a saddle flashing that overflashes the joist and backflashes the cladding, to ensure that water does not leak through the penetrations.

The cantilevered joists need to be treated to H3.2. (Refer to NZS 3602 for timber treatment requirements as modified by B2/AS1.)

Cantilevered timber-slatted decks have a very high weathertightness risk.

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Timber-slatted decks – disconnected

It is possible to build a timber-slatted deck that is completely independent of the building, but this may involve a support structure incorporating more posts and beams.

The advantage of these types of decks is that they do not require any connection to the building and therefore there are no penetrations through the cladding.

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Waterproof decks

Waterproof decks incorporate a waterproof membrane that is installed over a timber substrate. They have a high risk of weathertightness failure as they are really just a low pitch roof surface that can collect substantial amounts of water that needs to be drained.

Many waterproof decks are installed over habitable spaces, which means that any leak through them may result in water entering directly into the building.

They also incorporate a range of details, such as internal gutters, drainage scuppers and solid handrails, that can be difficult to design and build.

Open-ended waterproof decks are lower risk than fully enclosed decks as they can be drained off the open end into an external gutter and generally do not require complex drainage solutions.

The following criteria apply to waterproof deck design and construction. You can also find information in the book BRANZ Good Practice Guide: Waterproof decks and balconies.

Deck size

The smaller the deck area, the less water it will collect and have to drain. The size of the deck will dictate the extent of drainage that is required and the slope of the deck. Larger decks need more drainage capacity and greater slope to ensure that water is drained effectively.

E2/AS1 calls for waterproof decks to be a maximum area of 40 m2.

Deck threshold

There must be a threshold with a minimum height of 100 mm above the highest point of the waterproof deck. Where a level access is required, this should be formed by incorporating a removable raft as a level trafficable surface over the waterproofed deck surface.

A recessed drain can also be used across the intersection of the deck to the building to allow the deck surface to be at the floor level of the building. This needs to be accurately designed with sufficient capacity, fall, and drainage and overflow outlets.

Deck slope

Waterproof decks must be installed to a minimum slope of 1:40 (1.5°) away from the face of the building that they are accessed from. Increasing the fall will improve deck drainage but also may make the deck feel steep. This can be overcome by incorporating a timber or tile/paver raft as a level trafficable surface.

Deck substrate

Membrane deck surfaces require a solid substrate as support. Plywood is the most common type of substrate used and must meet the following guidelines:

  • Plywood substrate and timber deck joists must have a maximum moisture content of 20%.
  • CD grade ply with minimum CCA treatment and minimum 17 mm thick must be used.
  • Timber joists must have minimum H1.2 treatment (H3.2 if joists are cantilvered). Refer to NZS 3602 for timber treatment requirements.
  • Sheets must be laid with staggered joints, and sheet edges need to be supported.
  • The maximum joist spacing must be 400 mm.
  • The ply must be laid with the face grain at right angles to the main support.
  • The ply must be fixed to joists with 10 g x 50 mm stainless steel screws at 150 mm centres along sheet edges and 200 mm centres elsewhere and with 3 mm gaps between sheets.
  • External edges must have a 45° chamfered radius.

Under E2/AS1, the void formed by the deck joists beneath the ply substrate may need to be ventilated, and a minimum gap of 20 mm between any insulation and the underside of the substrate needs to be maintained. Some proprietary systems require specific ventilation or allow the insulation to fit up to the underside of the substrate.

Plywood manufacturers may also have other installation criteria that the designer will need to consider and make the builder aware of.

Deck membrane

There is a wide variety of deck waterproofing membrane compositions. Common membrane types are:

  • sheet – various sheet sizes and thicknesses manufactured from synthetic rubber (EDPM and butyl), PVC, asphalt and modified bitumen (E2/AS1 only covers EPDM and butyl rubber membranes)
  • liquid-applied – acrylic and polyurethane.

Some membranes have a trafficable surface while others need to be protected with a removable trafficable surface (such as a timber raft or tiles on adjustable proprietary supports). Adhering a trafficable surface directly to the membrane (such as clay tiles) should be avoided as this makes it difficult to identify the source of any water leaks that may occur and also makes it more difficult to carry out repairs. It is also outside the scope of E2/AS1.

At the edges of a deck, membranes need to be adequately turned up behind wall or balustrade claddings and turned down over the deck edge or into internal gutters and drainage scuppers. The membrane should:

  • turn up behind any cladding a minimum of 150 mm or to the manufacturer’s specification
  • turn down a minimum of 70 mm at open deck edges, and the turn-down should be packed out from the cladding to ensure that a drip edge is formed.

Each type of membrane has specific performance criteria and installation requirements. Designers need to select and detail a membrane that is most suited to the deck design and complies with the manufacturer’s requirements.

All waterproof deck membranes must be installed by manufacturer-approved specialist contractors.

Deck drainage

Membrane decks need to incorporate adequate drainage. Depending on the design of the waterproof deck, this can be formed by:

  • external gutters (to open-ended decks)
  • internal gutters draining into integrated outlet pipes
  • internal gutters draining through scuppers in solid balustrades into rainwater heads.

Internal gutters must:

  • have a minimum fall of 1:100
  • have a minimum width of 300 mm
  • be located to collect water draining across the deck
  • be capable of managing the level of discharge for the deck area
  • have no membrane laps within the gutter.

Every internal gutter must have at least one outlet and one overflow. Where the gutter discharges into an outlet pipe, it must be a minimum diameter of 75 mm and sized to suit the overall gutter capacity.

Outlets need to be protected from blockage.

Where the gutter discharges through a scupper into a rainwater head, the scupper must be a minimum width of 200 mm with 75 mm minimum depth. The rainwater head must be 100 mm wider than the scupper.

Overflows need to be incorporated to discharge any water that the gutter and drainage system can’t handle in heavy rainfall situations. These need to be 1.5 times the cross-sectional area of the discharge pipe and installed to drain water from the deck to an outlet 50 mm clear of the exterior cladding. Overflows need to be located so that the top of the overflow is a minimum 50 mm below the building floor level.

Open deck balustrades

Supports for open deck balustrades must not be fixed through the horizontal surface of a membrane deck unless an approved waterproof fixing assembly has been specified. In all other cases, balustrade supports need to be fixed through the exterior face of the cladding and must incorporate rubber washer seals that seal off any penetration through the cladding, while ensuring that the cladding can drain freely.

Any connections between the ends of an open balustrade and a wall cladding need to be sealed in the same way. Any penetration through the cladding must be treated as a potential water leakage path.

Enclosed deck balustrades

The top of a solid balustrade is, in effect, a small roof that has the potential to capture water. This means that any junction within the balustrade cap and between the balustrade top and adjoining walls is critical, as poor junctions can result in water leaks.

Solid enclosed balustrades must be built out of minimum H1.2 treated timber framing and must be completely covered with wall underlay. (Refer to NZS 3602 for timber treatment requirements.)

Under E2/AS1, the cladding must be installed over a cavity and the enclosed deck wall offset from an adjacent wall in the same plane.

The top must be protected with a metal cap flashing that is set to a cross fall of 5° with an overlap to the balustrade cladding of 50 mm minimum in low wind zones and 70 mm in other wind zones.

As an alternative to a metal cap flashing, plaster finishes can incorporate a texture-finished coated plaster installed over a membrane underflashing, but in these situations, the cross fall must be increased to 10°.

A saddle flashing must be incorporated to overflash the cap flashing and underflash the wall cladding where the balustrade top intersects with adjoining full-height walls.

Where an open handrail is incorporated with a solid balustrade, the handrail connections must not be fixed through the top surface of the balustrade – they must be fixed through the vertical face of the cladding. As for open balustrade connections, the penetrations in the cladding for these handrail connections need to be sealed with rubber washer seals.

Waterproof decks and solid balustrades have a very high weathertightness risk – builders need to be familiar with all the associated construction details and ensure that these are followed. If the designer has not provided sufficient documentation, builders should ask for more.

If builders have any concerns about the details for a waterproof deck, they should raise these with the designer.

The deck membrane should be installed by a manufacturer-approved specialist installer, and the substrate should be of suitable rigidity and moisture content at the time of installation.

Also, the membrane should be protected from damage throughout the construction process. Even small punctures (such as those created by the flat head of a nail) can result in significant water leaks that are often hard to locate for repair.

Updated: 30 June 2015