Waterproof decks over habitable spaces are a common area of weathertightness problems. Any failure means that water gets into the habitable spaces below.
As decks carry loads from the occupants, there is also the risk of injury once the supporting framing becomes damaged.
Where deck joists are cantilevered (i.e. the floor joists extend through the building envelope to support the deck), water entry damages both the deck structure and the floor joists. Also, as one end of the deck is unsupported, damaged cantilevered decks can become unsafe quite quickly.
In any remediation design of decks:
- isolate the internal structural members from the external structural members if possible, and provide external support to the deck (a major consequence of leaks where deck joists have been cantilevered out from the structure is deterioration of the mid-floor joists over living spaces)
- provide some protection to the deck – is it possible to roof it over? (see figure below)
- ensure the highest point of the finished deck surface is at least 100 mm below any waterproofed threshold (see figure below)
- protect waterproof membranes from foot traffic
- fall the deck with sufficient slope (designs to E2/AS1 require 1.5° minimum; however, 3° is recommended to ensure fall is achieved to two (preferably) individual outlets to avoid the need for gutters to be incorporated into the deck)
- detail a permeable and removable walk-on surface – the availability of adjustable supports for timber and slab trafficable surface allows a steeper slope to be readily incorporated while maintaining a level walk-on surface for occupants
- consider open balustrades, which will allow excess surface water to flow off the edge to an external spouting, rather than fully enclosing the deck with barrier walls
- provide more than one outlet and overflow where the deck is contained within solid walls
- ensure there are no fixings or penetrations through the deck membrane
- avoid internal outlets if possible – provide two outlets where unavoidable.
As with parapets, balcony walls were commonly detailed with a flat top and finished with the wall cladding. Adding to the risk was the fixing of a handrail through the top of the wall.
Because of the construction that was used, significant deterioration of framing in balcony walls and the deck and wall framing below is relatively common, and significant repair is likely to be needed. Options that may be available include:
- replacing the solid balustrade wall with one that is open on at least one side of the deck
- ensuring any handrails are side mounted, not top mounted, with weatherproof fixings
- flashing the top of the wall and providing a saddle flashing at all balcony wall/full-height wall junctions.
Where allowed under district plan requirements, adding a veranda to an existing building provides benefits by:
- sheltering a large area of wall from the rain
- providing protection to critical junctions around windows and doors
- covering a risky waterproof deck structure.
Issues surrounding the addition of a veranda are:
- the structural connection to the building to ensure uplift is resisted
- support of the open side
- flashing of the roof to wall junction.
Updated: 9 September 2014