Tiles are often used as a wear surface and decorative finish for external waterproof decks. They are bonded directly to a synthetic rubber waterproof membrane system but there are other options.
Designs following the Building Code Acceptable Solution E2/AS1 can only have butyl rubber and EPDM (synthetic rubber) waterproofing membranes laid over a plywood substrate. This applies for decks that have:
- a minimum fall of 1°
- at least 100 mm set down at doorways to the waterproof surface
- a maximum area of 40 m2
- no steps level within the deck area
- no integral roof gardens
- no downpipe discharging onto the deck.
In E2/AS1, paragraph 8.5.1 (d) specifically states that ‘the application of directly-applied wearing or decorative surfaces to membranes is not covered in this Acceptable Solution’. The comment following 8.5.1 says that synthetic rubber membranes are subject to damage when walked on and a suitable wearing surface will help reduce this damage.
Removable raised wear surface
Paragraph 184.108.40.206 covering level access says that ‘raised removable surfaces of tiles or timber shall be provided over the underlying watertight enclosed deck surface’. This is so the membrane can be cleaned and regularly inspected for damage that may allow water to penetrate into the substrate and/or framing below. Options given in figure 17A of E2/AS1 include removable timber decking panels or paving slabs supported clear of the deck on proprietary chair supports.
Installing a removable raised wear surface that allows drainage through it, over the full deck area, has the benefit of a level transition from the inside surface to outside, while maintaining the 100 mm minimum set down to the waterproof surface. The slope of the waterproof surface can also be increased while a level walk-on surface at floor level is maintained. A 1° slope is easy to negate as a result of deck deflection or construction inaccuracy, so being able to increase the slope without it being noticeable to the users will ensure effective long-term drainage. In addition, the surface can be lifted for cleaning and inspection and gutters and outlets can be concealed.
Direct adhesion of tiles an Alternative Solution
Where it is proposed to adhere a tile finish directly to a synthetic rubber waterproofing membrane, the construction method and materials must be approved by the building consent authority (BCA) as an Alternative Solution to Clauses B2 Durability and E2 External moisture. Satisfactory evidence must be presented to the BCA that the performance requirements of both these clauses will be met. The design of the deck must also ensure that water is able to effectively drain from the surface over the life of the deck.
The following key design parameters should be considered when adding an adhered tiled finish to a waterproof deck.
- Ensure the substrate is sufficiently rigid – tiles are not tolerant of substrate deflection that can crack tiles and joints. The maximum recommended deflection in the substrate or framing is 1/360th of the span.
- Allow for thermal movement in the tiles, membrane and substrate – generally flexible control joints are required at no more than 4 m centres around the edge of the tiles and where deflection may occur (see Figure 1)
- Ensure the deck has sufficient slope and outlet/overflow capacity to allow effective drainage – E2/AS1 requires 1° minimum fall. However BRANZ believes a minimum of 1.5° fall should be provided to give effective drainage throughout the life of the deck and to cope with any deflection that may occur. Some BCAs are requesting a minimum fall of 2° for waterproof decks (slopes over 1.5° will generally be noticeable to users).
Use correct installation system
Manufacturers of synthetic rubber waterproofing material have combined with an adhesive manufacturer to develop suitable tile installation systems. Systems are designed to provide compatibility between the membrane, adhesives and grouts and allow effective curing of the adhesive, especially when dense impervious tiles are used.
When using these systems it is important to precisely follow the manufacturers’ requirements.
The adhesive used must be specifically formulated for bonding tiles to synthetic rubber – success of this method in the past has been variable. A bond coat may need to be applied to the membrane to allow adequate adhesive bonding when tiling.
The adhesive must be allowed to cure fully, particularly where tiles with low moisture absorption are used over the impermeable roofing membrane. In these cases an adhesive which uses a chemical setting or curing process should prove more successful than one which cures by evaporation.
The weather must be warm enough – the adhesive typically used must be applied in temperatures above 10°C or it will not cure.
It is also important that the membrane is not damaged between its application and the installation of the tiles – any damage must be repaired and allowed to dry/cure first. The membrane should be flood tested before the tiles are applied and the adhesive cured before grouting starts. Tiles shouldn’t be walked on until curing is complete.
Concrete topping slabs
When tiling over a synthetic rubber waterproof membrane, many in the tiling industry consider it safer to lay an unbonded concrete topping slab and adhere the tiles to it (see Figure 2).
The thickness of the topping slab is taken account of during design to ensure minimum deck set downs (100 mm minimum under E2/AS1) and drainage slopes are achieved. A minimum topping slab thickness of 75 mm is recommended for an unbonded topping to prevent curling of the slab during curing.
With this option it is important that the:
- deck structure will take the weight of the concrete topping (the supporting structure is likely to require specific design because of the additional weight)
- concrete topping must be allowed to dry and cure before the waterproofing membrane is applied
- roof must also incorporate sufficient fall to effectively drain the water – fall can be provided in either the substrate or in the topping slab
- tile adhesive must be compatible with the waterproofing system applied to the topping slab.
Articles are correct at the time of publication but may have since become outdated.