Cantilever Decks & Waterproof Membrane Decks

Article By: Style Plus | December 2016

The biggest issue to address is the provision of adequate slope on the membrane deck. Flat membrane decks do not drain, so proper sloping has to be designed into the deck structure.

-  Jan Antoni Glinkowski • Director • Style Plus

Cantilever Decks

Cantilever Decks

A cantilever deck is one where the support of the deck joists is formed from an extension of the floor joists from inside the house. This type of deck construction does not have posts and beam to support the joists.

The structural integrity of the deck is derived from the size of the joists, the type of wood, their projection from the house and how the floor joists are fixed within your building. Cantilever deck joists are very difficult to seal where they penetrate the cladding. This provides a means for external water to enter the building structure by capillary action through the timber.

Tip: Cantilever decks can present problems if not carefully designed, constructed and maintained.

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Rotted Cantilevered Joists

Typically cantilever decks are constructed during the original construction. It is easier to extend the floor joists out further to create the deck.

The problem with cantilever decks is the risk of the joists rotting over time. If the joists are rotted, and compromising the structural integrity of the deck, then the choices are limited in repairing the deck.

Rot in the cantilevered joists and water damage to the wall assembly are common problems.

Waterproof Membrane Decks

Waterproof Membrane Decks

Waterproof membrane decks incorporate a waterproof membrane that is installed over a timber substrate. In essence they are a low pitched roof surface that will collect water which needs to be drained.

Typically, they are constructed with timber framing, a plywood substrate and waterproof membrane and could be over a habitable living space.

To drain the water the design of the deck will incorporate a range of details such as internal gutters and drain points, and drainage scuppers.

Fully enclosed decks have a high risk factor compared to decks that are open ended. Where water does not effectively drain from a deck surface, the waterproofing will deteriorate faster and the risk of water entry through waterproofing membrane joints or a small area of damage is greater.

Open ended decks can be drained into an external gutter system therefore negating the need for a complex drainage system.

Tiles directly stuck to the membrane were quite common back in the 90’s. This created a problem of the membrane being damaged over time. However the Building Code E2/AS1 now requires as an acceptable solution is a cavity installed between the deck waterproofing membranes and trafficable surfaces to allow for inspection and maintenance of the membrane. This cavity requirement instantly rules out a normal tiling installation such as tiles fixed directly to the membrane surface.

Tip: Good design, detailing and construction practices can effectively address the known trouble spots.

Waterproof Membrane Decks

Typical Issues With Waterproof Membrane Decks

Waterproof membrane decks over an internal living area are classed as high risk and more expensive to repair because of the extent of damage to the deck and wall structure that could occur. Also the multiple trades involved in repairing a waterproofed deck and the internal linings add to the repair cost. The following are a few examples with failing membrane decks:

• Tiles directly fixed to the waterproofing membrane. This prevents maintenance of the membrane.

• Solid balustrades with handrail penetrations with top fixings.

• Substandard flashing detail or no flashing at all.

• A lack of fall to the deck surface causing water to pool on the membrane.

• A lack of up-stand for waterproofing membranes terminating under the joinery. The building Code requires a minimum up-stand of 100mm.

• Substandard waterproofing membranes and substandard application of the membrane. For example where the membrane has been joined.

• Solid balustrades with flat tops and inadequate waterproofing/flashing.

• Water outlets too small and/or no overflows. Waterproof decks that are fully enclosed should have a minimum of two outlets (if one blocks, water will still be able to drain off the deck) and the same number of overflows.

Tiles & timber decking with waterproof membranes

Previously, it was common practice to fix tiles directly onto membranes. Direct fixing does not allow for inspection or maintenance of the membrane. It also makes early leak detection difficult and repairs costly.

The council’s position is that they will not approve direct fixing of tiles over a timber framed substrate. If tiles are to be laid, they must be on a floating deck or be installed over an approved system such as Jacks. For example a design could be tiles resting on plastic screw jack pedestals or deck jacks.

A timber deck | membrane design must also be maintainable. One way of achieving this is by designing a removable proprietary floating timber system on pads or feet.

• Quickbuild sub-frame system for decking & tiles

• DVK for waterproofing flat-roofs, internal gutters, balconies, terraces and decks.

Tiles & timber desking with waterproof membranes Quickbuild sub-frame system for decking & tiles

Whilst all information is considered to be true and correct at the date of publication, changes in circumstances after the time of publication may impact on the accuracy of the information. The information may change without notice and Style Plus is not in any way liable for the accuracy of any information printed and stored or in any way interpreted and used by a user.



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