Mushrooms and mould, rotting timber and soggy carpet – these are all common signs of a leaking shower or waterproofing failure. Andrew Golle, an assessor and trainer in the areas of waterproofing and tiling, shares his expertise with Building Connection.

Shower recesses and bathrooms are subjected to daily stress through water attack, differential movements and chemical reactions. It only takes one mistake, or oversight in construction practices or design to result in a leak, which can then result in costly remediation and red faces for all involved. Most builders do not get the opportunity to re-visit their projects and observe bathrooms subjected to normal and extended use.

At handover, shower screens are shiny, tiles are all the one colour and silicones are mould free. After a relatively short period of time – one to two years – signs and symptoms appear which can prove that some wet areas are not so user friendly, if not failing completely.

The National Construction Code (Building Code of Australia) and AS 3740-2010 Waterproofing of Domestic Wet Areas are the minimum requirements to be followed in wet area design and construction. Too often, building practitioners treat these requirements as the maximum achievable standard of work and even try to avoid some mandatory steps and practices.

The omission of barrier stops at all shower recesses and wet area doorways, including laundries and W/Cs, fail to satisfy these minimum requirements. Best trade practices are achievable and will improve the usability of the amenity. Membrane application to the top of the mortar bed, for example, will reduce mould and bacteria build-up in otherwise saturated screeds.

Wet area construction needs to be viewed holistically. Many functional shower leaks are not due to membrane failure, but from moisture tracking through porous tiles, adhesives and screeds, or unsealed penetrations. All components of wet area construction should be considered in unison, in order to function effectively under normal use.

The following two key principles will assist building practitioners and designers to avoid waterproofing failure and provide serviceable and usable wet area facilities.

The first thing: keep water within the designated wet area

BCA Volume 2 P2.4.1 states: “To protect the structure of the building and maintain the amenity of the occupants, water must be prevented from penetrating: Behind fittings and linings, or into concealed spaces of sanitary facilities, bathrooms, laundries and the like.”

Restricting moisture movement through porous mediums such as mortar beds and tile adhesives should be incorporated in construction practices. Water will travel behind tiles during normal use through capillary action and will free flow where cavities exist. This is why impermeable membranes are applied beneath tiles and other linings.

Barrier stop angles must be incorporated as part of the waterproofing membrane system and finished to the top of the finished floor level (FFL). Cavity slider doors are a particular source of wet area functional failure, where water exits the wet area into the door cavity from beneath the tiles. The barrier stop angle must be returned, across the door cavity to form a continuous connection to the membrane system.

Where a penetration passes through a waterproof or water resistant construction, the penetration shall be waterproof. This includes all mechanical fixings at all stages of construction, such as substrate sheet fixings, shower screen fixings, and toilet pan fixings.

Tap penetrations must be sealed to allow for servicing of taps without affecting the sealant. This requires setting the plumbing combination forward of the wall substrate to allow for sealant application between substrate and combination, and not tap spindle. Mixer bodies and rose penetrations must also be sealed at the substrate interface.

Insert or island baths are another source of frustration for waterproofing practitioners. AS 3740 requires the installation of a barrier stop angle underneath the bath lip. This is not always possible, due to bath type and construction processes. Alternatively, a sealant type 3 (waterproof material to water resistant material) can be incorporated as part of the membrane system. The objective is to provide a barrier at the bath lip, incorporated into the membrane system, to stop water movement under the lip and into the unprotected bath cavity.

AS 3740 Amendment 1, published in 2012, incorporates Table C1. This table is also in BCA 3.8.1.1. The table provides direction as to which components must be either waterproof, or water resistant. These mandatory minimum requirements are an easy guide to follow and should be incorporated in conjunction with the detail supplied in the standard.

The second thing: only use compliant materials

AS 3740 Section 2 lists compliant waterproof and water resistant materials to be used in wet area substrates and linings. Materials not listed in the standard are deemed non-compliant and should not be included in scheduled zones within the wet area. For example, timber window mouldings should not be present within 1800mm from FFL in a shower bath. Table C1 requires the wall area, in a shower, to be water resistant to a minimum height of 1800mm from FFL. Timber is not listed as a compliant substrate or lining material, and as such, it should not be there.

A practical solution is to over sheet the timber sill and jambs with compliant fibre cement wet area sheeting as a substrate, waterproof junctions and horizontal surfaces, and lay compliant water resistant tiles as a lining material. Timber mouldings can then be cut and fitted over the widow head and jambs, above tiling height. The use of plaster setting to sheet joints within shower recesses is non-compliant and provides bonding problems for the waterproofing applicator.

Plaster base coat and topping compounds are not listed as water resistant substrate materials, and should not be within the scheduled protected zones as prescribed by Table C1. Tiled areas within wet area walls do not require plastered finishes to the same level as a painted finish. Setting should not be applied to sheet joints and fixing heads in these areas. In preference, horizontal sheet joints should be supported by back blocking, or noggins.

Nail and screw heads, sheet joints and other penetrations should be sealed, appropriate bond breakers applied and waterproofed as per Table C1. The standard requires that compliant substrate and lining materials must be used in conjunction. A water resistant lining with a non-compliant substrate will not comply with the standard.

A common construction practice, breaching this requirement, is where compliant laminated sheeting is installed onto plasterboard sheeting. In this instance, the substrate sheeting must be water resistant with junctions, penetrations and base of the sheeting waterproofed to the required height above FFL.

The fitting of shower screens to unprotected plaster sheeting and the use of timber skirting in bathrooms, laundries and toilets are other examples of non-compliance. The extent of waterproofing to shower enclosures is measured from the shower rose connection. Unenclosed showers require compliant materials to 1500mm from the rose connection.

The current trend of installing a shower rain head through the ceiling is a popular choice; however the extent of compliant substrate and lining materials still must be measured from the rose connection. The shower rose penetration should be sealed and the shower wall substrate and linings compliant to ceiling height. The walls should be tiled to the top and not finished at 2m height.

Leaking showers and failed waterproofing have long been at the top of recorded building defects lists throughout the country. Following these two key principles and paying attention to detail right through the construction process will help to reduce these costly defects. Be the water, out think the water and you will keep it where it belongs – in the shower.

Andrew is a registered builder in Queensland, New South Wales and Tasmania; a qualified Cert III Construction Waterproofer; and a waterproofing and tiling assessor and trainer.

His company, Armont Rectification Builders, specialises in solving waterproofing defects throughout the country and is the owner of the Tile Reglue Injection Method, a specialised service to positively refix loose floor and wall tiles without removal.