Andrew Golle clears up the confusion surrounding screeds and explains the difference between waterproofing above or below.

Providing surface and sub-surface drainage by implementing falls, to an outfall, is an integral part of waterproofing design. AS 3740-2010 Waterproofing of Domestic Wet Areas and AS 4654.2-2012 Waterproofing Membranes for External Above Ground Use mandate minimum fall requirements for wet areas to manage surface drainage and reduce the effects of wind driven rain. Bonded and un-bonded mortar screeds are often used in conjunction with an appropriate waterproofing membrane system to manage surface drainage, or act as a ballasted drainage cell.

The simple screed is often misunderstood though, and when partnered with the wrong waterproofing system, results in systemic failure and breach of BCA performance provisions.

One such mismarriage is currently occurring in NSW. In order to combat mechanical abrasion and re-emulsification of water based membranes beneath un-bonded screeds, someone has come up with a cunning plan to install solvent based membranes beneath the screed and dress incompatible water based membranes onto the solvent, to achieve compatibility with wall coverings. Some are even trying to join the two flexible membranes with an inflexible hydro epoxy.

We are yet to encounter a membrane manufacturer who will support such a practice, and some practitioners are actually using a cocktail of products from different manufacturers. It is becoming a generally accepted practice in commercial applications and doomed to fail.

Do we waterproof above or below the screed?

AS 3740-2010 3.2 states: Where a tile bed or screed is used, the waterproof membrane shall be installed above or below the tile bed or screed.

Both are correct, and have their various uses. Membrane application beneath the screed is common, in New South Wales, Queensland and Victoria as a transition from formed external metal trays and flashings, where the only option is to sit the tray under the bed. Other states such as Tasmania and South Australia will apply waterproofing membranes above their screeds. What are the pitfalls and how do we decide which system to adopt?

We first need to assess the area to be waterproofed and the membrane system used, in order to specify the appropriate design. Quite often this is approached with only half a thought to the serviceability of the waterproofing system, which often results in membrane failure or adverse collateral defects such as efflorescence and bacterial growth in the bed. Service factors include: substrate structure and movement; bond breakers and release zones; mechanical abrasion of membranes; bed saturation and substrate drainage, and compatibility with other materials.

Below screed application is applicable where extensive substrate movement or construction joints are present, requiring an unbonded sheet membrane such as PVC or torched bitumen. An unbonded screed is placed over the membrane as a ballast and porous drainage cell. Below screed application may not be appropriate in a disabled open shower bathroom, where the screed becomes coagulated with organic matter resulting in bacterial growths and restricted screed drainage.

Above screed membrane application is appropriate where a bonded membrane is applied onto a bonded screed with mandated bond breakers and release zones; maximising control of falls and movement accommodation of the membrane. However this is not appropriate where a solvent based polyurethane membrane is applied, not being compatible with tile adhesives and other finishes.

These membranes should only be installed beneath an unbonded screed/floating bed. They are flexible, abrasive resistant, do not re-emulsify but you can’t easily stick a tile to them without solvent compatible adhesive.

Bonded and unbonded screeds

An unbonded screed/ or floating bed is best used over membranes that cannot be bonded to, such as sheet membranes or solvent based membranes. These screeds are often relied upon as drainage cells. Membrane materials need to be abrasive resistant and accommodate building movement. Quite often this is misunderstood and unbonded screeds are placed over membranes that are applied too thin, resulting in mechanical abrasion.

This mechanical abrasion is akin to trying to wear through the membrane with a brick. Any thin spots, weak points or high spots can wear through; forming ruptures and blisters. Water based membranes can also re-emulsify where the screed is not drained.

AS 3740-2010 A3.5.1 requires that where a membrane is applied under the tile bed, a drainage system be provided within the tile bed, to drain the reservoir of moisture within the bed. This requirement refers to the rebating of drainage control flanges to accept membrane drainage at the lowest level, and providing falls at membrane level. This can be achieved with proprietary levelling systems, prior to membrane application.

AS 3958.1-2007 A2.3(b) Guide to the Installation of Ceramic Tiles nominates the minimum unbonded screed thickness at 40mm. This is often ignored and screeds often crack around floor wastes where minimum thicknesses are not achieved.

Re-enforcement wire is commonly incorporated within the screed to maintain its integrity. This is combined with slip layers or separation materials, such as corflute sheets between the screed and membrane. Failure to install these components correctly results in mechanical abrasion of the membrane and punctures where side cover and bottom cover of the wire is not provided.

Bonded screeds are utilised where screed thickness cannot be built up or to minimise dead loads on structure. AS 3958.1-2007 A2.3(a) nominates the minimum bonded screed thickness of 15mm at any isolated point. Also where bonded screeds are installed as part of a flexible waterproofing system in combination with a flexible bonding agents.

Bonded screeds are preferable where the membrane is applied above the screed to minimise substrate movement and screed curl beneath the membrane; and below the screed, as it minimises mechanical abrasion that the membrane would be subjected to under a floating bed.

Selection of bonding agents becomes critical and must be matched to the substrate and screed materials. A cement slurry with PVA additive is not appropriate in wet area construction. AS 3958.1-2007 2.8 (a) states that PVA bonding agents should not be used where they are affected by moisture.

Selecting a bonding agent requires assessment of the bonding agent’s properties as being suitable for the intended use. Water resistant acrylics, SBR (styrene butadiene rubber), latex and water based polyurethanes will not re-emulsify and are far more stable than PVA based liquids. A cement slurry with a selected polymer will ensure compatibility with the membrane substrate and also provide a cement matrix with the topping screed.

A cement based tile adhesive with C2S1 classification provides an excellent bonding agent with polymer adhesive, cement matrix and up to 5mm flexural resistance. This is generally applied with a notched trowel and the bedding material placed onto the glue while wet, like laying a big mortar tile.

Movement accommodation at bond breaker junctions is severely compromised when waterproofing is applied beneath a screed. The compacted screed restricts membrane movement when releasing from the bond breaker, as there is no active release zone. This can be addressed by installing compressible foam to the perimeters, prior to placing the screed. Alternatively, foam backing rod can be installed at corners, on top of the membrane, prior to placing the bed. This provides a compressible active release zone, not constricted by compacted concrete or mortar.

AS 4654.2-2012 2.16 requires that a mandatory inspection be conducted of the membrane system. This should be conducted by the waterproofer the day after application, as one cannot see pinholes, blisters or splits form until the membrane dries. The waterproofer can therefore install the foam backing rod during the completion inspection, and document this on waterproofing application certificates, as the release zone for bond breakers.

Application of membranes to the top of a bonded screed is preferable where the membrane requires functional falls, where movement accommodation at bond breakers needs to be maximised and where the bed needs to remain dry to reduce live loads.

Isolating the screed from saturation also limits bacterial contamination of the bed and helps in the control of efflorescence defects.

Screeds should be steel trowelled and not sponged, as removal of the top cement layer reduces the overall membrane bond. Use of a cementitious negative pressure membrane may be applied to a green screed reducing the standard curing times associated with top of bed membrane applications.

Fast drying proprietary screeds may also be used to reduce construction times. Movement accommodation at floor wall joints can be maximised by forming the Ellog Gutter into the green screed at perimeters. This gutter provides a greater surface area for membrane release from bond breakers and a functional release zone.

The gutter also avoids filleting out of the membrane, leaving a tidy, concave junction where floor finishes can be applied without stressing or damaging the membrane. The gutter should be filled with a compressible sealant, between floor finishes and the wall to promote an active release zone and reduce free standing water and mould concentrations. A good bit of gear, that Ellog Gutter.

To summarise

Render, bed, screeds are an essential part of waterproofing systems. Misused and mismatched with the wrong membrane can result in systemic failure. Unbonded screeds are best employed as a floating bed, over unbonded membranes, where bonding to solvent or sheet membranes is not possible, or where structural movement dictates unbonded systems with isolation layers.

Bonded screeds are best employed with liquid applied bonded membranes, compatible with water based adhesives and bonded finishes. Bonded screeds function best with Class III flexible membranes requiring 12mm bond breaker and release zones. Top of screed membrane application should always be onto a bonded screed.

Those of you who are trying to match up solvent and water based membranes, in combination, please stop it. Go and seek technical specifications from your manufacturer, follow compatibility chains and document on your waterproofing application certificates.

Andrew is a registered builder in Queensland, N.S.W and Tasmania, a Cert III Construction Waterproofer and Cert III Wall and Floor Tiler and Forensic Tiling Defect Investigator. His company, Armont Rectification Builders, specialise in solving waterproofing and tiling defects throughout the country, and are the owners of the Tile Reglue Injection Method. T.R.I.M is a specialised service to positively refix loose floor and wall tiles without removal.