What lies beneath.....

This question comes up frequently with architects, specifiers and builders asking our Technical Services about specifications for internal and external wet area applications and whether the waterproof membrane should be applied under or over the screed? 

No doubt many professionals in the industry will have their thoughts on this topic, but over the past decade many of our specifier friends have resorted of going with the safety option resulting in having waterproof membranes under and over the screed, just to be sure. Now as a manufacturer of waterproofing systems I'd be lying if this wasn't a good thing for the simple fact it means twice the amount of waterproofing sales however from my involvement in the industry over the past few decades I do believe this safety option of over and under the screed has become a method approached by many due to the way wet areas are viewed and has just as much to do with substrate, screeds and surface finishes as it does with the membranes and the method of waterproofing. 

Having had this discussion recently with a prominent national architecture firm they flatly stated the reason they specify under and over the screed was because they had some difficulty in trusting the workmanship of how membranes were applied (they weren't so kind with their words). In short they figured if the waterproofing was doubled up there was reduced risk of failure. I'm sure some of you reading this would agree if the applicator can't be trusted with the membrane application under the screed, there is every chance the same applies over the screed. Now in previous blogs I've had the discussion about certifying our waterproofing practitioners around the country so not re-visiting that but wanted to explore why we've arrived at this train of thought that under and over the screed is the only way of a sure thing?

I might go back a little and rewind how it once was. In the late 80s and early 90's when wet areas were starting to be waterproofed with membranes instead of just perimeter flashings, the trend was for the membrane to be applied directly to the floor substrate which started to alleviate the leaking issues in wet areas. Quality membranes applied directly to the substrate offer the additional benefit in tiled wet areas by providing underlay properties for the tile bed, protecting the tile bed from substrate movement. Sheet systems with good surface bonding properties for adhesives/screeds will provide crack resistance and anti-fracture properties in these applications, while some high quality liquid membranes also offer this property albeit not to the same extent. 

After wet area waterproofing became part of the building code, the phenomena of wet screed beds under the tile finish was beginning to rear and it was discovered that a membrane performing as intended was creating the accumulation of water sitting on the membrane surface, from water seeping through grout joints. Enter the evolution of the leak control flange which was designed to reduce the problem of water sitting on the membrane. It was a big improvement to what the industry had before, but as many found it was dependent on being correctly applied, that is not sitting proud of the floor substrate but flush with the floor. During this period, we started seeing the method of applying liquid membranes on top of the screed to overcome this issue, which in principle has merit with the objective of preventing water from being absorbed into the bed by guiding water directly into the floor grate with the leak control flange. 

Like all things that evolve we discovered a few issues with this method, some of which included tile beds that sounded drummy due to separation of the screed from the floor and also the failure of some liquid membranes due to them being applied onto screeds while they were damp or wet, affecting the curing process and compromising the waterproof properties from fully developing. Or worse still, these exposed membranes being tampered with or damaged from after trades resulting in the membrane integrity being compromised. This often leads to a debate on the benefits of a concealed membrane system vs exposed membrane over the screed. 

Now while it has been easy to point the finger at the waterproofing industry by isolating the membrane as causing the issues, somewhere along the lines there has been an oversight of how the screed and tile finish have contributed to these issues. Too many screeds formed are as absorbent as a kitchen sponge, however when screeds are formed with a quality waterproof additive, water absorption of a screed can be reduced to < 3% compared to the >20% that is often found when no additive is used or worse still when a PVA additive is used. Please anyone reading this, remind anyone forming a screed to keep PVAs for wood working classes; just because they can bond with cement they cannot stand up to an environment where water will be present. Try immersing a dried PVA film in a glass of water and see the Houdini act before your eyes. When screeds are formed with a quality waterproof additive at the correct mixing ratios the water absorption of the screed becomes significantly lower, so much so they should handle immersed conditions. 

Regardless of whether you believe membranes best go over or under the screed, they are being applied in applications where there will be water and should be able to handle constant wet conditions, acting as a 2nd line of defence to the membrane. And let's not forget about tile grouts, they too can be formed using a waterproof additive reducing the volume of water that is absorbed into the joint. There are numerous lab tests we've carried out with water absorption of tiling systems, but before even referring to the test results it is visible to the naked eye how slow the water uptake is into the tile bed when a water resistant grout is used. On the discussion of caulking and sealants, when using the appropriate grades and applied correctly, i.e. not on the surface but in the movement joint, this also reduces water absorption into the tile bed. 

There is no reason a standard 10-year warranty can't be provided for the use of one of these methods but we should remember waterproofing wet areas is not just about membranes, it is about systems and best working practices by all, including the after trades applying finishes over waterproofing membranes. When we stop the isolated silo view of these critical works and look at it collectively, we realise achieving the desired result isn't too hard to do. 

So do we go under and over the screed? I'm all for this method for the fact that it provides greater value to the client by providing a longer warranty period.