Waterproofing under or over the bathroom bedding? (Screed)

July 29, 2019

By: George Abou Haidar

Easily over fifty percent of our bathroom renovations were commissioned due to failed or incorrect waterproofing systems.  It is therefore no surprise that clients want to be assured that water will never strike from their bathroom again.

As such, I have had multiple conversations about water proofing systems, and often such queries revolve around the waterproofing application above or below the sand and cement bedding ,or “screed”.  In short, which is better? Are they both better? Is one method “wrong”, or illegal, as I’ve heard declared on more than occasion?  This post hopes to provide some insight in relation to those questions.

Questions, questions:

Are both methods legal?

Yes. The Australian Standard 3740-2010 (with Amendment 1 2012) addresses both methods, for both enclosed and unenclosed showers (Figure 3.5) Note that an important part of waterproofing is the creation of relevant “water stops”, a vertical extension of waterproofing that prevents water from leaving a certain area.  The water stops are the vital factor in both systems. The emphasis in both methods is on the water stops.

Are there advantages and disadvantages to either method?

Yes, here are some:

Advantages of waterproofing under bedding:

• Less chance of tiler damaging membrane
• Is applied directly to a substrate, protecting it directly.
• Powerful polyurethanes can be used without worrying about lack of tile adhesion

Disadvantages of waterproofing under bedding:

• If the water stops fail, the entire screed will be constantly wet
• Wet screeds are notorious for producing Efflorescence, whereby a crystalline deposit of salts forms on a surface. These salts generally travel up to grout lines and cause the grout to look terrible.

Advantages of waterproofing over bedding:

• Severe efflorescence is generally not an issue
• If done correctly, the water will never wet the bedding underneath
• Ideal for uneven substrates, since the bedding ends up smoothing out the flooring

Disadvantages of waterproofing over bedding:

• There is more of a chance that a careless tiler may damage the membrane since the tiling process occurs on it. One patch of damage can render the whole system useless
• Pooling becomes more of an issue, since the membrane does not absorb water. Pooled water will tend to travel under the tiles and find its way out and around water stops if it can.

Why not have both?

It’s becoming increasingly popular to waterproof under and over the bedding. I believe this is a must for bathrooms above ground floor.  However, the issues then relate to time and money. Waterproofing under and over the bedding will cost both time and money, and some clients simply cannot wait or pay for such an endeavour

In conclusion, there are pros and cons to both systems of waterproofing. It is key to remember that stops are vital either way, and that it’s still possible to have a working system one way or the other.

Reference: AS 3740-2010 (with Amendment 1 2012) Waterproofing of Domestic Wet Areas