The beauty of natural stone makes it a popular upgrade for both interior and exterior surfaces. Waterproofing is essential to protect areas that will be exposed to water and moisture. Photo courtesy Custom Building Products.


It may sound like an oxymoron to “keep wet areas dry,” but it’s no laughing matter when you consider the cost of mold and water damage in terms of dollars, time and effort, and potential harm to your reputation. Proper waterproofing measures are crucial, especially as natural stone continues to be the luxury surface of choice, and is showing up everywhere from kitchens and baths to exterior living areas, and even laundry rooms and finished basements. 

Even the densest stone (granite or quartz) is still porous enough to allow water molecules to pass through. Grout joints also serve as a thoroughfare for water moving from surface to substrate, so the substrate needs to be waterproofed.

Some liquid-applied membranes may be applied in a single layer using a sprayer, saving considerable time and labor. Photo courtesy Custom Building Products.

How Mold Grows

We may not realize it, but mold spores are in the air and often are attached to surfaces. To grow, these spores need food and moisture. The “food” can be anything from soap scum in a shower stall to the gypsum board used in construction. When moisture or humidity levels elevate, along with temperature, and a food source is readily available, mold spores bloom into mold and create potential health issues. Cases of surface mold, such as topical growth on shower stall grout, are easily treated with proper cleaners.

It is interior mold that is most insidious, dangerous and expensive to remedy – mold that grows uninhibited behind or inside the walls, feeds off gypsum, drywall or wood framing exposed to moisture. These problems are expensive and difficult to eradicate. Prevention is relatively easy with proper construction and installation methods. Simply put: keep water away from food sources. This usually means applying a waterproof barrier that will protect vulnerable materials by keeping them dry.

Types of Moisture Barriers - Sheet Membranes and Liquid Membranes

When selecting or specifying a waterproofing membrane, it is important to consider the type of substrate, the installation conditions, and the manufacturer’s recommendations for each product. You must also confirm that the product meets ANSI A118.10 requirements, which are the minimum standards for “load-bearing, bonded waterproof membrane for thin-set ceramic tile and dimensional stone installations.” 

Two types of waterproofing membranes are available: sheet membranes, and cold fluid or liquid applied membranes. Each has its advantages, and may be used according to jobsite requirements and installer preference. Sheet membranes are generally more expensive and labor intensive to install. Sheets must be cut to fit corners and contours, and seams will result. On the plus side, drying time is minimal, if needed at all, so stone installation can begin almost immediately once the membrane is in place.

Liquid applied membranes are available as one-part or two-part systems. One-part systems are much more common and economical, and can be applied in layers with a roller or trowel. Some can also be applied using a sprayer, which is much faster and can eliminate the need for multiple coats because it puts down a thicker, more uniform layer of product. A sprayer application is especially efficient in areas such as showers, where there might be benches, soap dishes and shelves that would take more time to maneuver around with a brush, roller or trowel. 

Two-part systems involve the same type of liquid membrane with an added layer of mesh fabric that must be applied separately between liquid coats. Typically, two-part waterproofing membranes are specified for areas that require additional reinforcement.

The most advanced liquid membranes provide crack suppression to help preserve the integrity of the membrane and thus its waterproofing capabilities during minor surface movement. Some are even listed with IAPMO as pan liner materials, saving further time and labor. 

Liquid membrane products are available in water-based or solvent-based formulations. Water-based membranes are environmentally-friendly and usually fume-free – an important consideration, especially in interior work areas.

Strataflex Membrane being positioned over concrete. Photo courtesy N.A.C. Products Inc.

Where to Waterproof

The Tile Council of America identifies “wet” areas as being soaked, saturated, or regularly and frequently subjected to moisture or liquid. The obvious places in a typical home are showers, tubs, saunas, steam rooms and swimming pools, but there are many others to consider, especially when installing stone. Laundry rooms, bathroom floors, kitchen floors and counters should always be waterproofed because water can seep through both stone and grout. Kitchens and baths over raised foundations or basements and outdoor balconies/decks are especially vulnerable to leaks and mold. These areas should be treated very carefully to ensure proper, long-term protection.

To ensure the best possible protection against water damage and mold in a stone installation, find a product that meets the standards and specifications of the job, and is protected by the manufacturer with a solid warranty program. When applying, follow the manufacturer’s instructions explicitly to ensure the product performs up to stated levels.

If you have any questions about waterproofing products, contact your local installation supplies distributor or manufacturer rep for technical support. You can discuss a situation by phone, email or even in person. Taking the time to use the right waterproofing product correctly will eliminate most mold threats to your installation and help ensure its long-term beauty and integrity.