Manufactured Stone Veneer: Best Practices for Installing

By Jeff Tew

Along with providing sought-after authenticity and rich beauty, manufactured stone veneer (MSV) offers versatility and light weight that make it suitable for a wide array of architectural applications, from foundation façades and chimneys to statement walls and outdoor landscaping projects. It’s also straightforward to install and can be used over both metal and wood-framed walls, masonry substrates of CMU (concrete masonry units/blocks), and poured and tilt-up concrete.

As with any building material, long-term performance of manufactured stone relies on proper installation. Along with following manufacturer instructions, here are some general guidelines for prepping the façade and protecting the walls from inside out.

Preparing the Envelope

Water management is a critical component of proper stone veneer installation. Achieving appropriate water management requires adherence to three principles:

1.      MSV Cladding: The purpose of this layer is to make our buildings beautiful and interesting while protecting from the elements like UV and debris, and slow the penetration of rainwater.

2.      Primary Weather Resistive Barrier (WRB): The primary WRB is used in conjunction with the cladding to protect the building from moisture penetration and in some cases can be designed to be part of an air barrier system. 

3.      Rainscreen: This is placed between the primary WRB and the MSV cladding and used to create an intentional airspace to help moisture and vapor escape the exterior envelope system. While not required in all climates, the rainscreen provides an additional layer of protection against water infiltration.

Before beginning manufactured stone installation, first ensure that the surface where the stone veneer will be applied is compliant with local building codes and does not exhibit any signs of structural failure or deterioration. With sheathing like OSB, ensure proper gapping (typically 1/8” at seams) to allow for expansion and avoid cracking the stone veneer.

When working with frame construction, make sure to apply two layers of weather resistive barrier and install a weep screed to allow moisture to escape. The primary layer of WRB is responsible for drainage of any incidental water that infiltrates the wall system, so investment in a good primary WRB is critical. The second WRB layer, often referred to as the “sacrificial layer,” protects the primary WRB from mortar adhesion while also establishing a small drainage plain between the two layers.

It’s critical to install a weep screed at the bottom of the MSV cladding system. The weep screed provides an exit point for any incidental water in the MSV system and capillary break to keep water from wicking up into the system. In most instances, the weep screed should be placed with 4” clearance above grade or 2” clearance from a paved surface. Install the weep screed first, so that WRB materials can overlap its 3-1/2” attachment flange in shingle fashion.

If you’re dealing with masonry, as opposed to frame, construction building codes don’t require a WRB or weep screed. If there are concerns with moisture, there is nothing that precludes installation of a WRB in masonry applications. When installing a WRB over masonry, you’ll lose the option to adhere the stone veneer directly to the masonry and will need to install either lath and a scratch coat or a high-quality exterior-grade cement board.

Installing the Lath

Lath represents the skeleton of the adhered stone veneer system, and its installation is critical.

To ensure proper installation, fasten lath to framing so the weight load of the adhered manufactured stone veneer system can be transferred to the framing. That means using appropriate-length fasteners (make sure they are galvanized and corrosion-resistant) to obtain a minimum 3/4” penetration into the framing. Space fasteners no more than 7” on center, and lap lath 1’ at vertical and 1/2” on the horizontal seams.

At both inside and outside corners, wrap lath past the corners to the next framing member approximately 16” beyond the corner.

Among the options for lath are expanded metal lath, woven wire lath, welded wire lath, fiberglass lath, and some types of plastic lath. As you determine what type of lath to use, remember that MSV, mortar, and lath can weigh up to 25 pounds per square foot. Lath with its attachments will carry this load to the framing. Be sure to use a quality, code-approved lath and don’t cut corners.

Applying the Scratch Coat

The scratch coat provides two critical functions: minimizing water and air infiltration (and decreasing corrosion susceptibility) and providing a solid base to adhere the MSV too.

For optimal performance, the scratch coat should be 1/2” to 3/4” thick. Using a self-furred or lath furring fastener, center the lath near the middle of the mortar thickness. Reinforcement at the center of the mortar offers optimal crack resistance. Failure to fur the lath will make it difficult to obtain the necessary scratch coat thickness and will also prevent reinforcement from functioning to its full extent.

Achieving a Solid Bond

To meet building code requirements, adhered stone veneer must achieve a minimum bond strength of 50 psi. To meet these standards, follow these key principles:

• Make sure the stone units are clean. Remove any loose material, dirt, and dust from the back of the MSV unit with a stiff bristle brush.

• Consider weather conditions. Hot and/or dry weather will require you to dampen the back of each manufactured stone unit as well as the scratch coat. This is a crucial step in helping the setting bed mortar achieve proper bond. Meanwhile, cold weather will require you to heat stone veneer materials, and also to tent and heat during installation and curing.

• Choose high-quality mortar that meets (or exceeds) building code and stone manufacturer requirements.

• Carefully mix mortar per the mortar manufacturer instructions and to a consistency that a trowel covered with it can be inverted and the mortar will not slide off easily. Mix only what you can use, as the bond is critical. Don’t take chances here.

Consider the best options for application:

• Method A: Burn or press a thin layer of mortar into the back of a clean and damp MSV before applying an additional amount of setting bed mortar. The total thickness should be approximately  1/2” to 3/4” thick on the back of the stone. Press the MSV unit onto the prepared scratch coat with sufficient movement and pressure to remove an air from behind the unit and achieve squeeze out on all sides of the unit.

• Method B: Trowel mortar onto the wall in an approximate 5-square-foot area 1/2” to 3/4” thick.  Don’t prepare too large an area that it begins to dry or skin over before you apply the MSV unit. Burn or press a thin layer of mortar into the back of a clean and damp MSV unit before applying the unit to the wall. Do so with enough pressure and movement to remove any air from behind the unit and achieve squeeze out on all sides of the unit.

  
  

For a complete list of surfaces onto which you can adhere manufactured stone, plus specific installation guidelines for each, refer to Cultured Stone’s Resources.

The goal is to achieve a uniform layer of mortar on the wall that supplements the mortar of the scratch coat and achieves a minimum 50-psi shear bond strength. You should have no voids within the mortar layers that could capture water, as captured water can freeze and result in de-bonding.

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About the author: Jeff Tew is Director of Technical & Training for Westlake Royal Stone Solutions. To learn more about Cultured Stone, visit www.culturedstone.com.