Begin with two WRB layers before manufactured stone veneer installation
Manufactured stone veneer installation over wood or metal framing requires two layers of water-resistive barrier, or WRB, as independent layers lapped in a shingle fashion. The first layer is known as the primary layer and acts as the drainage plane. This layer does the heavy lifting, manages water and air infiltration, and is integrated with all flashings prior to the installation of the second layer of WRB.
It's a good time to ensure that all penetrations, transitions, and openings have been flashed and properly integrated with the primary WRB per code and manufacturer requirements.
One WRB layer is sacrificial
The second layer is known as the sacrificial layer. Its purpose is to protect the primary layer from mortar contact. Direct mortar contact with many WRB materials negatively affects the water holdout capabilities of the material. These two layers create a small, continuous gap between them from top to bottom for incidental water to drain down and out.
Building papers referenced in the building codes set the base performance requirements for all WRBs. The building code sets the minimum bar at ASTM D226, Type 1. These materials are lapped two inches at horizontal and six inches at vertical overlaps. Fasten with WRB manufacturer-approved fasteners, typically roofing nails, staples, cap nails, or cap staples.
Some lath manufacturers offer lath with this sacrificial layer attached, aka paper-backed lath. House wrap or building wrap products will bear approval to ASTM E2556 or ICC ESAC 38 for equivalency to the minimum code requirement of ASTM D226, Type 1. Wrap manufacturers will have tested lapping and fastener requirements, but in general, ensure six-inch horizontal laps and 12-inch end laps. Fasten with cap nails or cap staples.
Install WRB per manufacturer instructions
There will also be seam tape specified as compatible with the house wrap or building wrap. Inspect primary WRB to ensure all penetrations are flashed properly. All openings, including windows and doors, should have head flashings and a drip screen. Even foam insulation can provide equivalent water-resistive barrier properties when seams are taped. The specific details for foam as WRB must be obtained from that foam manufacturer. Take note the vertical seams of these materials should be offset.
Fairly new are the liquid-applied WRBs, which will function as a primary WRB air barrier. Even newer is weight-bearing WRB, onto which a proprietary mortar bonds veneer directly to the WRB. These are proprietary systems, and their instructions must be followed.
You may need a rainscreen gap behind manufactured stone veneer
Your local building code may require a rain screen gap of 1/8 inch to 3/8 inch. The drainage plane gap can be increased with a drainage mat or furring strips. Framed applications also require the use of a foundation weep screed. Both layers of the water-resistive barrier must overlap the weep screed flange.
CMU, concrete masonry units, might be a suitable substrate but do not require a water-resistive barrier or weep screed. Poured concrete is also suitable but often requires cleaning to remove form release agents and sometimes adding texture.
If you have concerns about moisture migration or bonding to the masonry surface, you may add lath and WRB.
In the next installment of this series, we’ll discuss rain screen and lath details.
