Part 3: Surface Preparation
WEATHER RESISTIVE BARRIER (WRB)
Wood or Metal Studs
Prepare 2 layers of a Weather Resistive Barrier (WRB). WRB should be a #15 felt conforming with ASTM D226 or a Grade D 60 minute building paper (Super JumboTex is a popular Grade D 60 minute building paper). The first layer of WRB may be either traditional house wrap or a WRB meeting the above requirements. Various liquid WRB are also available. Consult the manufacturer of the WRB to ensure that it is suitable for use with natural thin stone veneer and to obtain installation instructions.
While installation over sheathing is preferred, installation over open studs is possible. When installing over OSB, it is important to ensure that the OSB sheets have been adhered with an appropriate gap between the pieces. This allows for expansion should the OSB swell. If OSB swells, it may cause cracking on the wall. For interior applications, a WRB is typically not required.
Drainage mat (Rainscreen) products are becoming increasingly popular. These are typically installed on top of the WRB to allow a cavity for water to drain. Consult applicable building codes to determine if a drainage mat is required in your area. Certain drainage mat products may be considered a WRB. Consult the manufacturer of the drainage mat product to determine if it qualifies as a WRB and if any specific installation procedures should be followed.
Tape all joints with a tape approved by the cement board manufacturer. Apply a commercially available WRB over top of the cement board. If using a liquid WRB, consult the manufacturer of the WRB to ensure that it is suitable for use with natural thin stone veneer. For interior applications, a WRB is typically not required.
Concrete Block (CMU) or Poured Concrete Walls
Over clean CMU block or poured concrete walls (CMU block or concrete that has not been sealed, painted, or otherwise coated), thin stone veneer may be installed without a WRB. While not required, liquid applied WRBs are often advantageous to prevent moisture from wicking through the wall. Natural stone veneers and manufactured concrete products are often porous, and without a liquid WRB, it may be possible for a small amount of moisture to wick through the wall in extreme conditions. Liquid applied WRBs provide a good protection again this wicking.
Over painted, sealed, or otherwise coated CMU block or concrete, liquid applied WRBs are not recommended, as they may not adhere well to the coating. If a WRB is desired, a single layer of WRB may be used. WRB should be a #15 felt conforming with ASTM D226 or a Grade D 60 minute building paper (Super JumboTex is a popular Grade D 60 minute building paper).
Certain products may qualify as WRBs that are outside the scope of this installation guide. Consult with the manufacturer of these items to ensure that they qualify as a WRB when used for the installation of natural thin stone veneer.
While drainage mat (Rainscreen) products are not currently required, they have strong advantages in mitigating moisture.
Weep screed, drip cap, and casing bead should be used as required by applicable building codes. Flashing should be installed prior to the WRB to ensure that the WRB overlaps the flange of the flashing element. All window, doors, and other intrusions should be properly flashed as per the requirements of window or door manufacturer. The MVMA installation guide has many flashing details that can be applied to the installation of natural stone.
Aluminum flashing is not recommended. Use either galvanized steel or plastic.
Installing the WRB
Refer to manufacturer’s instructions for the specific WRB being used. WRB should always be installed so that a plane is formed between the 2 layers.
WRB should be installed horizontally starting at the bottom of the wall, overlapping each layer 2" over the previous layer. The bottom layer of WRB should be overlapped over weep screed or present flashing. Vertical seams should be overlapped at least 6". Staple or nail WRB to the wall, fastening to studs where possible. Spacing for fasteners should be 6" to 12" vertically. Avoid unnecessary fasteners as they increase the potential for water penetration.
It is extremely important to properly overlap the layers of WRB and to install the WRB in the proper sequence with the other flashing materials present on the project.
When using liquid applied WRBs, follow the manufacturer’s recommendations to ensure proper installation.
Wood or Metal Studs
2.5 or 3.4 lbs./sq. yd. self-furring wire lath conforming to ASTM C847 should be used over both wood and metal studs. For open studs, 3.4 lbs./sq. yd. self-furring wire lath is recommended to provide additional rigidity and support.
For installation over wood framing, use 1.5" roofing nails spaced no more than 7" vertically on studs. Fasteners must penetrate stud no less than 0.75". For horizontal framing applications use 1.5" roofing nails driven flush to the plaster base. Wire staples may be used, provided they are 1.5" driven flush and contain a crown of not less than 0.75". They must penetrate studs no less than 0.75". Screws may be used for either vertical or horizontal framing applications provided they penetrate studs no less than 0.625".
For installation over metal studs, fasten screws no more than 7" vertically on studs. Use screws that will project no less than 0.375" into the metal studs.
No wire lath is required with cement board applications, provided proper mortars are used for bonding the stone veneer to the cement board.
Concrete Block (CMU) or Poured Concrete Walls
Over clean CMU block or concrete (CMU block or concrete that has not been sealed, painted, or otherwise coated), no wire lath is required. Over sealed, painted, or otherwise coated CMU block or concrete, 2.5 or 3.4 lbs./sq. yd. self-furring wire lath conforming to ASTM C847 should be used.
When attaching wire lath to CMU block or poured concrete walls, a power or powder actuated fastener is recommended at the corners as well as the long dimensional midpoint of the wire lath. Additional fasteners (either power or powder actuated or stub nails) should be fastened in rows no more than 16" on center and spaced no more than 7" vertically. They should be a minimum of 0.75" with a head size of at least 0.375".
Wire Lath should be installed horizontally with the cups facing up. All joints should overlap a minimum of 1" horizontally and vertically. Wire Lath should always be attached to studs and never to sheathing. Refer to ASTM C1063 for additional information. There may be other commercially available products that may be used in place of 2.5 or 3.4 lbs./sq. yd. self-furring wire lath. Consult the manufacturer of these products to ensure that they are appropriate for natural thin stone veneer installation. Ensure that the product used allows for the scratch coat to be adequately thick.
Ensure that proper corrosion resistant fasteners are being used for the type of lath being used. Like coatings should be used on the fasteners and the lath. For example, galvanized fasteners should be used with galvanized wire lath. Consult the manufacturer of the lath product to ensure that compatible products are used.
If installing over continuous insulation (CI) greater than 0.5" thick, consult the manufacturer of the insulation or an engineer to ensure that proper fasteners are used.
Over Wire Lath
A scratch coat with a thickness of approximately 0.5" should be applied to the wire lath. It is extremely important that the lath be entirely encapsulated in the mortar mix. The scratch coat may or may not be grooved prior to drying. Whether the scratch coat should be grooved or not depends on the type of bonding mortar being used. See the manufacturer's recommendations to determine whether the mortar being used works best with a smooth or rough scratch coat. A smooth scratch coat is often used for liquid applied WRBs.
Various mortars may be used for the scratch coat, but typically Type N or Type S mortars conforming to ASTM C270 are used. For Type N or Type S mortars, the mortar should be mixed thoroughly adding enough water to achieve the proper consistency. The mortar mix should be firm enough that it can be held at a 45° angle from horizontal without sliding off the trowel. Once properly mixed, the mortar should be troweled onto the wall, fully embedding the lath as it is applied.
If bonding mortar requires a rough scratch coat, a scarifier or otherwise similar tool may be used to roughen the scratch coat before it is dried. The scratch coat should be nearly dry at the time the scarifier is used, but should still be soft enough for the scarifier to create small grooves approximately 0.125" deep. Grooves should be created horizontally and should not expose the wire lath.
Allow the scratch coat to dry (a minimum of 24 hours is recommended) before applying the stone veneer.
Over Cement Board
No scratch coat is required over cement board; however, joints should be taped and sealed according to the manufacturer’s recommended practices. Also, ensure that the bonding mortar being used is compatible with cement board installation. Not all mortars will bond to cement board.
Over Concrete Block (CMU) or Poured Concrete Walls
A scratch coat is not necessary for installation over clean CMU or poured concrete walls. If the surface is not clean and wire lath is required, follow the preparation steps for a scratch coat over wire lath. If wire lath is not being used, It is extremely important that all sealers, release agents, etc. have been removed from the wall before starting the installation of stone veneer. This may be achievable by simply cleaning the wall with a water or sand blasting technique, but if the wall cannot be completely cleaned, the installation of wire lath may be required. It is recommended in all instances to clean the wall before starting the installation even if the wall appears visually clean. A liquid applied WRB is recommended but not required over CMU or poured concrete walls.
When adhering directly to a poured concrete wall without a scratch coat or wire lath, it is extremely important to ensure that the bonding mortar being used is compatible with installation over these substrates. Consult the mortar manufacturer to confirm compatibility.