Designing an Adhered Masonry Veneer – Part 3

Words: Steven Fechino
Photo: Bartco

Over these few months we have looked at the design of adhered masonry veneer walls that would be found in many residential and commercial projects are the country. We started with covering several components of the structural substrate. We focused on a stud wall construction, Oriented Strand Board (OSB), plywood, bituminous and gypsum sheathing. We also discussed the performance of each product and how they relate to the wall system.

In case you missed part two of this series, we discussed the weather resistant barriers commonly found in light commercial and residential projects. Our discussion gave insight on how you can accurately select the correct material for the application you are building.

In part three of this series, we will look at the requirements associated with expanded wire lath, continuous rigid insulation, and both scratch and top coat options for the materials and installation methods you can choose from for the construction of your wall.

Continuous Rigid Insulation

Continuous insulation is just exactly that, it goes around the face of the wall on the outside of the WRB.  This installation will nearly eliminate any thermal transmission paths, such as those caused by metal studs that transmit heat past the batt insulation inside the walls. Rigid insulation in sheets can qualify as a water barrier and many code requirements will allow the elimination of one WRB layer if it’s used.

Continuous rigid Insulation that is placed between the substrate and the thin masonry veneer that is ½ inch thick or less may be installed as a non-engineered system, while insulation greater than ½ inch (not very common) will require an engineered anchoring system.  Many manufacturers can offer properly designed anchors based off of published standards. This will eliminate the need to hire an engineer to calculate the anchor size for a fee.

Designed and tested specialty washers used for anchoring metal lath over rigid insulation are commonly available at distributors that provide rigid insulation. The allowable non-corrosive or corrosion resistant anchors used for the following anchoring of lath or lath systems are as follows:

Wood frame: staples, roofing nails and screws can all be used.  The minimum design for the embedment is ¾ inch, however a minimum of 1 inch embedment is a good practice to follow.

Steel Stud: The most common anchor is the self-tapping screw or hex head anchor with a neoprene washer attached to the anchor. Aerosmith Fasteners makes a pin that can be installed directly into the steel stud with or without a washer.  Minimum embedment is 3/8 inch, but again a greater depth will increase your odds of greater success.

Concrete or concrete masonry units (CMU):  Powder-actuated fasteners, also known as cap anchors, are allowable and do not need pilot holes, but they are not that commonly used. Concrete masonry screws are a good choice as they can be monitored for embedment, will not blow through the substrate and are typically more economical for the project than cap anchors.

Expanded Wire Lath

Lath is manufactured in three different weights, 1.75 pounds per square yard for interior applications, 2.5 and 3.4 pounds per square yard for exterior applications. Lath is typically placed horizontally, and the vertical laps of the lath are staggered from one layer to the next. Proper placement of the metal lath is stated in the trade as “cups up, smooth down”, meaning when you slide your hand up the lath from the bottom the lath will feel rough, and when you run your hand down from the top of the lath it will be much smoother to the hand.

Fiberglass laths do not necessarily fit into the previous description for placement. Self-furring lath includes dimples that hold the lath roughly ¼ inch out from the substrate to allow for full mortar encapsulation of the base coat. Typical metal lath for use in exterior applications is rated as “G-60 galvanized”, which will hold up under North American climate conditions.

Installation of metal lath is important and taking the steps to ensure a tight-fitting expanded metal lath installation are well worth the effort. It is very important that the first course of wire is installed level and attached properly. Install the weep screed before installing the WRB and metal lath so they can lap over the weep screed nailing strip.

When installing lath against framing, mark stud locations to ensure anchoring of the lath is attached to a structural member. This will speed the installation process as you will be able to place your fasteners (screw and washers, nails or staples) every 6 inches vertically and at every horizontal stud more efficiently and accurately.  If your layout is not square, correct it as much as possible in your initial product placement of the first course or two. Lath can be terminated at an inside corner; however, it must extend 12 inches in both directions on an outside corner.  Cutting metal lath is typically performed with tin snips, sheet metal shears or a 4 ½ inch grinder with a 1/8 inch carborundum blade.

Metal lath systems can incorporate two or more products in one, eliminating the need to perform labor on similar steps over and over.

One product offered as a lath system joins 2.5 pound per square yard metal lath with a ¼ inch thick three-dimensional polyester mesh (LathNet by Mortar Net Solutions). Installing these two products at the same time reduces the weather resistant barrier penetrations by 50 percent since it is an all in one application. Another lath system is a lath and paper combination (Paper-Back-Lath) which temporally protects the stud cavity and sheathing walls during and after construction.

All current lath systems come with an overlap on each sheet which offers the contractor the ability to install quickly with the shingle design, allowing correct overlap each time you place the product.

Scratch Coat

The mortar scratch coat can be made from different combinations of materials that are readily sourced from local suppliers. Mortar can be purchased factory pre-mixed or can be mixed on-site. The following mixes create durable scratch coats that can fully embed into the wire reinforcing and perform positively on your project.

Field Mixed Mortar Mix Design Recommendations:

Mix 1:

• 1-part Portland Cement (ASTM C150)

• 1-part Lime (ASTM C207)

• 4.5 parts Sand (ASTM C144)

• Potable water

Mix 2:

• 1-part Type S Masonry Cement (ASTM C91)

• 2.25 parts Sand (ASTM C144)

• Potable water

Mix 3:

• 1-part Type N Masonry Cement (ASTM C91)

• 2.25 parts Sand (ASTM C144)

• Potable water

Bonding Agents

Bonding agents help the mortar bond to the veneer and may make the mortar easier to work with. They include liquid latex and liquid polymers. Mortars that contain these types of agents benefit by a rest period after initial mixing followed by a remixing of the material to increase board life.

Packaged pre-blended mortars requiring only the mixing of water ensure the installer that the correct cement-to-aggregate ratios are retained and that any admixtures are properly dosed.

Pre-blended, non-modified thin masonry veneer mortars are generally used for the scratch coat, bond coat and jointing material for manufactured stone veneer. In a lath-applied installation over wood or steel studs. Pre-blended polymer modified thin masonry veneer mortars are generally used as a full depth scratch coat over concrete, concrete masonry or lath and to adhere and joint natural thin cut stone as well as lightweight manufactured masonry veneer units in thin masonry veneer systems. These mortars contain performance admixtures that increase bond strength and reduce sag when installing natural thin cut and lightweight manufactured masonry units. Pre-blended polymer modified thin set mortars are used as a bond coat only between the stone veneer unit and hardened scratch coat or properly prepared concrete or masonry substrate.

Spot measure the depth of the base coat, and make sure it is ½ inch or slightly greater during installation. Once the scratch coat has begun to harden, scratch level horizontal and plumb vertical lines in it. Even though the scratches will be covered by the top coat, they can serve as a useful reference for aligning veneer materials and they increase the surface area of the setting bed to help ensure a stronger bond with the veneer.

To adhere the units, apply a thin application of topcoat over the scratch coat and allow it to cure, usually for at least several hours. Then place a ½ inch thick back butter layer of mortar over the entire back surface of each individual unit and press them into the topcoat.  An alternative is to butter (placing mortar on an individual unit) only with a ½ inch thick spread on individual units and bond each unit directly to a scratch coat that is not fully cured. I have personally tried installation with both methods and have found that the overall appearance of the final work, setting time and durability of bonded materials performs better when the topcoat method is utilized.