Designing an Adhered Masonry Veneer – Part Three
Consider the requirements associated with continuous rigid insulation, extruded wire lath and the scratch and top coat options for the materials and installation methods available for the construction of your wall.
By Steven Fechino
During the last few months, we have looked the design of an adhered masonry veneer wall that could be found in many of the residential and commercial projects around the country. We began, in part 1 of this series, by covering several of the components of the structural substrate. Focusing on a stud wall construction, Oriented Strand Board (OSB), plywood, gypsum and bituminous sheathing, we discussed the performance of each product and how it is related to the wall.
In part 2, we discussed the weather resistant barriers (WRB) commonly found on residential and light commercial construction projects. The discussion gave insight to how you can properly select the correct material for the application that you are building.
In part 3, we will look at the requirements that are associated with continuous rigid insulation, extruded wire lath and the scratch and top coat options for the materials and installation methods from which you can choose for the construction of your wall.
Continuous rigid insulation
Continuous insulation is applied continuously around the face of the wall and outside the WRB, which eliminates thermal transmission routes, 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 between the structural substrate wall and adhered masonry veneer that is ½ inch thick or less may be installed as a non-engineered system. Rigid insulation ½ inch and greater in thickness will require an engineered anchoring system.
Designed and tested specialty washers used for anchoring metal lath over rigid insulation are commonly available from distributors who offer rigid insulation. Distributors typically can offer charts showing which anchors and washers to use to secure the chosen veneer over a variety of wall details to simplify purchasing.
The allowable non-corrosive or corrosion-resistant anchors used for the following anchoring of lath or lath systems are:
Wood frame: Staples, roofing nails and screws can all be used. The minimum design for the embedment is ¾ inch, however a minimum of a 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.
Lath is offered in three different weights: 1.75 pounds per square yard for interior applications, and 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 metal lath installation is well worth the effort. It is 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 into one, eliminating the need to perform labor on similar steps repeatedly.
One product offered as a lath system joins 2.5 pounds per square yard metal lath with a ¼-inch-thick, three-dimensional polyester mesh. Installing these two products at the same time reduces the WRB penetrations by 50 percent, since it is an all in one application. Another lath system is a lath-and-paper combination, 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. This allows correct overlap each time you place the product.
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 reinforcement and perform positively on your project.
Field-mixed mortar mix design recommendations:
- 1 part Portland cement (ASTM C150)
- 1 part lime (ASTM C207)
- 4.5 parts sand (ASTM C144)
- Potable water
- 1 part Type S masonry cement (ASTM C91)
- 2.25 parts sand (ASTM C144)
- Potable water
- 1 part Type N masonry cement (ASTM C91)
- 2.25 parts sand (ASTM C144)
- Potable water
Bonding agents help the mortar bond to the lath and veneer, and may make the mortar easier to manipulate. 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 addition of water ensure the installer that the correct cement-to-aggregate ratios are retained, and that any admixtures are properly dosed.
Pre-blended, 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.