Designing An Adhered Masonry Veneer

Words: Steven Fechino  
Photo: Banks Photo  

Today we will conclude this series on design of adhered masonry veneers. We have looked the design of an adhered masonry veneer based on the structural substrate as a stud type wall. In this article will wrap up the wall with the discussion on types of veneer units, the use of a weep screed and a quick discussion on sealants for the use in adhered veneer walls. In part one, we began 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 two of this series on wall design for adhered masonry veneers we discussed the weather resistant barriers commonly found on residential and light commercial construction projects. The discussion gave insight on how you can properly select the correct material for the application that you are building. 

In part three 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 that you have to pick from in the construction of your wall. 

Veneer 

Lay out your veneer before you begin, blending materials from several boxes will minimize any color variances in the manufactured veneer 

The total base coat and top coat thickness must be a minimum of 1-inch-thick from the outer surface of the WRB to the back of the veneer unit.  

When setting the actual unit, as it is set in place slightly rotate the unit back and forth with a push to create a secure bond of the unit to the substrate, when your mortar is tempered properly this is actually a very quick process. 

There are two schools of thought on the next topic of placement of units.  I typically like to install my units from the top of the wall down, this will reduce the mortar droppings on finished work, reduce clean up.  Placement of all cut units should be at the base of the wall if possible because most people tend to look at a finished wall from the top down. Others I have had contact with in the industry prefer to set from the bottom up, stating that their joints are tighter when the veneer units can compress into the unit below.  This method uses the weep screed as a support as well to move drainage away from the structure.  Either way, one way is not correct for all applications, chances are you will end up doing it both ways over time. 

When you can install from outside corners into the field of units, you can put random-sized cut units where they will not be visible making it difficult to see how you made bond.   

Never mess with wet mortar droppings that fall on the thin veneer. Let them dry to the touch before removing them with a horse hair brush, just as you would do with any veneer material. This will reduce the amount of clouding that will result and minimize your clean up if even necessary. Do not use masonry cleaners, wire brushes or pressure washers when cleaning cultured stone thin veneer walls, in some cases they can alter the appearance of the veneer. 

Grout your joints once initial set has occurred using a tuck pointer and hawk or a half-filled grout bag, (you will achieve more with less), produces less waste, it is faster, and is easier to control in the tighter application of material between the joints. Always turn scaffold walk boards on edge at night to avoid staining from rain shower splatter. When tuck pointing, follow standard practice guidelines outlined by the Brick Industry Association (BIA).  

Weep Screed, Flashing Details 

Weep Screed is typically galvanized metal or a durable plastic strip that is placed at the base of the wall of a veneer. The galvanized weep screed must be at least 26 gauge or 0.018 inches thick. The plastic weep screed must be a minimum 0.050 inches thick. The weep screed creates a clean edge to transition the base of the veneer from the rest of the structure.   

Both products must extend up the wall a minimum of 3.5 inches, fastened to a stud in frame construction or directly in to the concrete or concrete unit substrate. This product is not mandatory, but really is worth the additional cost for exterior veneer placement. Though it is called a weep screed, it is not a true weep, it only allows the moisture that drains to the base of the veneer to drain out of one of the base holes and away from the wall. When placing the weep screed, it is critical that the WRB covers the nailing strip and extends to the bend where it extends away from the building.  This step keeps any accumulated moisture from wicking into the base plate of the framing, which can create rust or wood rot, damage that would be concealed from view after construction.  Many building code officials have more rigid standards about requiring weep screeds in recent years. 

The weep screed must be placed a minimum of 4 inches above grade and 2 inches above a roof line, and alterations to these dimensions will usually get you into trouble. On a roof, it is very important that you make sure the weep screed is far enough above the stepped flashings (found on residential roof pitches) before the screed is installed. This would allow a re-roof and a second layer of shingles to be installed without removing the veneer. At grade, it is extremely important that the landscaping, water sprinkler, gutter discharge, or splash of any sort be prevented from hitting the base of the veneer. This is a location that can be effected by staining when landscaping medium such as dirt, mud or mulch is allowed to spatter into the veneer during a wet weather event. If the veneer is saturated in cold weather, freeze/thaw can occur, resulting in unsightly or failed veneer walls. 

Sealants used for flashings 

Today, many adhered masonry veneers are constructed very much the same as a cavity wall and utilizes many similar products. 

Common types of sealants used in flashing installations: 

•Butyl Synthetic Rubber – Adaptable to most membranes and metal flashings, workable in most temperature environments, typically not a brand-specific product 

•Modified Polyether Sealant – low volatile organic compounds (VOCs), single component, generally matched to specific membranes, workable in most temperature environments and paintable 

•Polyurethane Elastomeric Sealants- The most commonly used sealant over the past 20 years, however, due to adhesive capabilities of many modern non-metallic flashing products, compatibility and performance should be evaluated prior to specifying on a project with flexible membranes 

•Silicone Sealants – Popular for granite and marble installations, but are not typically recommended for cavity wall through-wall flashing as the silicone may have adhesion issues with flexible membranes and can leach oils into adjacent masonry porous surfaces