What is this? A two-piece Hohmann and Barnard Masonry Tie!
Why do I care? Keep Reading!
I am sure that most people look at a stone or brick facade and think not only of how aesthetically pleasing it is but how strong and stable masonry must be. Stone and brick are great materials for the exteriors of building—they require little or no maintenance, they are very durable, and the last forever. However, most masonry done today is masonry veneer—where you have a 3”-4” stack of masonry separated from the main wall (usually wood or metal studs with sheathing) by an air gap.
The air gap is critically important to the system as it allows a space for moisture, which migrates through the masonry (both brick and stone allow some moisture to pass through), to be collected and removed through the weeps at the bottom of the wall. Typically the air gap is 1”, but many designers are using larger gaps to make sure the 1” clear is maintained—this is especially important with stone, which has less of a uniform thickness than brick and with masons who are not as careful.
This image from Hohmann and Barnard shows a typical masonry wall assmebly. It shows metal studs, but wood studs are similar. The white is the sheathing and the pink is thew rigid insulation. You can see the air gap between the masonry and the insulation and how the brick tie spans this gap. Note, the wire in the plain of the bricks is a for seismic loading.
Like solid masonry walls, the weight of the stone in masonry veneer is transmitted vertically through the stone to the foundation. The difference comes when you look at wind loads. Wind loading on a building is a very complicated and depends on the speed of the wind, the height of the wall, the area of the wall, the topography, and the surrounding buildings. Wind pressure can easily reach 20-60 pounds per square foot and can either be pushing in or sucking out depending on the direction of the wind. This may not seem like much but when multiplied by the area of a wall it can add up. Say you have a 20’ long two story façade, even with 20 pounds per square foot, this is 8,000 pounds of forces on the building on a stack of masonry 4” thick. To transmit this loading to the house structure that is designed to take these loads, we use small metal connectors called brick ties.
There are many different types and shapes of brick ties but they are all designed to transmit horizontal load on the masonry to the walls. If you are in a seismic zone, which fortunately Austin is not, the brick ties also have further requirements to help deal with the unique loading earthquakes put on buildings. The ties are fastened to the wall and then set in the mortar of the masonry to create the connection to transmit the horizontal forces. The ties must be able to handle compression (wall pushing in) and tension (wall sucking out). To help distribute the load evenly and to keep the loads on each individual tie low, code requires that brick ties be installed for every 2.67 SF of wall.
Traditionally, residential ties are 7/8” wide corrugated strips of galvanized steel that are nailed to the wall (hopefully to the stud and not just the sheathing) and then bent to fall into the masonry mortar joints. These ties are inexpensive and easy, but there are a couple of issues that must be addressed before using them. First, they are not recommended for air gaps larger than 1” and since air gaps by code are not supposed to be smaller than 1”—you better have a 1” gap. Second, if the bend in the tie is not right on the fastener to the wall, the loading can develop (especially with a sucking pressure) so that the ties bend rather than transit the load. Third, I have read that there are concerns with these ties being more susceptible to deteriorating over time than wire ties. Fourth, these ties were not designed for rigid insulation applied to the sheathing. On the plus side, the ties are mostly used on residential structure which have smaller wall areas and more ins and outs that allow the masonry wall to be able to support itself.
Residential tie on right, simple commercial tie on left.
Commercial ties usually are two part systems where there is an anchor that attaches to the wall and then a wire triangle that is mortared into the masonry. There is usually some sort of height adjustment between the two pieces so that the wire ties can be in the masonry joint and still be perfectly horizontal with the wall fastener. This ensures the loads are transmitted horizontally. These ties are more expensive that residential ties because the have more material and multiple parts. Many of the ties have been designed to accommodate rigid foam insulation.
Here is a Hohmann and Barnard Tie that install before the insulation. The advanatge here is that you can see the nail patern and make sure the ties are attached to the studs and not just the sheathing. The tie easily spans the rigid insulation, but the insulation must be pieced around the ties.
One of the biggest changes to masonry veneer has been the increasing use of rigid insulation on the outside of the walls to help increase the R-value of the walls and to decrease thermal bridging in the walls (it is code required now for metal stud buildings). This rigid insulation complicates the brick ties because it is putting a semi-compressible material between the load and the wall that receives the load. If you use residential ties and just nail through the insulation, the ties will compress the foam when the wall is pushed and this can cause the ties to loosen with time. There are a number of commercial ties that are designed for rigid insulation. If you are using rigid exterior foam insulation, I highly recommend you use commercial ties that are designed for the thickness of the insulation or at least consult your structural engineer.
At my house, we have tall (almost 30’ in places) masonry walls, ¾” rigid insulation, and a 1.25” to 1 1.5” air gap depending on the individual stones, so we are using a Hohmand and Barnard tie that has a screw that is applied through the insulation and sheathing into the studs and a wire ties that attaches to a slot in the screw assembly. Timing is very important with the masonry ties. If you have rigid foam outside you walls, it makes it difficult to see where the studs are located. This is further compounded if your dry wall is on the interior of the wall and you cannot inspect from the inside to make sure the ties are hitting stone. Some of the commercial tie systems are designed to attach to the sheathing before the insulation, which may a good idea -- though sequencing who installs the ties and fitting the insulation into the ties may be an issue.