A wythe is a single vertical layer of masonry units (bricks, concrete blocks, or stones) in a wall. If you slice through a brick wall and count the layers from front to back, each continuous layer one unit thick is one wythe. A wall can be a single wythe thick or built from two or more wythes bonded together, and this distinction shapes how the wall handles weight, water, and temperature.
Wythe vs. Course
These two terms get confused because both describe layers, but they run in different directions. A course is a horizontal row of bricks or blocks running left to right along the wall’s length. A wythe is a vertical layer running from front to back through the wall’s depth. Think of it this way: courses stack upward, wythes stack inward. A standard brick wall might have dozens of courses but only one, two, or three wythes.
Single Wythe Walls
A single wythe wall is exactly one masonry unit deep. In this setup, the brick or block serves as both the structure and the exterior face of the building. Single wythe construction is common for interior partitions, garden walls, and some low-rise commercial buildings where a single layer of concrete masonry units provides enough strength.
Because there’s no second layer or air gap behind it, keeping water out becomes critical. For most single wythe walls exposed to weather, builders typically apply a water repellent system or exterior coating as a second line of defense. Vertical and horizontal steel reinforcement can be grouted into the hollow cores of the blocks to boost structural performance when needed.
When insulation is placed inside the cores of a single wythe wall (granular fill, polystyrene inserts, or spray foam), each option handles moisture differently. Granular fill lets water travel downward through the insulation to flashing and weep holes, where it drains outside. Polystyrene inserts leave enough space around them for water to drain as well. Foam fill, however, is impervious to water and can redirect moisture sideways across the block webs toward the interior, which makes water management trickier.
Multi Wythe Walls
Multi wythe walls use two or more layers with either a filled joint or an air cavity between them. This design is the backbone of most exterior masonry construction on larger buildings because it separates the job of keeping weather out (outer wythe) from the job of supporting the structure (inner wythe).
The space between two wythes is called the collar joint when it’s narrow and filled with mortar or grout, or the cavity when it’s a wider air gap. In cavity walls, the air space acts as a drainage plane. Water that gets past the outer wythe runs down the back of that layer, hits a piece of metal flashing at the base, and exits through weep holes spaced 16 to 24 inches apart. For this system to work, architects specify a cavity with at least 1½ inches of clear airspace, giving masons room to scrape away excess mortar that could bridge the gap and channel water inward.
Rigid insulation boards can be placed inside the cavity against the inner wythe, and the drainage performance of these walls is generally excellent when flashing and weep holes are properly installed. The one exception: if the cavity is completely filled with foamed-in-place insulation, the wall may lose its ability to drain, since the foam blocks the water’s path down to the weep holes.
How Wythes Connect to Each Other
For a multi wythe wall to act as a single structural unit, the layers need to be tied together. There are two main approaches. Metal wall ties, typically corrugated strips or adjustable wire assemblies, are embedded in the mortar joints of both wythes at regular intervals. When wall ties are used, the collar joint between the wythes must be filled solid with mortar or grout so the layers transfer force between them.
The second approach uses steel reinforcement bars placed directly in the collar joint, which then gets filled with grout. Grouted collar joints can handle roughly twice the shear stress of mortared ones (10 psi versus 5 psi), making them the stronger connection. The choice between the two depends on how much lateral and vertical load the wall needs to resist.
Veneer vs. Load-Bearing Wythes
Not every wythe carries structural weight. In veneer construction, the outer wythe of brick is attached to a wood or steel frame behind it. The brick looks the same from the street, but it supports only its own weight. The frame does the structural work. This is how most brick homes built in the last several decades are constructed.
In a load-bearing wall, the masonry wythe itself carries the weight of floors, roofs, and everything above it down to the foundation. A single wythe bearing wall uses one layer of masonry as both the structural system and the exterior face. Multi wythe bearing walls distribute loads across two or more layers working together as a composite unit.
Wythes in Historic Buildings
Before steel and concrete frames became standard, thick multi wythe masonry was the primary way to build tall, durable structures. Historic buildings across the Mediterranean, for example, used a three-layer stone masonry system: an outer layer of dressed limestone blocks, an inner layer of similar stone, and a core filled with rubble stone and lime mortar. In the traditional trulli buildings of southern Italy, a UNESCO-recognized construction style, these three layers could add up to walls over 100 cm (about 40 inches) thick.
These old walls had no air cavity and no metal ties. Instead, they relied on sheer mass and geometry for stability. The three-layer system functioned as an integrated composite, with all layers deforming together under load. The thick walls also provided passive thermal insulation, keeping interiors cool in summer and warm in winter. It’s a different philosophy from modern cavity wall construction, but structurally effective enough that many of these buildings have stood for centuries.