Affordable Masonry Detailing – Introduction
Brick is made by molding blobs of clay into cubes and then firing the cubes into weather resistant, rock-hard construction modules. Bricks are typically small, easily handled bits of fired clay held together with a cementitious paste.
Concrete block units are made by forcing a paste of cement, sand and water into molds. Once the blocks are unmolded, they are steam cured until the concrete hardens. Blocks are larger and take less labor to set than bricks. Because they are about 50% solid, concrete blocks are lighter than other masonry products. The core holes are also an ideal place to run lines of steel reinforcing and grout. This steel reinforcing greatly adds to the strength of tall, thin walls.
Stone is a naturally occurring material that is quarried and cut to size rather than molded. The naturally occurring flaws and fault lines that appear in the stone add to its natural beauty. They also define planes of weakness which can fail prematurely. Stone is the most expensive masonry choice so it is usually installed on high-end buildings like cathedrals or civic structures which are expected to have a very long life span. There are literally hundreds of kinds of stone. Each has a different color, texture and strength. This wide range of values makes it difficult to find definitive information about handling and detailing stone.
Up until about 1950, most masonry walls were built thick and solid. They were constructed of several wythes (layers) of stone or brick. This thick, dense construction was very forgiving of temperature changes. It was also thick enough to resist moisture penetration. These historic walls were often deeply textured, allowing shadow lines to add interest to the wall. These heavy, thick barrier-type walls worked well but they were expensive.
Modern cavity walls are cheaper, more quickly built and, if constructed correctly, more resistant to water penetration than old-fashioned massive barrier walls. Cavity walls are built as two parallel walls separated by a narrow slot of air. The exterior skin of the wall (usually about 4” deep) is the decorative, weather-resisting layer of the wall. The interior layer of the wall can be built of poured-in-place concrete, concrete block or studs with insulation and sheathing. This interior layer provides the structural strength of the wall. The narrow air space serves as a drainage channel. Any water that penetrates the exterior wythe of the wall drains down the cavity and is directed harmlessly to the exterior face of the wall through the flashing and weep holes at the base of the cavity. It is important to keep this air space clean and free of mortar. Blobs of dropped mortar can block the path to the weep holes. If the weep holes cannot drain away trapped moisture, the bottom of the cavity can turn into a little masonry bathtub. Not a good idea.
Resource: Rocky Mountain Masonry Institute-www.rmmi.org