A drip edge is often one of the most overlooked parts of the chimney we see during our inspections, but what exactly is a drip edge? A drip edge is a two-inch overhang at the top of the chimney level with or just below the crown. This is most often seen made from corbelled (stairstep-like) brick that is parged (smoothed with mortar) to make it watertight, but it can also be formed by building the crown from a concrete slab that extends 2 inches over the side of the chimney.
Although this area of the chimney is often overlooked, it is required by IRC building code R1003.9.1., which states that “masonry chimneys shall have a concrete, metal or stone cap, a drip edge, and a caulked bond break”. Prefabricated chimneys don’t get off easy either. According to R1005.1, prefabricated chimneys “shall be installed and terminated in accordance with the manufacturer’s installation instructions”, and most prefabricated fireplace manufacturer’s instructions specify that the prefabricated chimney chase is topped with a chase cover that has both cross breaks and drip edges.
So, having your home built according to modern building codes is great and all, but you are most likely still wondering what exactly meeting this code will do for you and why this code even exists in the first place. The concept of a drip edge is that rainwater will flow off the crown, flow over the corbeled rows of brick, and drip from the last row of corbeled masonry down to the roof below.
This allows the water to drip from the top of the chimney instead of cascading down the sides. Why does that matter? Well, water that saturates the brick and brick joints can cause some big damage to the brick and mortar joints. Brick is porous, meaning it absorbs water, and when water saturates these bricks, it can expand and contract during freeze-thaw cycles.
This expansion and contraction can crack the faces of bricks, saturate the brick holes and crack the brick in half, and even cause the mortar joints to separate, leading to even more paths for water intrusion.
Another particular problem that we often encounter in homes that do have drip edges is that they are unparged. Here is why that is an issue. Bricks have center core brick holes, which are filled with grout to help bond the rows of brick to each other. When the brick mason corbels the brick outward to begin forming the drip edges, they sometimes forget to mortar over this transition. The result is that the water that flows off the crown and lands on the corbel drip edge can seep into the brick holes and cascade down the chimney through the interior brick holes. Since brick is very weak in tension, the large amount of water that can saturate these brick holes can cause spalling damage when the water freezes and crack the brick faces. It is very important to seal any unsealed masonry drip edges to prevent this problem from occurring.