A good rule of thumb is; if the leak appears on every rain or during melting snow then it’s a roof leak, if the leak appears intermittently, it’s a chimney or chimney flashing leak. An intermittent leak, which is a leak that does not appear on every rain, is caused when wind blows rain from a certain direction against a failure in the chimney or chimney flashing…
Damage from a leaky chimney often shows where the chimney abuts a wall. The ceiling or wall will be stained, the paint peeling or on plaster a white chalky substance called “efflorescence” will appear..
Moisture penetrating your chimney, outside moisture is rain, snow or melting ice, inside moisture is from the hot water heater or furnace. Outside moisture will penetrate through a damaged or missing chimney crown, failed pointing, spalling bricks, improper chimney flashing or a missing chimney cap.
The top of a chimney has an opening to exhaust the smoke and fumes. A terra cotta flu pipe should be installed and a thick layer of concrete, which is known as a chimney crown should be poured covering the opening and abutting to the flu pipe. The chimney crown should extend a few inches over the outside walls of the chimney so that water will drain away from and not run down the chimney walls. A properly constructed chimney crown is your best protection against chimney damage. A chimney without a proper crown is like a toothache, ignore it and it will turn into a root canal.
The lack of a chimney crown is the main cause allowing water to penetrate the bricks. On a warm winter day bricks will absorb moisture then during the colder night time temperatures moisture freezes and expands causing cracks and splits in the brick. This damage is called “Spalling”. Typical spalling is cracks, splits in the brick or flaking. Another type of spalling is caused when the wrong mortar for pointing is used with soft brick. Bricks absorb moisture but hard mortar does not. Bricks will expand as they absorb moisture, the hard mortar does not, causing the brick to split. Using the correct mortar is especially important when working with
older brick in historical chimney restorations.