1 Freeze -thaw cycles Primary Cause
Water seeps into tiny cracks in the bricks .When temperatures drop below freezing ,the water expands about 9%.The expansion creates, internal pressure cracking the bricks surface. Over repeated cycles chunks of the brick face break off—– this is called spalling
2. * Soft, porous brick(common in older chimney)*Over fired bricks,*Bricks not rated for exterior use.
3.Failed of missing crown.( Cement slab at top of the chimney that has a pitch to it so water doesn’t sit on top of the chimney.
4. Defective of no chimney cap.
5. Mortar joint deterioration Mortar is softer and more porous than brick by design. When mortar erodes due to age crack open up, allowing more water to reach the brick edge .Once water gets behind the brick face spalling accelerates.
6 Efflorescence& Salt attack** Efflorescence is a white powdery deposit which is a sign of soluble salts inside the brick .As water moves through ,it brings salts to the surface. When water evaporates ,salt crystals, grow inside the brick pores, exerting pressure and causing flaking.
Poor flashing. Chimney flashing should be water tight copper ,lead or aluminum flashing. If flashing is rotted rusted or improperly installed water will run down the brick between the roof and the chimney saturating bricks from the side.
Thigs you can do to stop repoint with new mortar, fix chimney crown, replace with new bricks, waterproof chimney with a breathable water repellent never seal bricks need to breathe.