Step flashing and counter flashing
Step flashing is layered between the shingles along the sides of the chimney. Counter flashing is cut into the masonry above it and folded down over the step flashing, so water can never reach the joint. Both pieces are required — one alone leaks.
Why tar is not a repair
Roofing cement is a common shortcut. It bonds, then bakes in Texas sun, shrinks and cracks. Every tar patch we find is a leak waiting to resume, usually with hidden rot underneath.
When you need a cricket
On wider chimneys, a cricket — a small peaked structure behind the chimney — diverts water and debris around it instead of letting it pile against the back wall. Without one, wide chimneys on sloped roofs leak repeatedly no matter how good the flashing is.