Chimney tops are subject to brutal conditions, and it shows over time.
First it's a small mortar crack, then moisture gets in, freezes ... and the cracks grow.
Then it's a seed, a sapling ... and the roots find the easiest place to grow.
Then, bricks loosen and fall ... hurting the roof or maybe even someone below as it plummets and bounces it's way to the ground 30 or more feet below.

We've rebuilt the tops of dozens of chimneys, in different ways.

Called out to investigate a leak near this chimney ... we decided to look closely at the top.

We took the cap off and discovered 2 flues ... not 1, and they were both covered by the chimney cap with a "flue vent" in the center.  A prior worker made or ordered the wrong cap but decided to put it on anyway rather than remake it correctly.  So, neither of the chimney flues were actually venting.  It was more like they were leaking fumes at the top.  Besides causing a condensation problem, it's extremely dangerous to have a chimney that does not vent properly.

Test fitting the new custom stainless steel before welding and powder coating.

All done.  We prefer stainless steel to any other method.  It's lifespan is unsurpassed.

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Here, we did this one the old fashioned way.
Remove the crumbling top and loose bricks.
Build a form and add new perimeter bricks where necessary.
Put metal lathe work in the form, add fiber to the mortar mixture, pour, taper and cure.