A chimney cap will limit the amount of moisture that gets in and on your chimney. Less moisture means less problems with the freeze/thaw cycle...which breaks apart your masonry.

A cap will keep rain out of your chimney but will do nothing to protect the outside where spalling takes place. Many chimneys do have caps. Are they necessary? One of my chimneys was built in 1810; my neighbor's in 1774. Neither has ever had a cap. Both are still in fine shape in a damp coastal climate.

There are many chimney caps and top dampers available. Only the installer knows what he was proposing. He should give you a web reference or a photo and spec sheet.

Here is a link that might be useful: caps

Seems a little strange to me as well, but I know very little about chimneys. Like everything else get multiple people to look at it. Our roofer wanted to replace our chimney to the tune of $3 - 4k. The guy who we had do the work charged us $1k and is one of the top rated in our city for quality work. I'm just glad it's another thing I don't have to worry about!

Keeping birds and small animals out is more than reason enough to cap all chimneys. (Unless you enjoy a room full of maggots feeding off decomposing flesh and the delightful odor of death everywhere. I speak from experience.)

Unless someone has the gift of intimate experience with chimneys in this area, diagramming the situation might be a good idea.

Description of the situation

1. A brick on the angled exterior flank of the chimney dislodged.

1. The old lime mortar on the flank of that brick was broken where it went under the stucco of the house wall and then met with the 15 pound felt paper facing the exterior studs of the house wall.

2. The 15 pound felt paper on the exterior studs has a U channel flashing at the bottom which it overlaps vertically a bit on the inward side. The flashing runs diagonally aupward toward the center of the chimney and appears to function as a drain for water collected from the felt paper and the chimney mortar which extends down into the U.

3. There appeared to be a 1/8" or less air gap between the felt paper and the outer brick mortar face. Was this the unimportant result of natural shrinkage of the old lime mortar? Or is this air gap a design element that must be maintained when replacing the old lime mortar?

4. If this air gap is an important design feature, how do I establish it when putting in new type S mortar when I only have restricted access from the side of the chimney where I can remove the old lime mortar under the dislodged brick and through the top where some of the stucco wall has been broken? Is there any material I can temporarily insert between the wall felt and new mortar to establish the gap and then remove without disturbing things?

5. Note that the felt paper in place was ripped when removing some of the old lime mortar. Although the old felt paper is no longer tight, I have repaired the tear with waterproof caulking and it seems fairly strong. If I can maintain an air gap between the wall with its 15 pound asphalt felt and the new mortar, a problem is unlikely. However, if the weight of the setting mortar bears directly on the patched paper which is no longer taut, I am more concerned.

6. There is about 3/4" room to push a new patch of 15 pound paper under the old paper beneath the upper section of opened stucco wall. There are no pieces of felt in place for this patch to side lap under. The sides of the patch would have to be caulked and stapled to upright studs and the bottom left to overlap the top part of the U channel flashing below. Gaining access to the the two upright studs would require extensive removal of the old lime mortar bed.

Any and all thoughts and observations would be appreciated. If someone is familiar with the best approach in this situation from intimate experience, I would be more than grateful for any advice.

I apologize to the OP. My intent was to post my writing as
a new thread instead of a response to his but I made a beginner's mistake, which I'll fix by separating my question from
the thread.

Would it not be simpler to ask the guy to be more specific maybe even describe in an email then ask us if it is necessary?

My 1908 brick chimneys have caps at the t0p, but the only one with a damper (top-mounted) is the one which has the only fireplace...the rest are used for venting the kitchen stove, the hot-water heater and the old furnace which has been replaced with a new model which vents through the side wall.

Both of my chimneys have stucco covering them, and from what I can tell, it is original. Perhaps that is what your man is proposing, although the price seems very cheap for that. :)

He is probably proposing a coating for the crown that remains flexible.

That price is cheap either way.