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 and upward 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.