Masonry chimneys without mortar caps and hats can let lots of water into the flue and structure of the chimney. Over time, chimneys can be destroyed by this water infiltration. One of the most obvious signs of moisture infiltration is efflorescence. Efflorescence is the whitish powder that forms on the face of the chimney or in the fire-box of the fireplace. As moisture in the brick evaporates from the surface of the brick it leaves these salt deposits we call efflorescence.
Efflorescence itself doesn't represent more than a cosmetic issue but it does tell us how much moisture is being introduced to the chimney. Right now I am only talking about "outside" sources of moisture. There can also be interior sources of moisture that create or add to the efflorescence. Inside sources are gas and oil heating appliances----especially gas furnaces. These internal sources can actually add more water to a chimney structure than outside sources if the chimney isn't properly lined.
This chimney has no mortar cap and one can easily imagine how much rain it could collect and channel into the chimney structure. Of course the open flues obviously collect a lot of water.
The efflorescence on this chimney almost looks like failed paint---especially on the left side. This chimney is near my house, so I get to watch it annually. The owners frequently clean off the efflorescence and sometimes it looks much worse than it does in this picture. While it would appear to have a mortar cap, there is obviously a great deal of moisture finding its way into the structure----and of course there is no hat on the flue itself. So while a mortar cap is essential to protect the chimney, the cap itself must be functional. They often crack and have gaps around the flue. Keeping these mortar caps well sealed is essential.
If you don't keep the moisture out you will sometimes----under very severe conditions----find efflorescence on the inside of the home as well.
The flue itself should have a proper hat and spark arrestor/vermin screen. This "hat" can keep a lot of water out of the chimney structure.
Sometimes homeowners get very creative about keeping moisture out of the chimney structure and for providing a hat to keep moisture out of the flues. This next picture shows some of the ingenuity I get a kick out of.
My favorite "hat" of all time has to be this functional hat on the chimney for an oil fired furnace.
Charles Buell
PS, for those of you that are new to my blog (or for some other "unexplained" reason have never noticed)
all pictures and smiley-face inserts (emoticons) (when I use them) have messages that show up when you point at them with your cursor.
* * * * * * * * * * * * * * * * * * * * * * * * *
Click on the Rose 
to check out: AHA!---A Forum of Landmark Proportions---your Group
PS, for those of you that are new to my blog (or for some other "unexplained" reason have never noticed)
all pictures and smiley-face inserts (emoticons) (when I use them) have messages that show up when you point at them with your cursor.
DeCroe, is my "ethereal" home inspector assistant and occasionally flies into my blog and other people's blogs to offer assistance. To find out more about her beginnings just click on Raven.
