Sooner or later all brick chimneys need to be tuck pointed----you know, where the mortar joints have to be repaired. This is especially true of older homes where the mortar itself might not have been very good, and where age and missing hats has taken their toll on the chimney from the inside-out. Chimneys 
actually fair quite well from normal weather conditions. But, leave the chimney caps off so that water can get into the structure, deterioration can happen more quickly. Add to this, improper venting of gas and oil appliances and deterioration is guaranteed.
Sometimes the whole chimney needs to be tuck-pointed. Other times just doing portions of the chimney is enough----like the portion above the roof perhaps. Some times it is just the portion near the ground that is in poor condition. If the joints easily scrape away with a pen knife repairs are warranted. If the mortar is completely missing it is a "no brainer," like in this picture, where the pen knife not only scrapes the mortar away easily, but actually fits in the joint.
Deterioration of this chimney was at the point where some of the bricks will need to be replaced along with proper tuck-pointing.
Sometimes I am able to get close to the chimney on the roof, but I am not able to see down the flues. I can usually reach my camera up over the top and snap pictures of the condition of the inside of the flues. In this case the camera was able to see that bricks were falling inside the chimney where they may have already damaged, or will damage, the gas furnace metal liner. These metal furnace liners are difficult to assess by the home inspector. There really isn't an easy way to know whether a brick has fallen inside the flue and ripped a hole in the liner or not. When we see a brick leaning against the liner as in this next picture (and ready to fall) we do have to wonder though if one hasn't already fallen, and have to recommend that continuity of the liner be verified by the heating contractor.
If, in the Winter, we can see the furnace exhaust coming out of the chimney around the outside of the metal vent----sometimes even out though the missing mortar joints----instead of coming out of the cap----it is a pretty good indication that the liner is not continuous. Maintaining brick chimneys can be expensive but not nearly as expensive as replacement.
Charles Buell
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PS, for those of you that are new to my blog (or for some other "unexplained" reason have never noticed)
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