Let’s talk about chimneys, masonry chimneys to be specific. At the top of the chimney there is intended to be a concrete top, called a “chimney cap” or “wash cap”. The purpose of these caps it to allow water to drain over the top of the chimney, thus preventing it water from getting down into the masonry itself, which can cause significant damage. The issue of water flowing into the masonry is of particular concern when temperatures drop and the water freezes, as this can cause the chimney to heave, damaging the chimney and/or flue liner. Therefore, in order to protect the chimney from this kind of damage it is important to have a good chimney or wash cap, and to ensure that it is mortared at the seams to prevent leaking.
Above, you can see two photos of chimneys. In one of the photos, the chimney has a concrete wash cap and the mortar joints are sealed. In the other photo the chimney does not have a wash cap, and you can clearly see that the bricks have worn mortar joints, and some of the bricks themselves have shifted. To further complicate matters, the flue is not shown in this photo because the top of the flue is broken and now missing. In this case, the flue did not have a flue cover. The purpose of a flue cover is to protect the flue from water leaking in, which can cause damage, as well as to prevent rodents or birds from entering the flue and nesting there. All chimney flues should have a proper cap that is high enough for good drafting, and screened to prevent animal entry.