Brick joints are basically just what they sound like. These are all the spaces you see between the bricks in your fireplace.
There are three main types of brick joints. First, you have the head joint. This is the vertical space between two bricks seen to the left and right of each brick. Next, you have the bed joints. These are the horizontal spaces seen above and below each brick. Finally, you have collar joints. These joints are not always as visible as the others, but they are where two freestanding brick sections meet.
It may seem a bit extreme, but even these joints have specifications outlined in the building codes. In accordance with IRC code R 606.3.1 “Bed and Head Joints”, “head and bed joints shall be ⅜ inch thick, except that the thickness of the bed joint of the starting course placed over the foundations shall not be less than ¼ inch and not more than ¾ inch”. Are you lost yet? Translated into plain English, that basically means you should see a line of mortar between ¼ inch and ¾ in thick between your foundation and the first brick, and then all other joints should have lines of mortar ⅜ in thick. So, you’re probably wondering something like “why should I care about these little lines of concrete?”. Brick joints exist to bond individual bricks to each other, but even more importantly, they keep water from entering the chimney through these spaces between the bricks.
Brick and mortar are not the strongest under tension stress, and these joints often end up cracking as the chimney ages. Once these cracks start, it only gets worse. In cold months, water enters these cracks, freezes, expands in the crack, and thaws, leaving behind a bigger crack. This is what you may have heard called the freeze-thaw effect, and it happens over and over all winter. Summer months are not much better, however. These damp, dark openings are a perfect home for mold, fungal, and plant growth. As these plants take root, they increase the pressure inside the openings and can lead to further cracking. As you can imagine, they also don’t stop growing once they fill the cracked joint. If you have ever seen a chimney covered in moss or algae, there is a good chance it all started in one little crack.
Depending on the size of the crack, there are several different repair methods. First, any plant growth would need to be wire brushed off the chimney. Next, any small cracks would be filled with a clear, elastomeric (flexible rubber) sealant. Larger cracks may require a mason to repoint the chimney, which is where a trowel is used to insert new mortar into the gaps. Finally, if a large amount of growth was present on the chimney, a chimney water repellent may be applied.