On Mortar

Stewart Bridge

Mortar has been used in masonry for centuries. For that matter, mortarless masonry has also been around for a long, long time; the Romans used it extensively in their stone bridges. Both methods of constructing masonry have distinct advantages and disadvantages. In this post, we will look into the whats and whys of mortar.

Why Mortar is Used

Mortar serves several purposes. First and foremost, mortar is a simple means of ensuring maximum contact between stones. Maximum contact means maximum friction, which means maximum strength. Secondly, mortar helps the masonry handle vibrations. Mortarless masonry, particularly that which relies heavily on stone fragments to maximize friction between the main stones, can be vibrated apart. However, mortared masonry is very good at resisting vibration, as the mortar is one solid mass and actually adheres to the stones. This brings up advantage three of mortar: mortar adheres to the stones, helping to prevent them from being dislodged by impacts, as well as vibration, as mentioned above. This leads to yet another advantage of mortar, for mortar seals the masonry joints from water, making freeze-thaw cycles far less of an issue. All in all, new mortared masonry tends to be much stronger than mortarless construction.

An Unusual Case

With some very high-quality stonework, the joints are cut so tight that there is no room left for mortar worth speaking of. In these cases, the structure could be left dry-laid, and would no doubt do very well. However, it was generally accepted historically that mortared stonework was superior to dry-laid; in fact, it was even contended by at least one historic stonework authority that mortarless stonework did not merit being called a class of masonry at all, mortarless work being bad practice. Hence, precision stonework of more recent centuries was usually mortared. In order to leave a place for the mortar to go, the stone could be left with small pockmarks and cavities on the mortared surfaces, or perhaps mildly hollowed out on these inside surfaces. Alternatively, everything could be cut a hair too small to leave a tiny gap between each stone for mortar to fill. In short, the masonry was purposely made slightly less than perfect in order to leave a place for mortar to go.

The Demise of Mortar

Though it can and is used to achieve stronger masonry, mortar deteriorates over time. In fact, generally speaking, mortar is expected to deteriorate. This is fine and good, but the masonry becomes weaker and weaker over time as the mortar crumbles and leaches out. Repointing regularly helps keep the mortar joints sound, and is especially effective at helping prevent deterioration of interior mortar joints. However, once the interior joints have deteriorated, repointing will become more of a superficial repair job. Fortunately, grout can be pressure injected into the masonry to replace disintegrated interior mortar. Regardless, as the mortar is expected to deteriorate over time, quality mortared stonework is built such that it can and will stand without mortar, the mortar being added for further strength and stability.