The Point of Repointing

Repointing masonry is a regular maintenance item that is generally necessary every few decades or so. So what is the purpose of this exercise, anyway?

What Repointing Does

Repointing is the repairing of surface deterioration to mortar. Mortar does decay over time. In extreme cases, it simply crumbles away as the lime and/or cement washes away. More commonly, it tends to break away from the stones, leaving cracks in the structure.

The Degrees of Mortar Deterioration

Cracks in the joints of masonry causes two problems. First, it loosens the individual stones, making the structure more vulnerable to damage. In extreme cases, settlement and severe cracking damage will result as the stones fall out of the wall. Secondly, severe cracking can cause water penetration, leading to more extensive damage later on.

Though overall in reasonably good condition, the cracks in this culvert (including the less obvious ones) should be repointed to prevent further damage. That said, when looking at major cracks like this structure has, it is always best to evaluate whether or not continued movement is present, and, if so, address the underlying problem. If, after repointing, the crack opens up again, movement is evident.

Even if the structure is not cracked, the mortar can decay over time. If left untouched, the mortar will eventually entirely leach out of the structure. This, in turn, will loosen up the stones, resulting in weakening and possible crumbling of the wall. How severe the damage will be depends on how reliant the structure is on the mortar.

The mortar has leached out of this stone culvert, with the end result that the stones were loosened, hence why they have been knocked out of the structure.

A less extreme form of mortar deterioration that is common is where the surface mortar has decayed. Perhaps some of the surface mortar has fallen out, perhaps not, but in either case on close examination it will be found that the mortar is very soft and crumbly, providing no structural integrity, though further back in the wall the mortar may be sound.

Mortar is visible in this stone culvert, but large amounts of it have leached out. A thorough repointing would be required to fully restore this structure.

All of the cases described above are repaired by repointing

The Principles of Repointing

Repointing is done by cutting out the damaged section of mortar deep enough to achieve a good bond and the reach solid material behind the questionable mortar. The old mortar is cleared away with a chisel until solid mortar below is found. Cracked mortar is also cleared away in the same fashion at least deep enough to ensure the new mortar on top can thoroughly bond and seal the joint.

There are two basic rules of thumb to follow. First, never cut off a piece of stone if at all possible, and, second, as much as practical, leave the sound mortar alone; the sound mortar will always bond better than any repointing ever can.