When it comes right down to it, the whole art of masonry seems to revolve around the masonry joints. Masonry joints are the areas between the stones or whatever masonry units are used. The masonry units themselves rarely cause problems in their own right. Masonry units can be either bricks, concrete blocks, or (of course) stone. As long as the quality of these materials are good, they should cause no problems in and of themselves. (If the quality of the stone is very poor, by the way, it is possible for the stones to turn into a very interesting form of sludge in a disturbingly short amount of time.)
The Masonry Joint Problem
Understanding how the forces and weight are carried through masonry is a big help to building better structures. Masonry relies primarily on friction and compression to hold together, regardless of whether the masonry is in the form of an arch or wall. It follows, then, that wherever any two stones meet, they should contact each other as much as possible. And here is where the difficulties begin. If you try stacking a few stones together, you will probably find that they don’t really meet up very evenly. Of course, careful selection helps considerably, but it is not probable that you will consistently find stones that naturally fit perfectly together.
Filling in the Gaps with Mortar
One way you can fill in the gap is by using mortar. Mortar creates essentially a custom-shaped hard rock that fits precisely into all the cracks and crevices between a pair of stones. Good as far as it goes, but mortar can become expensive for a big project, and deteriorates steadily over time. Of course, the mortar can be repointed, but repointing is only a superficial fix (though it does help prevent further deterioration of interior mortar) and is a maintenance item. Furthermore, if the structure actually relies on mortar for strength, the masonry will tend to disintegrate once the mortar fails. A much better compromise is to try wedging small stone slabs into the joint with the mortar. This way, the small stones in the joints help carry the load, reducing the reliance on mortar. Then, too, this will also cut down on the amount of mortar needed.
Filling in the Gaps with Stone Fragments
If you are really careful and thorough with the stone fragments and shims in the masonry joints, you may not need mortar at all. The trick here is to do the job thoroughly, shoving the shims into the wall to fill in as many gaps as possible. The shims should wedge into place, yet mustn’t be pushed in so hard that you accidentally lever a stone out of place. A good way to think of this is to treat the shimming as though you are actually building a micro-masonry wall within the joints between two big stones. A good wall can be built using extensive shimming if care is used.
Still, you should be careful to select tightly stones to begin with, and, as shims can work loose over time, ensure that the shims fit tightly and the joints are reasonably tight to begin with.
Trimming and Cutting
Trimming and cutting stone is an art all of its own. Some stones, like sandstone, are easy to cut and use. Not only is sandstone usually found in nice slabs naturally, but sandstone actually cuts easily along a straight line when you take a chisel to it. Most stones, however, are far less agreeable to being cut. When you take a chisel to these stones, they seem to have a decided tendency to break in some line that appears to have little relation to where you were trying to cut. The key to cutting is to find the right tools for the stone type being worked with. For example, sandstone will cut well with a conventional chisel, while limestone does far better with a toothed chisel. The next thing to determine is how each stone wants trimmed.
For beginners, the best bet is to start with an easy-to-use stone like sandstone (if possible) or to start by building walls that rely more on shims then precision cutting. Then, over time, keep practicing trimming the stones to fit, starting with easy cuts, like knocking off a projecting edge or something. If you have reasonably flat stones to work with and want a simple rectangular slab, try using feathers and wedges to cut it. Feathers and wedges are by far the most consistently reliable means to cut a rock, though they cannot be used to fine-tune any surface.
Masonry Joint Standards
While traditionally there were several standards used for stone masonry, the general rule of thumb for masonry joints was that there should not ever be a gap larger than 1″ between any two rocks, with no gap greater than 1/2″ being far better. Also, vertical joints should ideally be spanned across with the next stone above to avoid so-called “running joints” where the vertical joint remains unbroken through several courses of masonry. It is generally grudgingly conceded that a running joint between only two courses is acceptable.
And Now, in Practice….
Fortunately, stone masonry is far more forgiving than one might think; there are many old structures that break various rules and standards of masonry yet are still in use.
Best of all, you will find that as you practice and continue to find ways to improve, each masonry project will be better than the last. Just remember: Except for major structural problems (such as a stone that has been accidentally broken free, for example), do not take down what you have finished; doing so only leads to discouragement and in the long run does little towards improving your skills. If you take something apart again for every perceived flaw, you may find that you never finish building! The best rule of thumb is to keep on building, and, if you see a noncritical flaw (and, believe it or not, most flaws in masonry are, in fact, noncritical) in something already erected, make a mental note of how to prevent the problem from happening next time. Then make changes in your construction methods as needed. And if you are a beginner and unsure about how to build with masonry, try building something temporary or unimportant the first few times to get the hang of building with stone, and maybe consider using mortar for your first permanent project.
Stone Arch Bridges 