Building Rubble Arches

Rubble Arch Detail

Rubble masonry is a versatile class of masonry, for it allows for the use of a variety of sizes and shapes of stones with minimal trimming. Just as rubble masonry can be used to build stone walls, rubble masonry can also be used to build an arch.

Rubble Arches: Overview

Rubble arches can be quite strong, even though a strange variety of stone sizes and shapes are used.

Mortarless Stone Bridge
A stone arch culvert using a rubble arch being tested for strength. It proved to be strong. Though the faces of this arch are fairly regular, the interior section of this arch consists of a variety of stone sizes and shapes.

Rubble arches still rely on the arch stones being placed neatly along the curve of the arch. The fact that the stones are largely uncut and are therefore a variety of shapes and sizes is what makes the rubble arch different from other arches with well-cut wedges of stone. A good rubble arch can actually be built much like a rubble wall, the only main difference being that an arch is, of course, curved, while a good stone wall is straight. Rubble arches can be surprisingly strong, as the stones, being either largely uncut or roughly trimmed, tend to break along their natural weak lines. Thus, the stones in a rubble arch will typically be used in their strongest natural form.

Laying Rubble Arches

Building rubble arches is easy. Just as for a rubble wall, shims are allowed. A rubble arch can be built with a variety of randomly shaped stones, but using flat slabs makes building the arch far easier.

A rubble arch under construction. Note that the arch stones are flat slabs, though they are a variety of different sizes and shapes. Using slabs like this makes construction easier. The fact that the slabs are thin helps to turn the arch. Note also how the arch stones are being built up in a stair-like fashion rather than one course at a time. This method of construction proved to make laying the arch easier.

The main consideration in building a rubble arch is to place all stones with their natural tapered side down. For those in limestone country where thin slabs of limestone are what is available, one cannot help but to notice that the stones are flat. Actually though, the stones are not as flat as they seem. Almost all stones have a natural, distinct taper, even if this taper is slight. The trick, then, is to place the thin end down, and shim up as needed.

Shimming Up

Shimming up a rubble arch is important, regardless of whether or not mortar is used. The shims ensure solid contact between stones. Furthermore, shims can be used to help make the curve of the arch. In fact, where heavy shimming is needed to create the round shape of the arch, the shims themselves essentially form small-scale masonry between the larger arch stones.

A closeup of a rubble arch under construction. Note how the larger shims are held in with smaller shims, forming a solid mass of masonry. Thus, the large arch stones are propped up at the requisite angle with the aid of many smaller, carefully chosen stones, which themselves are laid in a true masonry fashion despite being only inches in size. The smallest shims are mere millimeters in size. All these stones, large and small, collectively add up to achieve solid, high-contact surfaces within the arch.

To ensure the shims stay in place, care needs to be taken in stone selection to ensure there are no large gaps between the ends of the stones placed against the centering.

A side profile of the arch stones showing how the shims are laid to achieve the taper. Note how the ends of the arch stones resting against the centering form tight fits, which prevent the shims from being able to work out of the underside of the arch. One other detail to note is how the upper arch stones are not shimmed as meticulously as the lower ones; until all the major arch stones are laid, it is difficult to shim up the upper portions of the arch without inadvertently dislodging them. Once all the main arch stones are in place, you can go back and finish shimming much more easily, as the arch stones are better wedged in place and can’t accidentally be shifted out of line.

It may be found that it is difficult to find shims that will satisfactory stay in the last inch or two at the outer ends of the arch. This is fine, just increase the thickness of the arch as needed to compensate, to ensure adequate effective arch thickness. How much shimming needs to be done will depend on a variety of conditions. That said, some good, broad rules to remember are that a Roman arch will likely depend more on shims than a segmental arch; the thinner the arch stones the fewer shims will be needed; and the larger the span, the less each individual arch stone will need to be shimmed up to an angle. The shims are easily created by breaking up small stones.


It may be desirable to add solid backing to at least the lower parts of the arch if it is heavily shimmed up. Solid backing consists of masonry laid up against (and perhaps interleaving between) the stones of the arch. This solid material helps prevent shims from being vibrated back out of the arch, and can also act as shims in its own right, depending on how it is laid.

Arch Backing
A rubble arch showing the solid backing in place. The topmost stones of the arch have not been thoroughly shimmed up yet in this picture. Incidentally, this is the same bridge that later carried the bulldozer in the first picture shown in this post.

One other advantage of solid backing is that it can significantly improve the stability of the spandrel walls of a stone arch bridge.