It is often desired to build a stone arch bridge. Such a structure is scenic and, of course, lends a fine touch to landscaping. The question is how do you go about doing it?
We have provided this short guide as a very general rule of thumb; the details of your construction will vary. We’ve based it on a small footbridge we built, including some lessons learned afterwards from the process. We assume no liability for use or misuse of this information, but we do hope this will be helpful to the DIY enthusiast.
Finding a Location
The first thing to do, of course, is to determine the location of the bridge and the amount of waterway it is to handle. If possible, during heavy floods, determine how much water there is roughly. For small ditches it will not hurt anything for water to flow around or over the bridge. This may, however, be very inconvenient as, not only will it tend to muddy up any paths but also the bridge itself, and it is during heavy flows when a bridge is the most useful.
Roughly determining the area of water in the ditch at a given point allows for a calculation of how much total open surface area the arch(es) must have to accommodate maximum flows. It is typically desirable for the underside of the top of the arch to be level with or higher than the banks; however, this can cause a steep grade. We might add that, while with a bit of determination it can certainly be done, we would strongly discourage the amateur from undertaking long spans! About 8 feet is probably as long of a span as a first-timer would want to build. An 8-foot span is also convenient — plywood, which is a popular choice for building arch forms, is sold in 8’- by 4’-sheets — the perfect choice for an 8-foot-span Roman arch or anything flatter.
Choosing an Arch Shape and Design
The next step is to create a design, determining the number of arches and the shape they will take. Once this is decided, a little bit of geometry will allow one to determine the shape of the temporary formwork.
A wooden form can be cut out of plywood with slats nailed on top, or, if you have a lot of scrap plywood, you can do what we did and attach numerous plywood pieces cut to the right shape together, each spaced a few inchesapart. Alternatively, rebar or even PVC pipe can be bent and held into an arch shape, saving the need for cutting wood, though the resultant shape will be parabolic.
For arches flatter than a Roman arch, “skewbacks” of some sort will be needed; these are triangular stones on which the arch rests. Laying the stones such that a sloping face is created for the arch to rest on is a valid alternative. Regardless, we’d recommend making a template of the angle needed for this purpose.
Laying the Foundations
The next thing to do is to lay the foundations. If you are blessed with bedrock that is only a couple of feet or so under the ground you can simply lay the foundations on this; otherwise, for mortared work at least, the foundations must be placed below the frost line. For motarless work the arch can simply be laid on the hard subsoil; dirt seeps between the joints over time and will hold water, causing frost heaving anyway. If well laid, the masonry should be able to handle this.
When laying the bridge foundations on soil, as opposed to rock, it is strongly recommended to have the foundations wider at the base than the top so that they present a larger surface area to the soil. Also, we like packing gravel into the soil in some cases to help make a hard, compact resting surface for the foundations.
When mortaring, please note that it takes practice. For looks, avoid globing it on; it is virtually impossible to remove once it’s dried (as we, unfortunately, discovered too late) and, well, sometimes the structure would be more attractive without it.
Also use mortar — not cement! Hard cements can crack the stones because, when freezing and thawing, the stone will have more give than the mortar. We used type N mortar for our structure — and even it appears to have been a little too hard.
One observation about concrete, by the way, is that its ability to damage stonework appears to be governed by how and where it is used. The concrete scour aprons often used around stone arch bridges, for instance, rarely (if ever) seem to be a cause of damage to the bridge. On the other hand, indiscriminately smearing cement over the surface of stonework appears to greatly speed up the deterioration of the stones. Stuffing concrete into a gap in a masonry structure is also generally considered a bad idea in the long run.
Creating the Formwork
Once the foundations have been built up to the level where the arch is to start, some provision must be made for placing the temporary formwork the arch rests on during construction.
We added a narrow ledge on each abutment. Don’t forget provision for removing the form later! For our small 4.5’-span arch we rested the formwork on 2x4s which were laid with the narrow ends on the ledges. Then we rested the form on top of these. When the time came, it was very easy to topple the boards out, thereby dropping the form down.
Remember, that when building a flat arch, the flatter the arch the more stones behind the skewbacks there must be to prevent collapse. We would not recommend building an arch flatter than a 90-degree segment of a circle for a first-time attempt. We built a 120-degree segment of a circle, and it seems to be about as flat as you can go without a major sliding problem; it only needed one stone behind the skewbacks for complete stability.
When building the arch, we used numerous thinner rocks, as mentioned in our article about turning an arch, making the arch easier to make.
Also, build up the arch from both ends fairly evenly, otherwise it is occasionally possible to have some form of dramatic accident from the grossly imbalanced weight on the arch form.
We did not cut the stones to fit; we picked out and laid the stones so that they would naturally curve the arch. We then used numerous stone chips to help stabilize them.
If you are using mortar, we’d recommend making a thinner (though not liquid) batch, placing it on the arch stones, and shoving stone chips in once the next course of masonry is laid. Be sure to use thick, strong, waterproof gloves to prevent chemical burns, or better yet, gloves and some tool. Note that, when you get to the top of the arch, the mortar will want to fall out; some method of keeping it from seeping through any and all gaps and cracks in the formwork is necessary.
What we did for mortar was to dry-lay the arch, and, when it was finished, remove the formwork. We next pointed the joints — filled in all surface cracks on the arch with mortar — and then, when that was finished, made a liquid batch of mortar which we poured in on top (pouring in the mortar was the traditional method of arch building). Frankly, I don’t recommend this. Besides being an absolute mess, I must confess to being unsure how the old-time masons poured the mortar on the more horizontal sections of the arch. The mortar, of course, wants to pour down, and the best one could do was to use a board flushed up against the arch and use it to try to guide the mortar into the joints. I’m sticking with this is not a great idea — although one does get some comfort out of seeing the arch freestanding before the mortar is added!
Removing the Form
Once the arch and sufficient weight behind the skewbacks are in place, the form can be removed. Then all that’s left is finishing up the abutments, adding approaches as needed, and, for the finishing touch, adding “coping” stones along the top of the entire arch. The coping stones provide some curbing and also allow for hold in the walkway.
For safety, some form of guardrail can be added; just note that it must be mounted to the stones and not into mortar joints.