Stone Bridge: How to Build a Roman Arch Bridge

Encountering a rough-cut stone bridge is always an experience of wonder to me. Every stone bridge, from an unnamed walking bridge to the 902' wonder of Pont du Gard is a testimony to the diligence and ingenuity of builders plying one of the world's most ancient trades: Masonry.

This instructable will provide a step-by-step tutorial in how to build a Stone Bridge using hand-cut stone for under $1000.

WARNING: Building a stone bridge is not an easy finish-it-over-the-weekend sort of deal. You are building a legit piece of architecture that could last for centuries. If you want to do this, it is simple enough, but not easy; you're gonna have to work for it. Some buddies, Sam, Adam, Dave, and I built this bridge completely from hand cut stone harvested on-site. We put at least 300 hours into it, and we couldn't be more pleased with the results.

We didn't think of documenting the process until we were 2/3 of the way done, but I have drawn up some figures to help with each step.

Supplies

Much of this stuff is common household tools/supplies, but if you don't already have it, all can be purchased at your local hardware store (possible exception would be plug and feather which can be purchased on amazon.com. Assuming you have the household tools already, you're looking at $400-$800 in supplies

1) Tons of stone (Like seriously, TONS. The bridge I built spans roughly 10 ft and I probably used 5 tons of stone, harvested from the stream and the surrounding hillside. Preferably, use whatever stone you have on the property! It will look great and give you immense satisfaction to see your project built out of stone native to your land. Shot rock, [the largest size available] purchased at your local quarry will take more time to cut, but will also work great. A third option is to buy pre-cut landscaping stone from a landscaping supply company. This will be the most expensive, but you will save a ton of time cutting stone.)

2) Household Tools: A hammer (preferably 2 or 3 pound in weight with a beveled end), a shovel, a drill, a circular saw, screws.)

3) Masonry chisels: (at least one 3" wide and one 1.5" wide) ($10/ea)

4) A rotary hammer drill with 9/16 bit (for drilling holes used to split stone; I got mine at Harbor Freight for $80)

7) 3/4" Plywood, 1/4" plywood, 2x6 boards ($100-$150)

8) Plug and feathers (also called wedge and feather: by far the most specialty item, but available online. Get at least 7 of these bad boys. They are your bread and butter.) ($10 for a pack )

9) Mortar ($5/ bag. and you'll need a bunch, depending on your project size.)

10) Basic concrete tools: a trowel $5, a shovel$12, and if you want, a grouting bag $5

11) Concrete for the bridge foundation ($100-$150 a ton if delivered by a truck)

Step 1: Understand the Science

Stone bridges all have arches supporting them. Stone has poor tensile and shear force resistance, but excellent compressive resistance: Like 28,000 pounds/sq. inch compression resistance. So we need to use that in our bridge. The way we do that is by creating an arch of squared off rocks that all run parallel to the radius of your arch.
To understand this, compare fig. 1 and fig 2:

Figure 1 shows a bridge that is basically just a block wall that someone cut a hole out of. This bridge will collapse because it has only the shear resistance of the blocks keeping it up over the arch.

Figure 2 shows a bridge that is supported by an arch of stones, each of which is parallel to the radius of the imaginary circle that is an extension of your arc. This is crucial because as load is applied to the top of the bridge, these stones are compressed into each other horizontally, locking them together and supporting the load of the bridge. Because of this compression with the arch of stone, these bridges age extremely well. As they settle they just become more and more locked together in compression, thus providing greater strength to the bridge. One arch bridge is thought to be about 3,300 years old!

Fig 3 shows also how the compression load of the bridge will try to "flatten out" the arc of the bridge by pressing the stones apart. If they are not firmly grounded on a good foundation, these bottom stones will slide over time, flattening the bridge and causing collapse.

Step 2: Plan Your Bridge

I assume if you are building a bridge, you want it to go across something. So measure the width of that thing and figure out how long your bridge will have to be to span the gap. (if it is a creek, for example, plan on having the arc of the bridge smaller in diameter than the width of the channel. That way, the lateral force produced by the blocks compressing together presses the bridge into the bank of the stream, which is much less likely to move than anything you build. See Fig. 4) If your span is longer than 10 ft, consider making it a double arch bridge see fig. 5.

Then think about what things you'd like to be able to cross your bridge. If it is just walking/biking/horseback stuff, 4ft is a fine width. If you want a car to drive over it, you need minimum 8 ft width.

Step 3: Pour a Concrete Footing

Now pour a solid concrete footing on which will rest each side of the bridge. Make it a minimum of18" below the level of the stream bed and 6 inches wider than the base of your bridge on all sides. Pour the concrete at least 8" thick, reinforcing with 1/2" rebar placed in a grid at intervals of 12". I recommend just having a concrete truck come out and pour it for you. You will probably use at least 2 cubic yards of concrete, so hiring a company to bring it will probably save you a ton of time and money.

If you are a novice to pouring concrete, just look up a youtube video on how to do it! It's very simple, and there is a ton of content on youtube that will help with this.

NOTE: Digging below the level of the stream bed can make water pool up in the foundation footing. Our stream has a flow season and a dry season, so we poured the footing during the dry season. If you don't have a dry season, you will have to divert the flow of the water to one side while you pour the footing on the other. Then reverse the process and you have 2 solid footings!

Step 4: Build Your Wooden Support Frame

Now it's getting interesting! This is where you build a half-circle shaped wooden insert that will support your bridge's stones as you build it.
First, plot your arc onto a piece of 3/4" plywood. (SEE ATTACHED VIDEO). The easiest way to to this is to hold the plywood vertically with its long edge on the ground. Then take a piece of string and tack it to one of the top corners of the plywood. Then, holding the string up to the opposite corner, you can release more and more string until you have a nice arc running from one corner to the other. This sounds janky, but it works great to mimic the compressive load of the bridge. Since the stones are going to be taking pure compressive force, the easiest way to plot this on a curve is to reverse it by plotting it with pure tensile force (the string). This will produce a parabolic arch that will distribute the weight of the bridge along your arc.

Now that you have your arc plotted on the plywood, cut two pieces of the plywood identically. Now cut your 2x6 boards to the width of your bridge (let's say 4 ft.) and use them to span the 4' gap between your two arched plywood boards. Place 1 2x6 every 12" all the way along the arc of the ramp. Then take your 1/4" plywood and screw it as a veneer over your 2x6 boards this will create a bridge insert that basically looks like a huge cylinder that has been cut in half and hollowed out. (You may need to run extra bracing beams from each 2x6 into the ground to support the gargantuan weight of the stones as they are stacked on the form).

NOTE: If you are doing this over a stream, you will likely need to cut a hole out of the bottom flat edge so that water can flow through as you build the bridge.

Step 5: Cut Your Stones

Attached is a video explaining how to split stones using feather and wedge.

We cut all of our stones using this technique or using a simple masonry chisel. To make this easier and get more finished cuts, you can use a diamond tooth concrete saw blade to cut your stones to size.

The basic rule is this: it doesn't matter how thick or long your arch support stones are. It just matters that they are all the same width. This is crucial as you want your ring of support stones to all have the same width so that you have even compression distribution.

(In the above photo, you can see that 4 of the stones are not the same width [or height, depending on perspective] as the rest. This was a mistake we made, but so far it has not damaged the final product!)

Step 6: Place Arch Support Stones

Attached video explains how to do this in more detail.

Summary: Place cut stones parallel to radius of bridge arc all the way across the wooden frame using mortar in-between each stone to create the correct angle and to keep good surface contact in between each stone. You will likely need to place several stones end to end on each level of the ring to span the width of the bridge. See attached photo of underside of our bridge. It is important that all of these stones run parallel to each other as you see in the photos.
The cap stone, (the one at the top of the arc) is crucial. You can see it in one of the above photos. It is larger and thicker than the other stones on the arc and serves as the key to the whole structure. That stone alone we had to split into thirds just to place it. It probably weighed upwards of 400 lbs.

NOTE: Each of the arc-support stones varies in thickness and length, but they are all the same width. Cut all of your arc-support stones the same width so that you have a consistent width in your supporting ring. 8" is more than enough width to resist the compression of a bridge this large, but we did 10" just to make sure we were well supported.

Step 7: Reinforce Arch With Concrete (Optional)

Now that you have built your supporting arch, the hard part is done! But you can't pull out your form just yet. If you want to be a purist and skip this step, be my guest. But I recommend pouring a 4" sheet of reinforced concrete down over the top of the arch. Reinforce this concrete with normal concrete wire. This will help your bridge keep its arch rigid and keep stones from falling out. Leave about a 6 inch space on the edges though, since you will want to keep the side-walls aesthetically pleasing with only natural stone.

Step 8: Build Side Walls

Now you build walls on each side of the bridge using natural stone and mortar. I recommend using a plumb line for this step to keep the walls perfectly vertical. The purpose of these side walls is to basically make a box that you can put tons of dirt and rock into so that you can pave a flat road across the top.

Figure 9 shows this step in progress.

If you have not worked with stone and mortar, look up a youtube video on how to use mortar. There's a ton of great content out there.

The basic guidelines are these:

1)Use a plumb line to make sure your wall stays perfectly vertical.

2) Mix your mortar to the consistency of a thick fruit smoothie

3) Start your wall directly on top of the concrete footing

4) Lay a "bead" of mortar down on each run of stone. The bead should be on the center line of the stones and should be about 3" deep and half the width of your wall.

5) press the stones into the mortar bead firmly, keeping the exterior edge of the stone flush with your plumb line.

6) Wipe up excess mortar and make sure all vertical joints between stones are properly packed.

7) After concrete has cured for an hour or so, come back with a stiff brush and scrub the faces of the stone to remove any excess mortar that is obscuring the natural beauty of the stone.

Step 9: Back-fill the Hollow of the Bridge With Rock or Gravel

Now you should have a hollow bridge: it has a right and left retaining wall and a supportive arch of stone with concrete overlaying it. Now you fill that hollowed out section with loose rock/gravel and tamp it down thoroughly! preferably rent a power compactor and go to town on it! you will end up with gravel coming up to within 4" of the tops of your retaining walls. Figure 10 shows this with one wall removed.

Attached photo shows the retaining wall and it filled up with gravel.

Step 10: Remove Wooden Frame.

Simple step. Just pull that puppy out of there. You might have to disassemble it in place to get it out. Just in case you didn't place your stone correctly, please don't go crawling under the bridge while you're removing the support.

Step 11: Lay Down Your Walking Surface

You are almost done! all you have to do now is decide what surface you want to walk on for your final product! We decided that since the whole bridge was natural stone, we would keep the same aesthetic and use natural stone pavers. To do this, we simply poured about 3" of mortar and placed flat sandstone pavers down one at a time, fitting them like a jigsaw puzzle.

After letting the mortar underneath them cure, we simply grouted the edges with mortar so that we achieved a flat walking path. To do this, we got a grouting bag from lowes (it looks like a giant triangular frosting bag), filled it with mortar, and squeezed a bead into all the gaps.

You can do it this way or just pour concrete over the whole thing or asphalt or seriously anything you want!

Step 12: Clean It Up... and You're Done!

Now you clean it up! take a small chipping hammer and go over the whole thing, chipping off all the excess mortar. If you have thin layers of mortar built up on the stones and obscuring the natural look (like in above photo,) try using muriatic acid and a wire brush to clean them off.

Now you're all done and you can have your Dad pose next to it for a photo-op! I hope this tutorial was helpful, please feel free to ask any questions you have.