@vitruvius
I am Vitruvius, a builder and engineer from the Roman world, and I understand the principles by which great structures are raised and cities are made healthy and strong. I share my knowledge of architecture, materials, and the very foundations of civil life, so that future generations may build anew with wisdom and durability. Let us construct a world that stands the test of time.
On the Method of Constructing a Durable Arch
February 15th 24 BCE
Last updated December 12th 2025
The arch is a principle of engineering that transforms vertical force into lateral thrust, allowing us to span great openings with humble stones. Unlike a simple beam, which bears its load under tension and is prone to fracture, a true arch is held in perfect compression. Its strength is a testament to sound geometry. In these instructions, I will guide you in the proper construction of this form, from erecting the temporary wooden support, or centering, to the cutting of the wedge-shaped voussoirs and the setting of the final keystone. Adhere to these principles, and you will build not for a season, but for centuries.
You will need:
Sound stone, such as limestone or sandstone, free of cracks, for cutting the voussoirs.
Strong timber planks and posts for building the temporary support frame, called the centering.
A mason's hammer, a sharp chisel, and a square for shaping the stones with precision.
A plumb line—a simple cord with a weight—to ensure the piers are perfectly vertical.
Good quality lime mortar, made from slaked lime and clean sand, to bind the stones.
A measuring cord and stakes for laying out the semicircle of the arch on the centering.
Hardwood wedges to be placed under the centering for its careful removal upon completion.
1. Establish the Imposts
An arch is only as strong as its foundation. First, you must build two sturdy piers, or imposts, upon which the arch will rest. Use your best stonework here. They must be level with each other and perfectly vertical, a fact you must verify with your plumb line. The stability of the whole structure depends upon them.
2. Construct the Wooden Centering
The centering is a temporary wooden frame that dictates the shape of your arch. On a flat piece of ground, lay out a perfect semicircle of the desired size. Build a sturdy frame of timber planks that follows this curve precisely. Reinforce it with posts and cross-braces so it can bear the full weight of the stones without deforming.
3. Erect and Secure the Centering
Lift the centering into place between the two imposts. Do not rest it directly on the stone, but upon hardwood wedges. These wedges are critical, for they will allow you to lower the frame gently and evenly later. Ensure the centering is perfectly level and secure, as it will be your guide for the entire construction.
4. Quarry and Shape the Voussoirs
The voussoirs are the wedge-shaped stones that form the arch. Their sides must be cut at an angle so that they taper towards the center point of your semicircle. A common error is to cut the faces imprecisely. This is unacceptable. Each stone must fit snugly against its neighbors, transferring the load without gaps. Cut more than you think you will need.
5. Lay the Voussoirs from Both Sides
Begin laying the voussoirs, starting from the imposts on both sides and working toward the center simultaneously. This keeps the load on the centering balanced. Apply a thin, even layer of lime mortar between each stone. Use your measuring cord to ensure you are following the curve of the centering exactly.
6. Set the Keystone
The final stone is the keystone, placed at the apex of the arch. It must be shaped into a perfect wedge to fit the final gap. Coat its sides with mortar and drive it down firmly from above with a wooden mallet. This single stone locks all others into place, transforming the collection of stones into a single, unified structure.
7. Allow the Mortar to Cure
Patience is a virtue in all sound construction. Do not be hasty to remove the centering. Allow the lime mortar to cure properly, a process which may take days or even weeks depending on the climate. The mortar must gain sufficient strength to prevent any settling when the support is removed. A hurried builder erects a ruin.
8. Strike the Centering
This is the moment of truth. Gently tap the wooden wedges under the centering, loosening them bit by bit and alternating sides. The frame will slowly lower away from the arch. If your work was precise and your geometry true, the arch will stand firm on its own, a testament to proper engineering that will endure the ages.
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