Dome Building

Part of Structural Engineering

Constructing curved masonry roofs that span large spaces without internal columns by carrying loads in compression along curved surfaces.

Why This Matters

A dome is the three-dimensional version of an arch — instead of spanning a rectangular opening, it spans a circular one. A dome covers the maximum floor area with the minimum structural material of any masonry form. It creates vast, column-free interior spaces that are nearly impossible to achieve with flat construction. The Pantheon in Rome, built around 125 AD with a 43-meter span, stood as the world’s largest dome for more than 1,300 years. It used no steel reinforcement and is still structurally sound today.

For rebuilding communities, domes offer a path to large communal spaces — assembly halls, granaries, cisterns, furnace covers, oven roofs — using only masonry and simple tools. The construction methods require no sophisticated equipment, just careful geometry, patient work, and an understanding of the forces involved. Even a small dome (3–5 meters diameter) over a community oven or cistern provides valuable practical experience for larger structures.

The unique challenge of domes is that they produce outward-pushing horizontal forces (hoop tension) at the base, which masonry cannot resist. Every dome design must address this with thick walls, buttresses, an iron or timber tension ring, or some combination.

How a Dome Carries Load

Meridional forces: Vertical loads on the dome create compressive forces that run along the meridians — lines from the crown down to the base, like lines of longitude on a globe. These forces are always compressive throughout the dome — the dome acts like a series of arches spanning from crown to base.

Hoop forces: The meridional forces push outward at the dome’s equator. This outward thrust is resisted by the masonry running around the circumference (the hoop direction). In the upper portion of the dome, hoop forces are compressive — the dome is being squeezed together. In the lower portion (below approximately 52° from the crown for a full hemisphere), hoop forces become tensile — the dome is being pulled apart. Masonry handles the upper compression well but cannot handle the lower tension.

The tension ring: At the base of the dome where tension hoop forces are greatest, a tension ring (iron chain, timber ring, or reinforced concrete ring) runs around the circumference. This ring is the key element that contains the outward thrust and allows the dome to stand. Without it, the base of the dome cracks vertically (meridional cracks) and the dome splits.

Dome Geometry

Full hemisphere: 180° arc from base to base. Very strong but very tall for a given span.

Segmental dome (less than full hemisphere): The most common practical form — a shallow dome on a drum wall. Reduces height, but increases horizontal thrust at the base.

Pointed dome: Like a pointed arch, reduces horizontal thrust at the base compared to a hemisphere. Common in Byzantine and Islamic architecture.

Saucer dome: Very shallow segment of a sphere. Low height, but extremely high horizontal thrust — requires very strong tension ring.

Setting out on the ground:

1. Mark the center point of the plan
2. Strike the full circle of the base
3. Calculate the dome radius from desired span and height: if span = D and height = H, radius R = (D²/8H) + H/2
4. Use this radius to set out the curved template for masonry courses

Constructing the Dome

Method 1: Without centering (corbeled dome) The simplest dome construction — each course of masonry projects slightly inward over the one below, like a stepped pyramid but with the steps on the inside. No temporary support needed.

Steps:

1. Build a circular drum wall (the base ring)
2. Begin corbeling inward — each course overhangs the previous by 1–2 inches
3. Each stone must be long enough that its outer end (outside the wall) is heavier than its projecting inner end, or it must be weighted or mortared to prevent tipping
4. As the courses spiral upward, the opening closes
5. Top with a single capstone or small hole for smoke/light

Corbeled domes (like Irish passage tomb chambers) can span up to 5–6 meters without any centering, but they are not true domes — they carry load in stepped compression, not as a smooth shell.

Method 2: With centering (true smooth dome) A rotating centering frame guides each block into position. At the base, a rotating arm (like a compass arm) can swing around the center point and guide block placement. As the dome rises, adjust the rotating guide to the correct height and radius.

Construction sequence:

1. Set a pivot post at the dome’s center point
2. Attach a rotating arm of exactly the dome’s radius
3. Use the arm’s end as a guide — every stone’s inner face must touch the arm’s endpoint
4. Work in courses around the full circle before moving to the next course
5. Apply mortar between courses; allow to set before adding the next course
6. At the crown, lower the tension on the arm and place the final courses without guide

Corbel rings: For each course, tilt the stones slightly inward so gravity holds them in place while the mortar sets. The inner (intrados) face of each course tilts toward the dome’s center. Stones that tilt outward will slide before the mortar sets — always tilt inward.

The Tension Ring

Every true dome base needs a tension ring. Size it for the horizontal thrust from the dome.

Estimating hoop tension (approximate):

Hoop force (lb) ≈ W × r / (2 × h)

Where W = total dome weight (lb), r = dome radius (ft), h = dome height (ft)

For a dome 20 feet diameter, 8 feet rise, weighing 50,000 lb: Hoop force = 50,000 × 10 / (2 × 8) = 31,250 lb

An iron chain or wrought iron rod of suitable cross-section to carry this tension in the ring beam provides the necessary restraint.

Ring beam construction: The easiest approach is to build the base drum wall with an embedded iron chain or rod running around the inside circumference at the dome springing level. Alternative: cast an iron ring in sections and bolt together. Or: use a heavy timber ring (half-lap jointed at least 4 times around the circumference, with iron bolts at each joint).

Oculus and Openings

A circular opening at the crown (the oculus) — as in the Pantheon — actually improves dome stability by reducing tensile hoop forces near the crown. The edge of the oculus must be reinforced with a compression ring (an iron or masonry ring that ties the radiating meridional forces together at the top).

Doorways in a dome drum: Openings in the drum wall interrupt the tension ring. Each opening must be bridged by an arch, and the arch must transfer the interrupted tension ring force around itself. Use iron tie rods from each side of the opening to reconnect the tension ring through the arch.

Small Practical Applications

Before attempting a building-scale dome, practice on smaller structures:

Clay oven dome: Build from wet clay over a sand form. The sand is removed after the clay dries and is fired by the first fire. Easy to test the technique and understand the geometry.

Cistern dome: 3–4 meter dome over a water cistern provides a test of real masonry dome construction. Important: an underground cistern dome carries soil surcharge load (weight of earth above it), which changes the load distribution — the surcharge actually helps by increasing the compressive forces and reducing tension.

Granary cover: Small dome over a grain storage pit protects grain from weather and rodents. A corbeled dome from local stone is suitable for spans up to 4 meters.