A field guide to building a traditional brick barrel vault — from centring and springer courses through to keying the crown, the way Kerala's master masons have always done it.
A brick barrel vault carries its load entirely through compression, the same principle behind a stone arch extruded into a tunnel form. Get the geometry wrong and the vault either won't stand on its own once the centring is struck, or it will develop cracking along the haunches within a few seasons.
Construction begins with the centring — a timber formwork built to the exact curve of the vault's underside. Bricks are then laid in radial courses, each brick angled so its bed joint points toward the vault's notional centre, not laid flat as in ordinary wall construction. This is the detail most commonly got wrong by crews unfamiliar with vault work.
The springer courses at either haunch carry the highest thrust and are laid first and most carefully, often in a stronger mortar mix than the rest of the vault. Work proceeds upward from both haunches simultaneously and symmetrically, meeting at the crown.
Keying the crown — closing the final gap at the top of the vault — is the moment of truth. Done correctly, the vault becomes self-supporting the instant the last brick is keyed in; the centring can then be struck (removed) days later once the lime mortar has set. We strike centring gradually, easing it down over several hours, to let the vault settle into its final bearing without shock loading.
