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Because of the thrust of the arches making up the barrel vault, walls supporting it would have to be very thick or strongly buttressed. It was dangerous to make large openings in the walls, and impossible to do so in the vault itself, as removing any voussoir would cause it to collapse. This meant that little light would enter the structure, other than at either end of the barrel vault.
below diagram of
a barrel vault showing weight thrust
© Elizabeth Rankin and Amanda Skeet, Faculty of Arts, University of Auckland, 2004.