2If, however, there be not space for another wall, channels should nevertheless be made, and holes therefrom to the open air. Then tiles of the size of two feet are placed on one side, over the side of the channel, and, on the other side, piers are built, of eight-inch bricks, on which the angles of two tiles may lie, that they may not be more distant than one palm from each other. Over them other tiles, with returning edges, are fixed upright, from the bottom to the top of the wall; and the inner surfaces of these are to be carefully pitched over, that they may resist the moisture; they are, moreover, to have air-holes at bottom, and at top above the vault.
2But if there is not room enough for the construction of a wall, make channels with their vents extending to the open air. Then lay two-foot tiles resting on the margin of the channel on one side, and on the other side construct a foundation of pillars for them, made of eight-inch bricks, on top of each of which the edges of two tiles may be supported, each pillar being not more than a hand’s breadth distant from the wall. Then, above, set hooked tiles fastened to the wall from bottom to top, carefully covering the inner sides of them with pitch so that they will reject moisture. Both at the bottom and at the top above the vaulting they should have airholes.