All arches, whether of stone or brick, should be built on wooden centres made to exactly fit the curve of the arch and carefully set. The centres should have ample strength to support the weight of the arch and much of the wall above, as it is undesirable to put any weight on the arch until the mortar in the joints has become hard. Centres are usually made with two ribs cut out of plank and securely spiked together, and the bearing surface formed of cross pieces about 1x2 inches in size nailed to the top of the ribs, as shown in Fig. 109. The ribs forming the supports for the cross pieces should be placed under each edge of the arch, and if the depth of the arch exceeds 12 inches three ribs should be used. The centre should be supported on wooden posts resting on blocks set on the sill or some sufficient support below. It should not be removed until the mortar in the arch joints has had ample time to set.

Centres for spans of considerable extent are framed together with heavier timbers and in a variety of ways. The general method is shown by Fig. no, which represents a centre for a 10-foot span. The framework, indicated by the straight pieces, is made of 6x6 or 4x8 timbers, and to these are spiked pieces of plank cut to the outline of the arch. The cross pieces are then nailed to the top edge of the planks, as in Fig. 109. Such a centre should have a support under the middle as well as at the sides. As the centres are only required for temporary use, architects generally allow the carpenter to construct them as he deems best, but the superintendent should satisfy himself that they are of ample strength and well supported before the masons commence building the arch.

Fig. 109.