This section is from the book "A Treatise On Architecture And Building Construction Vol3: Stair Building, Ornamental Ironwork, Roofing, Sheet-Metal Work, Electric-Light Wiring And Bellwork", by The Colliery Engineer Co.. Also available from Amazon: A Treatise On Architecture And Building Construction.
143. Stone roofs are not much used at the present time, except for covering small vestibules, porches, turrets, towers, and monumental buildings. They may be flat or pitched. Large slabs are used for flat roofs, and small ones for pitched roofs. They may be laid either with lap or butt joints on a filling resting on the vaulting beneath. For towers and spires, the stone is laid in tiers, either with or without a lap joint.
144. In Fig. 127 is shown the construction of a flat roof, which may be applied either to vaulting or stone purlins, the latter being shown. The purlins a are rabbeted to receive the ceiling slabs b, and the roof slabs c are laid and bedded in cement concrete d.
The Greeks, and the Romans after them, used the same method of supporting and laying the roof slab, with this difference, that they cut the slabs, as shown at a, Fig. 128, so that they formed a channel with fillets b on each side, and covered the joint with a cap c.
For pitched roofs, ashlar slabs are used, laid as shown at (a), Fig. 129, in the same manner as shingles with broken joints; the slabs are bedded or laid in cement on the filling over the vault.
At (b), Fig. 129, is shown a much better slab roof; the slabs are cut with a rabbet and a lap joint a. The cap or ridge stone is made with a rabbet and a lap joint b on each side at the edge of the wing, the same as the lower edge of the slabs. The advantages of the rabbet and lap joint are, that the stones cannot leave the roof if they should work loose, as they would if laid with a simple lap, and there is no open vertical joint for the passage of water.
The plain butt-joint roof is shown at (c), Fig. 129.
The gutters are cut out of a single stone, as at a, Fig. 130, the roof slab overlapping it. The water is carried from the gutter through a channel cut in a stone extending the full width of the wall. This channel b meets the outlet of the gutter, and extends through the stone to a point outside the wall and discharges into the leader head c.
145. For turrets, towers, and spires, the roof, when built of stone, is complete without the interposition of framing. At (a), Fig. 131, are shown a one-half elevation and a one-half section of a smooth, flush-faced, conical roof; the beds of the stone are kept level as in regular walling, while the vertical joints radiate from the center of the plan, as shown at (6), and bonding as indicated by the dotted lines. The outer surface of the stones used for such roofs may advance, giving the suggestion of a lap only, as at (c); or they may be rabbeted and lapped, as at (d). The terminal, cap, or finial should commence at a point where the roof stones become too small to look well, as at e, and should be, preferably, of one piece of stone.