The old cast lead is much thicker than the modern milled lead, being as much as twelve or thirteen pounds to the foot of surface. It is certainly not quite even in thickness, and is subject to faults in the casting, but it is not so liable to crack as is milled lead. The old lead employed has also a considerable quantity of silver and arsenic in it, which was the cause of the beautiful white oxide it obtained. Modern lead blackens as the preparation of lead now includes its “de-silverisation.” The acid of timber which has not lost its sap decomposes lead; old building timber was water-seasoned as only ship timber now is.
The chief difficulties that had to be overcome in the use of lead were the weight of the sheets of lead to be maintained in position, and the great dilatation of the metal under the heat of the sun, so that it had to be at once strongly attached and free to move. The method followed was to nail it at the top and roll the lateral edges together.
The roofing at Canterbury was of twelve-pound lead and about 2.0 between the rolls. The thirteenth century lead of Chartres Cathedral, “covered externally by time with a patina hard, brown, and wrinkled, and shining in the sun,” was in sheets eight feet long, attached at the top by nails with very large heads and held at the bottom by clips of iron that passed down between the sheets and turned over the bottom edge of the upper one. The rolls were formed by turning over the margins one in the other without a wood roll; they were much smaller than the modern ones.
Our milled lead is rolled out in sheets about 16 × 6 feet and is usually cut in half lengthways, and 41⁄2 inches is allowed in each edge to form the rolls which are thus 2′-3″ apart. Lead one inch thick is sixty pounds to the square foot, so six-pounds lead is 1⁄10th of an inch in thickness. We generally make the mistake of putting a longitudinal roll along the ridge, but it is not so done in our old roofs, nor should it be, for the running out of the rolls frets the ridge into a simple decoration.
The lead covering of old roofs should be jealously maintained—its loss is irreparable. If repair becomes absolutely necessary for the protection of the building, such lead should be recast, it should never be replaced by milled lead. The old metal is easily recast on the ground, and this is now frequently done, but not frequently enough. It was cast on a wood table with a projecting margin or curb all round; on this slid up and down a cross piece notched down to give the proper gauge to the lead which it levelled.
Where lead was applied to the vertical or steep planes of dormers or spires the interlocking of the sheets in herring-bone was a practical as well as an artistic expedient. Where nails had to be driven through exposed lead, in repairs or otherwise, flaps like little shields were laid over them soldered on the top edge. Lead, where used to incase wood tracery, as in the open work of spires or dormers, was secured by means of laps and rolls without solder so that it was free to expand and contract. The modern plumber is much too apt to employ soldered joints even in structural work.
Small openings were made like little dormers, for ventilation of the roof timbers, by dressing a stout piece of lead up into a triangle or half circle in front dying back on the roof with the back turned up under the tiles or slates.
Sometimes cast ornaments were applied to a slated roof; the disc with undulating rays on the slated apex of the north-west tower at Rouen is an instance.