CHAPTER XIII.
EFFECTS OF SHOCK UPON BUILDINGS—THE 4TH MODIFYING CONDITION CONTINUED—RELATIONS OF ROOFING—MODES OF FALL.
As respects roofing of the ordinary class, of heavy timber framing, with or without common rafters superposed, or with heavy lathing only, to carry the ponderous ridge and furrow tiling already described; all that has been stated of the modes of giving way of flooring applies also, in like cases of wave transit, to it; with this addition, that it frequently happens, owing to the low pitch of Italian roofs, and the enormous weight of the tiling, added to the rudeness of workmanship in the framing, and the want of sufficient iron work in fastenings (iron being a dear commodity, and none made in the kingdom but by the old Catalan process, and all imported heavily loaded with duty); from all this, the roof frames often give way at the tye beams, and the principal rafters then thrust out the side walls, as the roof falls by the shock, if either emergent steeply or vertical. This is almost always the case when the roof is of considerable span, as over churches. In such roofs, "common rafters," are laid upon longitudinal "purlins," or beams stretching across the principals; and when the direction of wave transit is anything nearly horizontal, and in or near the line of the ridge of the roof the
