The " roods " themselves were not The simplest form is the " flat roof " consisting of horizontal wood joists laid from wall to wall as in floor construction.
The joists are covered with a waterproof material such as asphalt, lead, zinc or copper, the three last materials being usually laid upon boarding, which stiffens the structure and forms a good surface to fix the weatherproof covering upon.
" Lean-to," " shed," or " pent " roofs are practically developments of the flat roof, one end of the joists (which are now called " rafters ") being tipped up to form a decided slope, which enables slates, tiles, corrugated iron and other materials to be employed which cannot be used upon a " flat " roof.
Among the ancients it was in request for poles, rafters, joists, and for the construction of winepresses, tables and musical instruments; and on that account was so valuable that a plantation of cypresses was considered a sufficient dowry for a daughter.
The larger trees are sawn up into planks and battens, much used for the purposes of the builder, especially for flooring, joists and rafters.
First, he made a number of leathern tubes the ends of which he contrived to fix among the joists and flooring of a fine upper-room in which Zeno entertained his friends, and then subjected it to a miniature earthquake by sending steam through the tubes.
As a substitute for timber props at the face, pieces of steel joists, with the web cut out for a short distance on either end, with the flanges turned back to give a square bearing surface, have been introduced.
In large levels only the cap pieces for the roof are made of steel joists, but in smaller ones complete arches made of pieces of rails fish-jointed at the crown are used.
For shaft linings steel rings of H or channel section supported by intermediate struts are also used, and cross-bearers or buntons of steel joists and rail guides are now generally substituted for wood.
Steel-rolled joists, spaced 3 ft.
Probably the weight of steel used in this way is excessive, but the joists are cheap, readily procurable and easy to handle.
In the Matrai system thin wires are used instead of rods, and are securely fastened to rolled steel joists, which form the beams on which the slabs rest; moreover, the wires instead of being stretched tight from side to side of the slab are allowed to sag as much as the thickness of the concrete will allow.
The Williams system small flat bars are used, which are not quite horizontal, but pass alternately over and under the rolled joists which support the slabs.
Joists with flanges 7 and 7 in.
If the loads to be carried are largely stationary, and if the span is small, the floor joists can be readily proportioned by means of tables given in the handbooks issued by many steel companies.
The distance between joists should be limited to 5 or 6 ft.; horizontal bracing by means of diagonal rods is sometimes used, but should be avoided.