Chap. XXII.

TIMBER ROOFS.

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Fig. 2127.

Pfeiffer executed several roofsin Germany , which derive con-siderable strength from the intro-duction of inclined timbers, whichserve the double purpose of hold-ing up the tie-beam to the prin-cipal rafter, and strutting them :where timbers act as struts andties an iron bolt is almost indis-pensable to secure the joints; butas the carpenters of the middle

Fig. 2128. aoop at st. Paul, romb.

ages seldom introduced iron into their framing, they were obliged to formtheir joints in such a manner as to answer the same purpose; the shouldersof the timbers were accurately cut, to fit both the principaland tie-beam, into which they were framed and secured byan oak pin ; as long as the latter resisted fracture or draw-^ing down, the framing remained entire,and from the observations made uponseveral ancient roofs so constructed werarely find that anyinjury has taken place.

Fig. 2129. ROOP AT RIDING-SCHOOL, MOSCOW , AS IT IS AT PRESENT.

The celebrated roof of St. Paul’s-out-of-the-walls at Rome exhibited an admirable con-struction, and was perhaps one of the earliest where the upholding the tie-beams by ironrods was introduced.

The riding-house at Moscow , which at present exists, covers a considerable area; it isframed in an admirable manner, upon the same principles; a series of ties and struts insuccession are united into one simple and complete truss.

King Post Roofs , or their principle, we find adopted very early in Italy , and particularlyin the basilicas at Rome : in the church of St. Paul-without-the-walls, where the span isnearly 79 feet, was a finely constructed roof destroyed by fire a few years ago: it was con-structed of fir timber, and consisted of pairs of trusses, placed 15 inches apart, and each pairrested on the walls, with a distance of 10 feet 6 inches between them. The manner in whichthe trusses were framed together was as simple as it was ingenious, and a remarkably strongroof was formed by it: a king-post received the upper ends of the principal rafters, whichwere about 22 inches by 15; a collar or straining piece 15 by 12 abutted against the headsof two queen-posts, the position of which was maintained by additional timbers, placed underthe principals, and forming a double thickness where the greatest strength was required.

The Tie-beams were formed by scarfing two lengths of timber together, and securelystrapping them with three stout irons ; their scantling was 23 inches by 15.

The Collar , or upper tie-beam, was placed at two-thirds the height of the king-post, andby the introduction of the pairs of queens the tie-beam was bung up in three places to pre-vent its sagging; such a suspension of the tie-beam by the middle, and again at 14 feet oneach side of it, exhibits a thorough knowledge of the true rules which should direct the car-penters : no tie-beam should exceed the length of 15 feet, without some precautionssimilar to these being adopted, to prevent it from drawing in the wall plates and cripplingthe truss. In this example we have an early instance of suspending the tie by means of theheads of a king and a pair of queens, and which is now universally adopted, though theirons which have been used for the purpose have been variously formed; sometimes boltsand at others stirrup-irons secured by keys have served the purpose.

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