Chap. XXII.

TIMBER ROOFS.

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both equal in weight and strength, when properly proportioned. Roofs which are de-pendent upon the principle of a truss and straight tie-beam should have all their tim-bers proportioned to the duties they perform, and few subjects deserve more attention,as it seldom happens, that in large buildings we can place the several timbers exactly wherethe strength of the roof requires them.

The Theatre at Bordeaux has a fine example of roof, which is adapted to all the purposesrequired for the arrangement of the scenes: the whole, strutted and braced in a judiciousmanner, is partly curbed, and the plate, where the bend is made, is secured by a piece oftimber bolted to the tie-beam.

A curb roof is sometimes preferred,both on account of a greater spacebeing contained within it than a plainroof of the same height, and of itsexerting less strain on the tie-beam orabutments: the carpenters of themiddle ages thoroughly understoodthe mechanical prin-ciples by which theequilibrium of a roofcould be maintained,and the examples theyhave left us are de-serving of ourparticular at-tention andstudy.

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Fig. 2119. THEATRE AT BORDEAUX.

Roofs of theatres, or any building where the opening is too wide to be spanned with oneor even two pieces of timber, are generally framed by a combination of pieces, which in somedegree assume the property of an arch constructed in masonry, but their principles of sta-bility differ: all carpenter’s work is dependent upon the strength of the material and mannerin which it is framed or put together. When timber is strained by a force that destroys thebalance existing throughout its parts, that strain should be examined, and it should bedecided whether it is produced by stretching or compressing : in all framing there will beone point in a neutral state, as in a solid piece of timber.