Trusses

Roofs in truss form developed using carpentry joints and some steel strapping, until the latter part of the eighteenth century when bolts, and even glues, started to be used to create large truss forms from lighter timber members. Such truss forms often used softwoods, as distinct from the hardwoods more frequently used in the shapes previously described. The large timber sections in oak particularly were becoming very scarce and of course very expensive. Whilst some significant advances in span were achieved, using the techniques described above, the domestic roof did not require very large spans and changed very little from the collared coupled roof. Indeed many small terraced houses built during the eighteenth and nineteenth century required no principals at all. The dividing walls between the houses were close enough to allow the purlins to rest on these walls, effectively using them as principals. Figure 1.10 illustrates a typical terraced house roof construction.

The larger properties where the span of the purlin was too long for one piece of timber, or where hip ends were involved, continued to use the established methods of construction using principals, collars and purlins, but it was common practice to omit the principals and to support the purlins off the walls below with posts or struts.

Fig. 1.10 Purlin and common roof.

Purlin strut

Load bearing walls -;

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