Extended chord roof trusses occur in two principal forms, either extended top chords or extended bottom chords. In both cases, the support occurs on the extended member. They are predominantly used in conjunction with the fink or queen post truss but, with the exception of scissor and flat trusses, can be applied to other truss families.
Since the bottom chord, or tie member, does not occur at wall plate level but is raised, extended top chords are commonly referred to as raised tie trusses. The lower eaves height produces a cottage effect, allowing the new structure to blend in with period properties and, consequently, is attractive to some planning authorities. Other applications are to allow increased internal room height or as a design feature.
The extended bottom chord is normally used over dormer windows in raised tie roofs. It is also useful where the distance between the support wall varies. Designed for the maximum case, the extended leg can be cut back on site to take up the reducing span.
Hip end and roof intersections present specific problems and should be discussed at an early stage.
Structurally, the extended legs support the weight of the roof and resist the large bending forces imposed upon them. Additionally, the horizontal deflections that occur on raised tie trusses must be contained to a figure the supporting structure can accept. (For most structures it is recognised that 6mm can be tolerated at each support.) To achieve this, the extended legs must be strengthened using one of the following three methods:
(1) Increased Depth of Chord
The simplest solution is to increase the depth of the extended chord member, talk to our team for advice.
(2) Add Scabs
The extended chord can be reinforced by nailing or bolting additional members (scabs) to it.
Preferably the scabs should be nailed to the truss by the us, since they are a vital part of the structure and represent over 50% of the bending strength of the extended leg. Where they are to be site fixed, a nailing pattern must be obtained from the fabricator and strictly adhered to.
For some designs, bolts may be specified instead of nails. Bolts should be treated against corrosion and supplied complete with two plate washers to prevent the nut and bolt head from being drawn into the timber.
The term superchord describes deep chord structural members that are formed by stitching two smaller timber sections together using connector plates.. Whereas for solid timber the maximum sections available are 35mm x 197mm and 47mm x 244mm, superchords 35mm x 314mm and 47mm x 388mm can be produced from stock timber sizes.
It is particularly suited to extended chord trussed rafters offering not only significant economies in manufacture and delivery but also providing the architect with greater freedom in design.
As an alternative to reinforcing the extended legs, it may be preferable to use cross wall construction. By supporting the body of the truss on the beams, the load is relieved from the extended leg allowing smaller timber sections to be used and longer rafter extensions. Try to avoid extended legs greater than 0.9m, unless alternative methods of support can be provided.
To specify extended chord trusses provide the dimensions to our expert Design team or call us on 01933 279597.