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Ridged Roof Transitions

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Transitions are points in a roof geometry where a gabled or hip roof joins to accommodate a ‘T’ or L’ form in the room layout. These can sometimes become quite complex where two portions of a structure have different slopes and ridge heights. Generally, Utilihab does not favor the use of varying ridge heights and the standard system will only accommodate transition ‘kits’ for matching ridge heights or between the varying heights of the standard gabled and hip roof kits.

For same-height transitions using the standard rafter and ridge beam frame system, the perpendicular ridge beam attaches directly to the other ridge beam and two dihedral hip rafters are linked at the same point using gusset plate connection to form the ‘valley’ along the roof slopes. Transitional rafters along the merging slopes then link at right angles along the dihedral hip rafters. Depending on the roof cladding system, the dihedral valley rafters may employ a special gutter mounted continuously along them.

For same-height transitions using a truss roof system, transitional truss modules along the line of one roof section are fashioned to ride over the trusses of the adjoining roof section. Where the trusses have sloped bottom chords to create a vaulted ceiling, two hip trusses join at a half-truss along a king post to create a three-legged truss along the join of the roof sections and supporting transitional partial truss modules. Where a full cross of roof lines meet in a ‘+’ form, a full cross of dihedral hip truss are connected at a common king post and use to make a four legged truss module akin to a groin vault.

For differing-height transitions using the standard rafter and ridge frame system, a combination of dihedral rafters and the lower ridge beam connect at a mediating main rafter of the higher ridge roof. The transitional or ‘creeper’ rafters then link along the dihedral rafters. Additional support rafters may be needed along either side of the mediating main rafter of the higher roof. Note that this arrangement does not support the vaulted ceiling space of the lower height roof section along the volume of the roof transition and so a gabled ceiling enclosure is used to mediate the ceiling transition. However, if a hip roof is used on the higher roof section, both employ the same slope, and the transition forms an ‘L’ rather than ‘T’ shape in the structure layout, the lower roof section can employ the higher roof’s hip rafter as a one of its dihedral rafters and link it’s other dihedral rafter and ridge beam to it. This will neatly merge the interior vault topology of lower and higher ceiling sections allowing for a simple transition. This is generally an easier interface for differing height roof sections.

For differing height transitions using truss modules, the lower height roof section rides over the higher roof section using transitional truss modules along the merging volume. Extra truss modules in the higher roof section may sometimes be needed. An advantage of this form is that the transitional truss modules, which are sized to the general structural grid of the Utilihab system, can be freely placed along any point within the grid matching up with the rafter of the larger roof segment. However, where trusses with sloped lower chords, such as scissor trusses, are used for the sake of a vaulted interior ceiling a more complex transitional structure becomes necessary. A pair of dihedral hip trusses in the end of the lower height roof section linked to a king post on a partial main truss in the higher roof section create a three-point truss structure which supports transitional truss modules.

As we can see, these transition structure represent what could be the most complex of structures used in the Utilihab system.

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