Exposed trusses are in simple terms just visible trusses. This means that the ceiling is placed on top of the trusses and the trusses are open and visible from underneath. Exposed trusses are also known as architectural trusses or decorative trusses. (Please also read our ‘Exposed Truss Confusion’ article under BLOGS or ROOF INFO).
Roof truss details such as vaulted ceilings (raised tie or scissors trusses) create more head room and improve the appearance of mezzanine or upper floor areas. Leaving these trusses exposed and placing the ceiling on top creates the feeling of openness which increases the comfort level of the room. Manufacturing these open trusses as bare nail plated trusses does not look neat, thus the need for architectural trusses with decorative connector plates.
Exposed Raised Tie Truss & Raised Tie Ceiling Effect
Please note that even though these trusses look decorative, they are structural trusses and designed to carry the roof loads. They are slender trusses and need to braced, forming a roof system of trusses. The bracing is very important, as its purpose is to ensure that the trusses and truss members remain straight and do not bow out of plane. Inadequate bracing can lead to collapses and serious accidents. An engineered bracing system will ensure the structural integrity of the truss system, and will remain installed for the life of the roof system.
The biggest problems with bracing arise when people have exposed trusses and do not want to see the ‘ugly’ exposed bracing. Unfortunately the bracing forms part of the truss system and may not be omitted. If the client decides to remove the bracing after installation, then he is taking full responsibility for the roof structure. There are alternatives available to the standard bracing e.g. stainless steel cables can be used instead of timber or steel runners, and the runners can be fixed to the gable wall instead of using diagonal timber or steel bracing in the centre of the room.
Using modern 3D roof engineering software, truss designers can produce engineered truss shapes that satisfy virtually any aesthetic and functional specification by the building design professional.
Exposed truss options and material used
Exposed trusses can have any, or a combination of, the following options:
- normal nail-plated trusses
- nail-plated combined with decorative cover plates and bolts
- bolted with connector plates on some joints
- steel trusses cladded with timber
Exposed trusses are structural trusses, and as such must be manufactured using structurally graded timber. In South Africa this is mostly graded (S5 or S7) SA Pine, or laminated beams. SA Pine is available in widths = 38, 52 & 76mm, and depths = 76, 114, 152 & 228mm. For use as exposed trusses, these timbers are planed down to smaller standard sizes (except for 38mm). The PAR (planed all round) sizes are: widths = 38, 44 & 70mm, depths = 70, 108, 144 & 220mm. As 38mm timber may not be planed down to a smaller size, this timber normally gets selected to find the best ones to use as exposed trusses. (In my opinion the New Zealand imports are the best to use for the latter option). Please be aware that timber is a natural product and will always have a limited degree of warp or twist.
We use 3mm mild steel for our structural, decorative connector plates. We do sometimes manufacture the plates from architect’s designs, but in most cases the clients ask us to do a simple design. When a design is agreed upon, we have them manufactured and painted (please allow 2-3 weeks for this process).
When the exposed trusses are manufactured as normal nail-plated trusses, the cover plates are not structural, but merely decorative. In this case thinner steel plates (e.g. 2mm) can be used to cover the gang nail plates. This will hide the nail plates and give the impression that the cover plates are the actual connector plates
Bolted exposed trusses are assembled by means of 12mm bolts (also known as M12 bolts), or cut-to-length M12 threaded bar, together with M12 nuts. Where the head of the bolts or the nuts are in direct contact with the face of the timber, structural washers (round or square) must be used to prevent the timber from being damaged. Bolts & nuts being bolted through cover plates do not need washers.