What do you know about them ? Types Shapes / Limitations ? What supports them ? How are they joined ?
Definition / Design A truss is a self contained frame, designed to transfer roof loads, typically to external walls Most modern roofs can be adapted to a trussed system Members are typically joined by gangnail plates
Terminology – worksheet answers Bottom chord – the horizontal member forming the base of the truss, which will also support the ceiling below Top chord – the angled member at the top of the truss, in a standard truss this is where a rafter would be Web – these are the internal members of the truss that help distribute the load to the external walls Camber – An upward curvature built into the bottom chord to compensate for deflection
Terminology continued Girder truss – A truss that supports other trusses or beams, differs in shape depending on where it is in the roof Nail plates – the connectors made from a light gauge steel that join individual components of the truss Gable end truss – the first truss at a gable end Raking truss – a gable end truss altered to suit outriggers for a verge overhang
Terminology continued Panel points – these are the connection points in a truss, (eg) where a web meets a top chord Fish plate – an alternative bolted connection at a panel point Load bearing walls – in a trussed roof these are still the support walls, but are typically the external walls
Roof Trusses We mainly come across trusses in roof systems Now well have a look at the factors concerning them
Camber – P12 Trusses are built with a slight camber in the bottom chord This is designed to provide maximum calculated deflection
Minimum clearance Under no circumstances should trusses be supported along their span With a trussed roof, internal walls are merely partitions / non-load bearing Minimum clearance is 12mm
Support & Connections The structure supporting the trusses must be level & square External walls are load bearing with trussed roofs Or intermediate panel points for cantilevered trusses Either support directly over studs or change top plate
Support continued Where not supported by studs Use thicker top plate, or Double plate
Connections to trusses Trusses require either factory connectors or nail or bolted connections on site, some of these are – Nail plates (gang nail) Triple grippes Truss saddles Multi purpose anchors Various bolted brackets Joist hanger brackets Wall brackets
Worksheet two Handout Answer questions on handout to review section on support and connections Use your text book to assist you in your answers
Answers to worksheet two Parallel chord trusses – used in ground floor, suspended floors and roofs Camber – is there to provide maximum deflection when roof loads are placed upon them Minimum clearance for internal non-load bearing walls – 12mm Wall structure support – either directly over studs or change top plate to thicker or double one
Answers continued Connection methods used in trussed roofing – nail plates, triple grips, truss saddles, multi-purpose anchors, bolted brackets, joist hanger brackets, wall brackets.
Answers continued Changes for a cantilever It changes here
Lifting roof trusses If lifting manually, you should always use support timbers to drag them up If using a crane be careful to sling them correctly The advantage of a crane, many trusses can be lifted at once
Alternative wall bracing Mitek (gang nail) also make a timber wall bracing system they call Posi – brace Suitable for most timber framed houses
Worksheet three Answer questions to worksheet provided Review text for answers Then complete handout on roof shape and select types of roof trusses to use on the conventional shaped roof shown.
Answers to worksheet three Fixing to internal walls – by way of purpose made L- brackets, also, depending on layout of walls under, blocks may also have to be incorporated Types – standard, truncated, jack truss, girder, hip truss, truncated girder Support for saddle truss – timber ledger Typical bracing – speed bracing Picture for roof layout on next slide