4 What do you know about them ? TypesShapes / Limitations ?What supports them ?How are they joined ?
5 Definition / DesignA truss is a self contained frame, designed to transfer roof loads, typically to external wallsMost modern roofs can be adapted to a trussed systemMembers are typically joined by gangnail plates
15 Terminology – worksheet answers Bottom chord – the horizontal member forming the base of the truss, which will also support the ceiling belowTop chord – the angled member at the top of the truss, in a standard truss this is where a rafter would beWeb – these are the internal members of the truss that help distribute the load to the external wallsCamber – An upward curvature built into the bottom chord to compensate for deflection
16 Terminology continued Girder truss – A truss that supports other trusses or beams, differs in shape depending on where it is in the roofNail plates – the connectors made from a light gauge steel that join individual components of the trussGable end truss – the first truss at a gable endRaking truss – a gable end truss altered to suit outriggers for a verge overhang
17 Terminology continued Panel points – these are the connection points in a truss, (eg) where a web meets a top chordFish plate – an alternative bolted connection at a panel pointLoad bearing walls – in a trussed roof these are still the support walls, but are typically the external walls
28 Roof Trusses We mainly come across trusses in roof systems Now we’ll have a look at the factors concerning them
29 Camber – P12Trusses are built with a slight camber in the bottom chordThis is designed to provide maximum calculated deflection
30 Minimum clearanceUnder no circumstances should trusses be supported along their spanWith a trussed roof, internal walls are merely partitions / non-load bearingMinimum clearance is 12mm
31 Support & ConnectionsThe structure supporting the trusses must be level & squareExternal walls are load bearing with trussed roofsOr intermediate panel points for cantilevered trussesEither support directly over studs or change top plate
32 Support continued Where not supported by studs Use thicker top plate, orDouble plate
33 Connections to trusses Trusses require either factory connectors or nail or bolted connections on site, some of these are –Nail plates (gang nail)Triple grippesTruss saddlesMulti purpose anchorsVarious bolted bracketsJoist hanger bracketsWall brackets
41 Worksheet twoHandoutAnswer questions on handout to review section on support and connectionsUse your text book to assist you in your answers
42 Answers to worksheet two Parallel chord trusses – used in ground floor, suspended floors and roofsCamber – is there to provide maximum deflection when roof loads are placed upon themMinimum clearance for internal non-load bearing walls – 12mmWall structure support – either directly over studs or change top plate to thicker or double one
43 Answers continuedConnection methods used in trussed roofing – nail plates, triple grips, truss saddles, multi-purpose anchors, bolted brackets, joist hanger brackets, wall brackets.
44 Answers continuedChanges for a cantileverIt changes here
46 Lifting roof trussesIf lifting manually, you should always use support timbers to drag them upIf using a crane be careful to sling them correctlyThe advantage of a crane, many trusses can be lifted at once
60 Alternative wall bracing Mitek (gang nail) also make a timber wall bracing system they call Posi – braceSuitable for most timber framed houses
61 Worksheet three Answer questions to worksheet provided Review text for answersThen complete handout on roof shape and select types of roof trusses to use on the conventional shaped roof shown.
62 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 incorporatedTypes – standard, truncated, jack truss, girder, hip truss, truncated girderSupport for saddle truss – timber ledgerTypical bracing – speed bracingPicture for roof layout on next slide