Roof Trusses Created By M.S.Martin 11/04, reviewed 11/05, reviewed 11/07

Often used instead of pitched roofs

Not Always conventional shapes

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

Types

Types continued

Types continued

Types – Standard truss

Types - other There are many other types of trusses used in conventional roof shapes and we’ll look at them a little later First we need to look at the components and how a standard truss works

Hand out – worksheet one Identify the parts of a standard truss Summarize in your own words, the truss components, use the terminology section of carp 15 text for reference

Components

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

Truss panel points

Tension & Compression

Compression & Tension When trying to understand how this works its easier to look at half the truss

Handout – Tension & Compression Review handout to help understand how this process takes place

Parallel Chord trusses Used where large raking or level spans are required to bridge intermediate support They provide strength and versatility

Ground floor system

Alternative system

Roof Trusses We mainly come across trusses in roof systems Now we’ll 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

Types of connections

Types of connections

Types of connections

Types of connections

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

Fixing to non load bearing walls

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

Lifting continued

Roof truss types Its important to get familiar with the different types of trusses used in lieu conventional roof frames, some are – Standard Truncated Jack truss Hip truss Girder truss And more

Some typical roofs- handout

Roofs continued

Breakup – P28 text

Standard roof truss

Other types

Location of trusses

Review text Review text for other types of trusses. Saddle trusses Girder trusses Etc. Review from P29

Bracing and Erection Review text book for bracing requirements P24 Alternate roof styles P27 Cantilever trusses P32 Erection procedures P38

New alternative from Mitek

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