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 grippesTruss saddlesMulti purpose anchorsVarious bolted bracketsJoist hanger bracketsWall 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 –StandardTruncatedJack trussHip trussGirder trussAnd 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