Roof Trusses
Content
ROOF TRUSSES The alternative to using conventional roof frames is to use roof trusses – prefabricated frames delivered to site ready for installation. They are specifically engineer designed for each project, consume large areas of otherwise useable roof space and are difficult to install over large spans (due to their fixed height and configuration). Eg on home units, etc. Steel roof frames are also normally prefabricated as trusses. The relevant Australian Standard is AS 4440 - Installation of nail plated timber roof trusses. There are many different shapes and configurations to trusses and these usually derive from their intended uses. For example, one truss may be designed to carry extra loads (eg.materials storage) at ceiling level whilst another may carry extra loads (eg. solar hot water heaters) at the roof covering level. The members The member that replaces the rafter is a top chord. The member that replaces the ceiling joist is a bottom chord. The members in between the chords that replace underpurlins and struts are called webs.
Top chord Bottom chord
Webs
The large number of gang nail plates that hold the pieces of timber together is always an easy indication of whether the roof is trussed or conventionally framed.
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Trusses are easily identifiable by the presence of internal web members – they are often visible as a “W” configuration.
Evolution of truss design or how trusses work A simple coupled roof design relies on the bending strength of the timber to determine the maximum span of the members. If the lengths are excessive, bending will occur. The first member to bend will be the bottom chord as it is longer than the top chords. To stop this, the centre of the bottom chord is connected with a web to the apex of the truss. This web is in tension as it is pulling down on the apex joint. The apex will not sag as the top chords are pushing against each other and are therefore self supporting.
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As the truss design widens the top chords are now bending due to excessive length. To stop this more webs are installed from the base of the vertical one to the middle of the top chord span. These webs are in compression like a roof strut. The bottom point of these web cannot move downwards as they are held up by the joint connected to the apex. Now the truss has all chords supported at mid span.
As the span continues to widen again the bottom chord exceeds its bending limits and starts to sag between the points of support. It now needs to be supported at 1/3 span points. The single vertical web in the centre is replaced by two webs from the apex to the 1/3 span points on the bottom chord. The webs running up to the centre of the top chord are connected to the 1/3 span points on the bottom chord. Now the span of the bottom chord member has been reduced to 1/3 the width of the truss and the top chord span reduced to ½ its original span.
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Advantages Fabricated off site whilst slab or floor framing is being constructed – reduces construction time. Engineered design. Cost effective Simple and quick to erect. Disadvantages Cannot be altered on site without affecting engineering certification. Difficult to use and handle on very large roof spans without use of a crane - due to height of truss. Cannot be used effectively on complicated roof shapes – less effective than conventional roof framing. A wide variety of truss types exists. The differences are based mainly on span capabilities and the style and shape of the proposed roof (see next page).
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This diagram shows only a fraction of the truss types commonly available. Fixing of trusses Trusses are designed to transfer the entire load they support to the external walls. This allows the internal walls to be non load bearing, saves on costs and allows freedom of planning. Internal walls can be removed safely provided they are not serving a bracing function for the external walls
Internal walls To ensure they do not carry any loads the internal walls are set 10mm below the external walls and their top plates are attached to the trusses via special fixings called internal wall or slip brackets. Slip bracket
The slots in the slip brackets allow the truss to settle after it has been loaded with tiles.
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Viewed from above this slip bracket is connected to a trimmer that stabilised the top of the internal wall and provides a fixing for the ceiling lining. Notice the different distances of the bottom chords from the top plate. The span from the right hand bottom chord may exceed that of the lining so the trimmers solve the problem.
External walls The trusses are fixed to external wall top plates using Trip L Grips ®, a special three way steel connection plate as shown below. It is nailed into the side and top of the top plate and into the side of the truss. A minimum of 10 nails is required (4 in each vertical face and 2 in the horizontal face) to comply with the relevant standards. They are very difficult to remove so ensure they are in the correct position before nailing them permanently.
Trip-L-Grips ®.
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Bracing of trusses Trusses are stabilised against racking forces by the use of a steel strap specially designed for roof trusses. This strap is called speed brace and is bent at an angle of 120 0 for rigidity in handling and to assist in tensioning the strap. It is pre punched for nailing. It is generally laid at an angle of 300 to the wall top plates. This angle is measured in the horizontal plane. Specific requirements exist for fixing bracing at the apex and heel of the truss to counter the overturning effect of the brace( generally uses blocking between trusses where brace ends) and also for splicing the bracing.
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Another indicator of trusses is the presence of large steel brackets called boot brackets. These hold standard trusses onto girder trusses. A girder truss is one that supports other trusses or larger than normal roof loads. They are often constructed from hardwoods whereas standard trusses are made from softwoods.
Outriggers
The verge overhang can be achieved by cantilevering members called outriggers beyond the last truss supported by the walls. A number of other methods can be used to achieve the same result.
7
Gable end studs are added to facilitate fixing of brick veneer skins or other gable cladding
Saddle trusses are smaller trusses that sit on the top chords of larger trusses. They are used on L and T shaped buildings or to build dutch gables. In the photo below the label A points to saddle trusses.
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These two photos show the stages of eave frame construction. The bottom photo includes soffit bearers and metal fascia and fascia brackets. The eave lining of fibre cement is nailed to the underside of the horizontal soffit bearer.
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Eave soffit lining installed.
In some new houses builders are not fixing linings to any framework. They slot the lining into the metal fascia as shown below and nail through into the brickwork. When gutters and fascias need replacing the eaves linings fall out.
Detail of metal fascia and connecting bracket.
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Top chord Bottom chord
Webs
The large number of gang nail plates that hold the pieces of timber together is always an easy indication of whether the roof is trussed or conventionally framed.
1
Trusses are easily identifiable by the presence of internal web members – they are often visible as a “W” configuration.
Evolution of truss design or how trusses work A simple coupled roof design relies on the bending strength of the timber to determine the maximum span of the members. If the lengths are excessive, bending will occur. The first member to bend will be the bottom chord as it is longer than the top chords. To stop this, the centre of the bottom chord is connected with a web to the apex of the truss. This web is in tension as it is pulling down on the apex joint. The apex will not sag as the top chords are pushing against each other and are therefore self supporting.
2
As the truss design widens the top chords are now bending due to excessive length. To stop this more webs are installed from the base of the vertical one to the middle of the top chord span. These webs are in compression like a roof strut. The bottom point of these web cannot move downwards as they are held up by the joint connected to the apex. Now the truss has all chords supported at mid span.
As the span continues to widen again the bottom chord exceeds its bending limits and starts to sag between the points of support. It now needs to be supported at 1/3 span points. The single vertical web in the centre is replaced by two webs from the apex to the 1/3 span points on the bottom chord. The webs running up to the centre of the top chord are connected to the 1/3 span points on the bottom chord. Now the span of the bottom chord member has been reduced to 1/3 the width of the truss and the top chord span reduced to ½ its original span.
C
T T
C
Advantages Fabricated off site whilst slab or floor framing is being constructed – reduces construction time. Engineered design. Cost effective Simple and quick to erect. Disadvantages Cannot be altered on site without affecting engineering certification. Difficult to use and handle on very large roof spans without use of a crane - due to height of truss. Cannot be used effectively on complicated roof shapes – less effective than conventional roof framing. A wide variety of truss types exists. The differences are based mainly on span capabilities and the style and shape of the proposed roof (see next page).
3
This diagram shows only a fraction of the truss types commonly available. Fixing of trusses Trusses are designed to transfer the entire load they support to the external walls. This allows the internal walls to be non load bearing, saves on costs and allows freedom of planning. Internal walls can be removed safely provided they are not serving a bracing function for the external walls
Internal walls To ensure they do not carry any loads the internal walls are set 10mm below the external walls and their top plates are attached to the trusses via special fixings called internal wall or slip brackets. Slip bracket
The slots in the slip brackets allow the truss to settle after it has been loaded with tiles.
4
Viewed from above this slip bracket is connected to a trimmer that stabilised the top of the internal wall and provides a fixing for the ceiling lining. Notice the different distances of the bottom chords from the top plate. The span from the right hand bottom chord may exceed that of the lining so the trimmers solve the problem.
External walls The trusses are fixed to external wall top plates using Trip L Grips ®, a special three way steel connection plate as shown below. It is nailed into the side and top of the top plate and into the side of the truss. A minimum of 10 nails is required (4 in each vertical face and 2 in the horizontal face) to comply with the relevant standards. They are very difficult to remove so ensure they are in the correct position before nailing them permanently.
Trip-L-Grips ®.
5
Bracing of trusses Trusses are stabilised against racking forces by the use of a steel strap specially designed for roof trusses. This strap is called speed brace and is bent at an angle of 120 0 for rigidity in handling and to assist in tensioning the strap. It is pre punched for nailing. It is generally laid at an angle of 300 to the wall top plates. This angle is measured in the horizontal plane. Specific requirements exist for fixing bracing at the apex and heel of the truss to counter the overturning effect of the brace( generally uses blocking between trusses where brace ends) and also for splicing the bracing.
6
Another indicator of trusses is the presence of large steel brackets called boot brackets. These hold standard trusses onto girder trusses. A girder truss is one that supports other trusses or larger than normal roof loads. They are often constructed from hardwoods whereas standard trusses are made from softwoods.
Outriggers
The verge overhang can be achieved by cantilevering members called outriggers beyond the last truss supported by the walls. A number of other methods can be used to achieve the same result.
7
Gable end studs are added to facilitate fixing of brick veneer skins or other gable cladding
Saddle trusses are smaller trusses that sit on the top chords of larger trusses. They are used on L and T shaped buildings or to build dutch gables. In the photo below the label A points to saddle trusses.
8
These two photos show the stages of eave frame construction. The bottom photo includes soffit bearers and metal fascia and fascia brackets. The eave lining of fibre cement is nailed to the underside of the horizontal soffit bearer.
9
Eave soffit lining installed.
In some new houses builders are not fixing linings to any framework. They slot the lining into the metal fascia as shown below and nail through into the brickwork. When gutters and fascias need replacing the eaves linings fall out.
Detail of metal fascia and connecting bracket.
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