What is Truss? Components of Truss

What is Truss?

Truss is a structural member which is in the form of a triangle and connected together with pin joints. All the external forces (loads and reactions) must be applied only at either end of the member, resulting in either compression or tension force. All the joints in the truss members have pinned connections such that no shear or moment forces are transfer from member to member. Truss members are in the form of either tension or compression.

Components of Roof Truss

Span of Truss

It is the distance between the Span of the two supports (i.e. Metal Column or Post or Built-up column etc.) of Roof truss is called the span of the truss.

Spacing of Roof Truss

  • It is the Horizontal distance between two consecution trusses is called as Spacing of Roof Truss.
  • ¬†Economical spacing of Roof truss = 1/3 to 1/5 of Span of Truss.
  • For example If the span of truss is 30 m then Economical spacing of Roof truss = 1/3 x 30 to 1/5 x 30 = 10 m to 6 m.
  • Cost of the truss =2 times the cost of purlin + Cost of roof covering.
S. NoSpan of Truss (m)Center-to Center Spacing (m)
115 -183.5 – 6
227 -304.5 -7.3
3>4215 -18

Principal Rafter (P.R) or top chord

It is the member of roof truss from End support to the Ridge Point is Known as Principal Rafter. The main function of Principal rafter is to support the purling and carry compression forces.

Main Tie/ Bottom Chord

It is one of the Most important Components of Truss. The bottom chord member of roof truss is known as Main Tie. The main function of Bottom chord is to take tension.

Sag Tie

The vertical member joining ridge point and Midpoint of main Te or Bottom Chord is known as Sag Tie.

Ridge Point

A highest Point of the truss is Known as Ridge Point.

Eaves Point

It is the point which at edge of the truss is called as eaves Point.

Panel Point:

It is the point on a Principal Rafter where vertical or inclined members connected to principal rafter are called as panel point or nodal Point.


  • It is the member-supported on the Panel Points of two consecutive roof truss is called as Purlin. or It is the Horizontal Beam spanning between the two adjacent truss
  • It support the roof and wall covering and transfer the load to the truss.
  • Purlin is design as Continuous Beam.
  • Purling acts as a lateral load resisting elements and serve as flange bracing for the rafters.
  • Channel, Angle, Square Hollow Sections (SHS), Rectangular Hollow Section (RHS), RHS, I- sections, etc are generally used as a purlin.
  • It acts as lateral bracing to the compression chords of the roof truss, which enhances the strength of compression members.

Components of Truss: Girts

  • It is the Horizontal components of Truss that provides lateral supports to the wall cladding and also resists wind loads. The functions of Girts are the same as Purlin.
  • It provides the strength and stability against external forces.
  • Channel, Angle, Square Hollow Sections (SHS), Rectangular Hollow Section (RHS), I- sections, etc are generally used as Girts.

Sag Rods

  • Sag Rods is the member which is used to connect two purlins. It controls the deflection o and transmits the gravity load to the supporting member.
  • Sag rods act as lateral support for Girts and Purlin.
  • It prevents the bending in the direction parallel to sloping roof due to dead loads.

Components of Truss: Web Members/Intermediate Chord

  • It is the Vertical or inclined Components of Truss members joining top chord and bottom chord members are known as Web members.
  • Somewhere there is a fashion to tell the web members as Intermediate chord or strut.


  • It is the portion of Truss between two consecutive panel points on principal rafter is called as Panel.


  • It is the Vertical distance from main tie to the Ridge point is called as Rise.


  • It is the Ratio of Rise and Span is called Pitch.
  • Pitch = Rise/Span. Normally value of Pitch is kept 1/3 to 1/5.
S. NoRoof CoveringPitch
1Corrugated GI Sheets1/3 to 1/6
2Corrugated AC Sheets1/6 to 1/12
3Lapped shingles (e.g. wood,asphalt,clay and tile)1/24 to 1/12
4Flat roof and trapezoidal trusses1/48 to 1/12
  • Pitch = Rise/Span. Normally a value of Pitch is kept 1/3 to 1/5.
  • Lower pitches are more advantageous because the effect of wind pressure on the roof truss gets reduced.


It is the Extension of the Top Chord beyond the Support.

To Know More About the Components of Truss and its Design Please follow this IS Codes SP: 398-1987

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