Truss Spacing and Bottom Chord:

Truss Spacing

The spacing of trusses is the distance (centre to centre) between adjacent trusses. This may vary between 4 m to 10 m depending upon their size. Normally they vary between 1/5 and 1/3 of the span.

Ridge Line

It is a line joining the vertices of the trusses.

Eaves Line

It is a line joining the lowest point of the roof trusses, on either side, where the drained water is collected or lead to rainwater pipes.

Top Chord

The uppermost line of members extending from the eaves to the ridge is the top chord. It is also called the principal rafter.

Bottom Chord

The lowermost line of members extending from support to support is known as the bottom chord. That is also known as main tie.

Ties

Members of the truss that are majorly under tension.

Struts

Those members of the truss that are principally under compression.

Joints

The point where more than one member meet; they are usually connected to a gusset plate by means of welds or rivets; and are known as welded or riveted joints.

Panel

It is the distance among two adjacent joints in the same line in a member.

Purlin

Purlins are horizontal members spanning across top chord of trusses and support the roof cladding.

In that case of tiles and slates these are supported on secondary members called rafters which are laid over purlins.

The purlins are generally placed at the adjacent panel points of the top chord (or principal rafter), hence the distance between these points is also the 'spacing of the purlins'.