By
131170106076 PATEL POOJA D
TRUSSED ROOFS
When the span of the roof exceeds 4.8 m and where there are no inside
walls to support the purlins, framed structures known as trusses are
provided at suitable interval along the length of the room.
The trusses carry the ridge piece and purlins on which the common rafters
Rest.
Spacing of trusses depends upon the load on the roof, position of cross
walls, span and material of the truss.
The spacing is generally 3m for wooden trusses
The roof consists of 3 major elements:
a) Rafters: to support the roofing material
b) Purlins: to provide intermediate support to the rafters
c) Trusses: to provide support to the ends of the purlins
TYPES OF TRUSSES
The various types of trusses are as follows:
• King-post truss
• Queen-post truss
• Combination of king-post and queen-post truss
• Mansard truss
• Truncated truss
• Bel-fast truss or Bow- string truss
• Steel truss
• Composite truss
KING-POST TRUSS
A king-post truss consists of the following components:
• Lower tie beam
• Two inclined principal rafters - supports the purlins
• Two struts - supports the principal rafters
• A king-post - prevents sagging of tie-beam at the centre
• Two purlins - placed at right angles to the principle rafters and
supports the common rafters
• Common Rafters - supports the roof covering
• Cleats - prevent the purlins from sliding (tilting)
The spacing of king-post truss is limited to 3 m centre to centre and it is
suitable for spans varying from 5 to 8 meter.
QUEEN POST ROOF TRUSS
A queen-post truss has two vertical posts known as Queen-posts unlike kingpost where there is only one.
A queen post roof truss has the following components:
• Straining Beam - horizontal beam that connects the top of the
queen posts
• Two struts - join the feet of the queen post to the principle rafter
• Straining sill - to counteract the thrust from the struts which are in
compression
• Purlins with cleats
• Principle rafters and common rafters
• Tie beams
Queen post roof truss are suitable for spans between 8-12 m.
COMBINATION OF KING POST AND QUEEN POST
TRUSSES
Queen post trusses are suitable for spans up to 12 metres. For greater
spans, the queen-post truss can be strengthened by one more upright
member, called princess-post to each side.
A combination of the king-post and queen-post is suitable up to 18 m
span.
MANSARD ROOF TRUSS
This truss is named after a French architect, Francois Mansard (1598-1666),
who designed this truss to satisfy the requirements of municipal laws.
designer and is a combination of king-post and queen-post truss.
• It is a 2- storey truss, with the upper portion consisting of king-post
truss and lower portion of queen-post truss.
• Mansard truss has two pitches. The upper pitch varies from 30⁰-40⁰
while that of lower pitch varies from 60⁰ - 70⁰.
• The use of this truss results in economy in space and a room may be
provided in the roof between two queen-posts.
• However it has now become obsolete because of it odd shape.
TRUNCATED TRUSS
It is similar to Mansard
truss, except that its top is
formed flat, with a gentle
slope to one side.
Generally used when it is
required to provide a room
in the roof, between the
two queen posts of the
truss.
BEL-FAST ROOF TRUSS
• It is also called Bow-string
roof truss since it is in the
form of a bow, with its top
chord being curved.
• This can be used up to a
span of 30 m if the roof
covering is light.
• This is also sometimes
known as the latticed roof
truss or low string truss.
COMPOSITE ROOF TRUSS
Roof made of two materials, such as timber and
steel, are known as composite roof truss.
Normally the tension members are made of
steel and the compression members of timber.
STEEL ROOF TRUSSES
• When the span exceeds
12m,
timber
trusses
become
heavy
and
uneconomical.
• Steel trusses are used
economically over larger
spans.
• The mild steel is easily
available in rolled sections
of standard shapes and
sizes.
ADVANTAGES OF STEEL TRUSSES
• Section comprising of steel trusses are readily available in
•
•
•
•
•
•
required dimensions resulting in minimum wastage of
material.
They are light in weight, easy to construct and can be
fabricated in any shape depending upon structural and
architectural requirements.
Steel trusses are stronger and more durable (rigid) in
comparison to timber trusses.
They can be used for any span.
Steel trusses are fire-proof & termite proof.
They are more resistant to other environmental agencies and
have longer life.
They are economical.
TYPES OF STEEL TRUSSES
• Open trusses
• North light trusses
• Bow string trusses
• Arched rib trusses and solid
arched ribs
THANK YOU