Definition of roof:-

Roof is the uppermost part of a building provided as a structural covering to protect building from atmospheric weather (sun, rain, wind etc.)

Functional purpose of roof

  • To keep out rain, wind, dust and snow.
  • For strength and stability of building.
  • For durability and free from maintenance.
  • For fire safety.
  • Preventing excessive heat loss in winter.
  • To keep the interior building cool in summer.
  • Provide resistance to the passage of sound.
  • Safety to occupants
  • Aesthetic beauty

    Types of Roof

    1. Pitched or Sloped roof

  • This is the roof that has certain gradient (slope) with respect to horizontal.
  • Suitable where rainfall/snowfall is very heavy.

        2. Flat or terraced roof

  • This is the roof that is relatively horizontal with nominal gradient (<10o ) to drain off rainwater.
  • Suitable in building in plain or in hot region where rainfall is mild.

    3. Curved roof-

    -       Top surface curved & opted to give architectural effects.

    –    Suitable for buildings like library, theatres, recreation halls etc.



    4. Shelled roof

    -This is the type of roof that has thin section with curved surfaces; hyperbola, parabola, etc.

    -The latest technology is being applied for the construction of this roof.


    5. Domed roof

  • This is the curved roof having the roof area more than semicircle (hemispherical shape).
  • Dome had largest roof area compare to all types of roofs.

    Terminology used timber Roof


Single timber roof

- consists of only common rafters which are supported at the ridge and at the wall plate.


- Used when span is less so that no intermediate support is required for rafters



a. Lean to roof


  1.           Span limits 2.4 m, rafters slope one side only
  2.           Wooden wall plate is supported either on a steel corbel or stone corbel which is provided 1mc/c.
  3.           Difference in elevation between two wall plates gives desired slope.(usual slope=30o)
  4.           Provided for sheds, verandas, outhouse, etc.

    b. Couple roof

    1. Span limit 3.6 m, formed by pair of rafters and rafters slope to both side of    ridge of roof.

         2. Not very much favored due to its tendency to spread out at the feet (wall   plate level).

c. Couple close roof


  1.  Span limit 5.0 m, two-side slope, additional tie in wall level.
  2.  Similar to couple roof except that ends of the couple of common rafters is connected by horizontal member called tie beam.
  3.  One tie-beam for each pair of rafters.
  4.  for longer span or for greater loads, the rafters may have tendency to sag can be checked by central vertical rod (king rod).

    d. Collar beam roof

    -          span limit 5.0 m, two-side slope, additional tie above wall level.

    when span increases and loads becomes more to avoid bending tendency  by introducing collar beam by raising tie beam at ½ to 1/3 of the vertical height.

    e. Collar and scissors beam

    -          Span limit 5.0 m, two-side slope, and scissors from wall level to center of roof slope.

    -          Similar to the collar roof except that two collar beams crossing each other to have an appearance of scissors is provided.

    2. Double or Purlin roof

    - Roof basically consists of rafters and additional structural member (purlin) at mid span.

    - Purlin provides intermediate support to rafters.

    Reduce the size of rafters, each rafter is

    supported 3-points (on wall, ridge and purlin)

    Trussed roof

Kingpost roof truss

-          Consists of following components: lower tie beam, two inclined principal rafters, two struts, a king post.


-           principal rafter support the purlins, purlins support the closely spaced common rafters which have the same slope as the principal rafters. The common rafters support the roof covering as usual.


-           Spacing of king post truss limited to 5-8 m/c.


-           Suitable for spans varying from 5-8 m.


-           struts connected to the tie beams and principal rafters in inclined direction, prevent the sagging of principal rafters.

b. Queen post roof truss

-          a queen post truss differs from a king post in having a two vertical posts rather than one.

-           such additional vertical posts is known as queen post, the tops of which are connected by a horizontal piece known as straining beam.

-           the queen post are the tension members.

-           Straining beam receives the thrust from the principal rafters and keeps the junction in stable position.


c. Combination of Kingpost & queen post roof truss

-          For span greater than 12m queen post truss can be strengthened by one more upright member called princess post to each side.

-           It seems combination of king post and queen post trusses  suitable up to 18m.

d. Mansard roof truss

-          It is two storey truss, with upper portion consisting of king –post truss and the lower portion of queen post truss.

-           The upper pitch (king post truss) varies from 30o to 40o while two lower pitch (queen post) varies from 60o to 70o .It is obsolete because of odd shape.

e. Truncated roof truss

-          Similar to mansard truss except that its top is formed flat, with a gentle slope to one side.

-           It is required to provide the room in the roof between the two queen posts.

f. Belfast roof truss

-          It is in the form of bow consisting thin sections of timber with its top chord curved.

-           For light roof covering it can be used up to 30m span.

g. composite roof truss

- made of two material timber and steel.

- tension members are  made of steel, while compression members are made of timber.

-          Special fitting are required at the junction of timber and steel members. For figure refer book (B.C. Punmia)

h. Steel roof truss

- when span exceeds 10 m, timber truss size becomes heavy and uneconomical.

-          It is widely used due to its properties like easy to construct and fabricate fire proof, more rigid, high strength, and durability, economical due to high load bearing capacity.

-           it is fabricated from rolled steel sections such as channels, angles, T-sections and plates.

Roof coverings

Roof covering is the material used as a protective barrier over the framework of roof structure.  Roof covering is essential to building from rain, snow, sun, wind etc.

  • Types of roof covering

CGI-Sheets-slope more than 250, lapping 15 cm on edge & 1 & 1/2 corrugation at side.

AC – Sheets- asbestos cement sheets are now increasingly used due to several varieties but white asbestos, which is compound of magnesia and silica are principally used.

Slates-slope more than 250, lapping 50-75%. Slate is hard, fine grained sedimentary clayey stone

Thatch-slope more than 400, thickness 10-15 cm. thatch is light and combustible vegetative roof cover.

Tiles-slope 200-450, lapping-single or double

Wood singles-singles are thin slabs of wood usedto cover roofs

Metal roofing

Glass covering

Bituminous felt

Plastic sheets, etc

Tile roofing


Lap and Gauge

  • The overlapping of roof coverings is called LAP and
  • The interval of roof covering on the roof is called GAUGE.
  • Calculation of gauge:

Gauge = (Length of tile-lap)/2


For a tile of size 265 mm length and a lap of 65 mm, Gauge may be, G =(265-65)/2  = 100 mm.

  • Single lap: tiles are laid with overlapping side joints to a pitch of 350(min), min. lap may be of 50 mm.
  • Double lap: tiles are laid like the butt jointed plain tiles, more than 50 % of the tile is overlapped. The third layer of the tile press the first layer.
  • Gauge distance for slate

    Head nailed gauge = [length of slate-(lap+25mm)]/2

    For the case of, center nailed gauge = [length of slate-lap]/2