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
Types of Roof
1. Pitched or Sloped roof
2. Flat or terraced roof
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
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
b. Couple roof
2. Not very much favored due to its tendency to spread out at the feet (wall plate level).
c. Couple close roof
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
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)
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 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.
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
Plastic sheets, etc
Lap and 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.
Head nailed gauge = [length of slate-(lap+25mm)]/2For the case of, center nailed gauge = [length of slate-lap]/2