Pitched Roof or Sloping Roof
The pitched roof generally have sloping top surface and these are useful at places where rainfall or snow fall is heavy. It is one of the cheapest and economical alternative for covering the structure. Pitched roof are generally used in factories and workshops, godowns and warehouse, theater and auditoriums and in hilly regions.
The climatic condition of the site, nature of the covering material, span and other factors are all responsible for the slope of the roof. It is designed keeping all these aspects into consideration.
Different Components Used in Pitched Roof :
The horizontal distance between the internal faces of walls or supports is called as span.
The vertical distance between the wall plate and top of the ridge is know as rise.
The inclination of the sides of a roof to the horizontal plane is called as pitch.
Apex line of the sloping roof is called as ridge.
The ridge formed by the intersection of 2 sloped surfaces having an exterior angle more than 180° is called as hip.
Wooden board which is fixed to the rafter at the eaves. It is also called as facia board. It is 15-20 cm wide and 20-5 mm thick
Purlins are nothing but horizontal wooden or steel members which are used to support the common ratters of a sloping rout when span is larger.
The triangular upper part of a wall formed at the end of a pitched roof is called as gable.
The edge of a gable running between the eaves and ridge is called as verge.
The small blocks of wood which are fixed on the rafters or ceiling are called as cleats.
A square or rectangular block made of stone or concrete which is provided under the end of the beam or truss to spread the load from the roof over a large area of bearing is called as template.
Common Rafters :
Inclined wooden members which are laid from the ridge to the eaves are called as common rafters. They are intermediate rafters which are supporting the roof coverings.
Hip Rafters :
Sloping rafters which forms the hip of a sloped roof are called as hip rafters. They are laid diagonally from the ridge to the corners of the wall for supporting the roof covers.
Jack Rafters :
Jack rafters are normally shorter than any common rafter. These rafters are generally laid from hip or valley to the eaves.
Valley Rafters :
Valley gutters are generally supported by these rafters. Valley rafters are sloping in nature and are laid diagonally from ridge to the eaves.
Framework which is generally made up of triangles is called as truss. It is build and designed to support the covers or ceilings of the rooms.
Types of Pitched Roof
There are three types pitched roof :
- Single Roofs.
- Double or Purlin Roofs.
- Trussed Roofs.
1. Single Roofs :
Single roofs consist of only common rafters to each slope without any inter-mediate support and which are secured at ridge and wall plates. The single roofs are generally of the following four types :
(a) Lean-to Roof or Verandah Roof or Shed Roof :
In this type of pitched rafters tend to slope only on one side only. It is also called as pent roof or aisle roof. It is generally adopted for maximum span of 2.4 metres.
(b) Couple Roof :
Couple or pair of common rafters generally form this type of roof, which normally slope to both the sides of the ridge of the roof abutting each other. Couple roof is adopted for spans upto 3.5 meters. Lower ends of rafter are nailed to the wall
plates placed on the walls. They are placed suitably over the walls. Purlins and battens support the roof coverings.
(c) Couple Close Roof :
Couple close roof is very much similar to couple roof, except that the legs of the common rafter are connected by a horizontal tie beam. To prevent the walls from overturning, the tie beam is connected to the feet of the common rafters. Tie beam is generally made of wood or steel. It is economically suitable for spans upto 5 meters.
(d) Collar Beam Roof :
Similar to couple close roof except that in the latter case, a tie beam is raised and placed at a one-third to one-half of vertical height from wall plate to ridge. This is usually done when the span increases or when the load is more. The tie beam is usually known as collar beam. Collar beam roof is suitable for spans upto 5 meters.
2. Double or Purlin Roof
It becomes uneconomical for the rafters when the span exceeds 2.4 meters, Hence in order to make the span economical, Intermediate supports which are called as purlins are provided under rafters.
3. Trussed Roofs :
Framed structure is generally known as trussed roof. When there is absence of inside supporting walls and partition for purlins and when the roof span exceeds more than 5 meters. Some of the mainly used Trusses are as follows :
- King-post truss.
- Queen-post truss.
- Mansard roof truss.
- Truncated roof truss.
- Bel-fast roof truss.
- Steel trusses.
(a) King-Post Truss :
As shown in the figure, the central vertical post is known as the king post. The main motto of the king post is to give support for tie member. To prevent the bending of principal rafter, inclined members known as struts are provided at the center of the principal rafter. It is generally adopted for span varying from 5-8 meters.
(b) Queen-Post Truss :
This truss differs from a king-post truss in having two vertical posts, known as queen posts. This truss is suitable for spans varying from 8 to 12 meters.
Queen post truss is little bit different from the king post truss. It consist of 2 vertical post supported at two different points as shown in figure. This modification is generally made to prevent the sagging of tie beam due to increased span length. By doing this the tie beam equally gets divided into 3 parts and the load is equally distributed among them.
(c) Mansard Roof Truss.
Francois Mansard was the man behind designing this truss so it was named on his name. As shown in the figure its basically 2-storey truss. Top part of the truss is same as king post and lower part is basically same as queen post truss. So it is the combination of both king and queen post truss. Now-a-days this type of truss is not in use due to it’s odd and ugly design and due to invention of new steel trusses.
(d) Truncated Roof Truss :
This truss is very much similar to mansard truss. Here the top portion of king post is absent and only queen post truss is present. This types of truss are adopted to give out a space for room in the roof.
(e) Bel-Fast Roof Truss :
Bel-fast roof truss is also known as latticed roof or bow string truss. It is shaped in the form of bow. This truss consist of thin sections of timber
and its top chord is made curved. It is generally used for the spans about 30 meters. Light roof coverings is also provided on the top chord.
(f) Steel Trusses :
Practically the main reason for the suspension of timber trusses in mild steel
roof trusses. They are more economical for spans greater than 12 meters as compared to timber. Generally most of the steel trusses are made out of angle sections as they can resist both tension as well as compression. Moreover they can be easily jointed.