TD6/3: ROOFS

ROOFS

Roof is the upper most portion of the building which protects the building from rain, wind and sun. Various types of roofs used may be divided broadly into three types:

1. Flat roofs
2. Pitched roofs
3. Shells and folded plates

Flat roofs are used in plains where rainfall is less and climate is moderate. Pitched roofs are preferred wherever rainfall is more.

Shells and folded plate roofs are used to cover large column free areas required for auditoriums, factories etc. Brief description of these roofs is presented below:

1. Flat Roofs: These roofs are nearly flat. However slight slope (not more than 10°) is given to drain out the rain water. All types of upper storey floors can serve as flat roofs. Many times top of these roofs are treated with water proofing materials. With advent of reliable water proofing techniques such roofs are constructed even in areas with heavy rain

The advantages of flat roofs

• The roof can be used as a terrace for playing and celebrating
• At any latter stage the roof can be converted as a floor by adding another
• They can suit to any shape of the
• Over-head water tanks and other services can be located
• They can be made fire proof easily compared to pitched

The disadvantages of flat roofs are:

• They cannot cover large column free
• Leakage problem may occur at latter date also due to development of cracks. Once leakage problem starts, it needs costly
• The dead weight of flat roofs is
• In places of snow fall flat roofs are to be avoided to reduce snow
• The initial cost of construction is
• Speed of construction of flat roofs is

Types of Flat Roofs:

All the types listed for upper floors can be used as flat roofs.

Pitched Roofs: In the areas of heavy rain falls and snow fall sloping roof are used. The slope of roof shall be more than 10°. They may have slopes as much as 45° to 60° also. The sloped roofs are known as pitched roofs. The sloping roofs are preferred in large spanned structures like workshops, factory buildings and ware houses. In all these roofs covering sheets like A.C. sheet, G.I. sheets, tiles, slates etc. are supported on suitable structures. The pitchedroofs are classified into

• • Single roofs
  • Double or purlin roofs
  • Trussed
    • Single Roof: If the span of roof is less than 5 m the following types of single roofs are used.
      • Lean to roofs
      • Coupled roofs
      • Coupled-close roof
      • Collar beam roof

In all these roofs rafters placed at 600 mm to 800 mm spacing are main members taking load of the roof. Battens run over the rafters to support tiles.

b) Double or Purlin Roofs: If span exceeds, the cost of rafters increases and single roof becomes uneconomical. For spans more than 5 m double purlin roofs are preferred. The intermediate support is given to rafters by purlins supported over collar beams.

c) Trussed Roof: If span is more, a frame work of slender members is used to support sloping roofs. These frames are known as trusses. A number of trusses may be placed lengthwise to get wall free longer halls. Purlins are provided over the trusses which in turn support roof sheets. For spans up to 9 m wooden trusses may be used but for larger spans steel trusses are a must.

Depending upon the span, trusses of different shapes are used. End of trusses are supported on walls or on column.

Roof Coverings for Pitched Roofs

Various types of covering materials are available for pitched roofs and their selection depends upon the climatic conditions, fabrication facility, availability of materials and affordability of the owner. Commonly used pitched roof covering materials are:

• Thatch
• Shingle
• Tiles
• Slates
• Asbestos cement (A.C.) sheets
• Galvanized iron (G.I.) sheets

1. Thatch Covering:

These coverings are provided for small spans, mainly for residential buildings in villages. Thatch is a roof covering of straw, reeds or similar materials. The thatch is well-soaked in water or fire resisting solution and packed bundles are laid with their butt ends pointing towards eves. Thickness varies from 150 mm to 300 mm.

The advantage of thatch roof is they are cheap and do not need skilled workers to build them.

The disadvantages are they are very poor fire resistant and harbor rats and other insects.

• Shingles: Wood shingles are nothing but the split or sawn thin pieces of wood. Their size varies from 300 mm to 400 mm and length from 60 mm to 250 Their thickness varies from 10 mm at one end to 3 mm at the other end. They are nailed to supporting structures.

They are commonly used in hilly areas for low cost housing. They have very poor fire and termite resistance.

• Tiles: Various clay tiles are manufactured in different localities. They serve as good covering materials. Tiles are supported over battens which are in turn supported by rafters/trusses etc. Allahabad tiles, Mangalore tiles are excellent inter-locking tiles. They give good
• Slates: A slate is a sedimentary rock. Its colour is gray. It can be easily split into thin

Slates of size 450 mm to 600 mm wide, 300 mm long and 4 to 8 mm thick are used as covering materials of pitched roofs in the areas where slate quarries are nearby. A good slate is hard, tough, durable. They are having rough texture and they give ringing bell like sound when struck. They do not absorb water.

• C. Sheets: Asbestos cement is a material that consists of 15 percent of asbestos fibres evenly distributed and pressed with cement. They are manufactured in a sufficiently large size.

To get sufficient strength with thin sections they are manufactured with corrugation or with traffords

• I. Sheets: Galvanized iron corrugated sheets are manufactured in the sizes 1.0 to 1.2 m wide and 1.65 m length. The Galvanization of iron makes them rustproof.

They are durable, fire proof, light in weight and need no maintenance.