What Are The Different Types Of Roofs?

The roof is the uppermost 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 rainwater. All types of upper-story floors can serve as flat roofs. Many times top of these roofs are treated with waterproofing materials-like mixing waterproofing chemicals in concrete, providing Coba concrete. With the advent of reliable waterproofing techniques, such roofs are constructed even in areas with heavy rainfall.

The advantages of flat roofs are:

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

The disadvantages of flat roofs are:

• They cannot cover large column-free areas.
• Leakage problems may occur at the latter date also due to the development of cracks. Once the leakage problem starts, it needs costly treatments.
• The deadweight of flat roofs is more.
• In places of snowfall flat roofs are to be avoided to reduce snow load.
• The initial cost of construction is more.
• The speed of construction of flat roofs is less.

Types of Flat Roofs: All the types listed for upper floors can be used as flat roofs.

2. Pitched Roofs

In the areas of heavy rainfall and snowfall sloping roofs are used. The slope of the 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 warehouses. In all these roofs covering sheets like A.C. sheet, G.I. sheets, tiles, slates, etc. are supported on suitable structures. The pitched roofs are classified into

(A) Single roofs

(B) Double or purlin roofs

(C) Trussed roofs

(A) Single Roof

If the span of the roof is less than 5 m the following types of single roofs are used.

(i) Lean-to roofs

(ii) Coupled roofs

(iii) Coupled-close roof

(iv) Collar beam roof

In all these roofs rafters placed at 600 mm to 800 mm spacing are main members

taking the load of the roof. Battens run over the rafters to support tiles. Fig: 1.3

shows various types of single roofs.

Fig: 1.3 Single Roofs

(B) Double or Purlin Roofs

If the span exceeds, the cost of rafters increases and a 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. Fig: 1.4

shows a typical double or purlin roof.

Fig: 1.4 Double or purlins roofs

(c) Trussed Roof

If the span is more, a framework of slender members are 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. In the case of wooden trusses, suitable carpentry joints are made to connect various members at a joint. Bolts and straps are also used. In the case of steel trusses joints are made using gusset plates and by providing bolts or rivets or welding.

Depending upon the span, trusses of different shapes are used. The end of trusses is supported on walls or on the column. Fig: 1.5 shows the different shapes of trusses used. Fig: 1.6 shows a typical wooden truss detail and Fig: 1.7 shows the details of a typical steel truss.

Fig: 1.5 Types of trusses

Fig: 1.6 A typical wooden truss 

Fig: 1.7 Steel roof truss

3. Shells and Folded Plate Roofs

Shell roof may be defined as a curved surface, the thickness of which is small compared to the other dimensions. In these roofs lot of load is transferred by membrane compression instead of by bending as in the case of conventional slab and beam constructions. Caves are having natural shell roofs. An examination of places of worships built in India, Europe, and Islamic nations show that shell structures were in usage for the last 800 to 1000 years. However, the shells of middle ages were massive masonry structures but nowadays thin R.C.C. shell roofs are built to cover large column-free areas. Fig: 1.8 shows commonly used shell roofs.

Advantages and Disadvantages of Shell Roofs

Advantages of shell roofs are:

(a) Good from an aesthetic point of view

(b) Material consumption is quite less

(c) Formwork can be removed early

(d) Large column-free areas can be covered.

Disadvantages are:

(a) The top surface is curved and hence the advantage of the terrace is lost.

(b) Formwork is costly.

Fig: 1.8 Types of shell roof

Fig: 1.9 Types of folded plate roofs

Folded plate roofs may be looked at as a slab with a number of folds. These roofs are also known as hipped plates, prismatic shells, and flatwork. In these structures also bending is reduced and a lot of load gets transferred as membrane compression. However folded plates are not so efficient as shells. Fig: 1.9 shows typically folded plate roofs.

Advantages and Disadvantages of Folded Plate Roofs Over Shell Roofs

Advantages are:

(a) Formwork required is relatively simpler.

(b) Movable formwork can be employed.

(c) Design involves simpler calculations.

Disadvantages are:

(a) Folded plates consume more material than shells.

(b) Formwork can be removed after 7 days while in case of shells it can be a little earlier.

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:

1. Thatch
2. Shingle
3. Tiles
4. Slates
5. Asbestos cement (A.C.) sheets
6. 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 end pointing towards eves. Thickness varies from 150 mm to 300mm. They are tied with ropes or twines to supporting structures. The supporting structure consists of round bamboo rafters spaced at 200 mm to 300 mm over which split bamboos laid at right angles at close spacing. It is claimed that reed thatch can last 50 to 60 years while straw thatch may last for 20–25 years.

The advantage of the 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.

(2) Shingles

Wood shingles are nothing but the split or sawn thin pieces of wood. Their size varies from 300 mm to 400 mm and lengths from 60 mm to 250 mm. 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.

(3) 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 appearance also.

(4) Slates

A slate is a sedimentary rock. Its color is gray. It can be easily split into thin sheets. 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 a rough texture and they give ringing bell-like sound when struck. They do not absorb water.

Fig: 1.10 A.C. sheet roofing

(5) A.C. Sheets

Asbestos cement is a material that consists of 15 percent of asbestos fibers evenly distributed and pressed with cement. They are manufactured in a sufficiently large size. The width of an A.C. sheet varies from 1.0 to 1.2 m and length from 1.75 to 3.0 m. To get sufficient strength with thin sections they are manufactured with corrugation or with Trafford's [Fig: 1.10]. They are fixed to the steel purlins using J-bolts. The roofing is quite economical, waterproof. However not very good thermal resistance. They are commonly used as covering materials in warehouses, godowns, or for larger halls. In auditorium etc., if these sheets are used, false ceilings are provided to get good thermal resistance.

(6) G.I. Sheets

Galvanised iron corrugated sheets are manufactured in sizes 1.0 to 1.2 m wide and 1.65 m length. The Galvanization of iron makes them rustproof. They are fixed to steel purlins using J-bolts and washers. They are durable, fireproof, light in weight, and need no maintenance. They are commonly used as covering materials for warehouses, godowns, sheds, etc. Table 1.11 gives a comparison between GI and AC sheets for roof covering.

Table 1.11 Comparison between GI and AC sheets