Timber Roofs.pdf

Timber Roofs

Roofs

© MZS Engineering Technologies

The uppermost part of a building which is constructed in the form of framework to give protection to the building against rain, heat, snow, wind, etc. A roof basically consists of structural elements provided at the top of the building for the support of roof coverings The structural elements consists of; - Trusses - Portals - Slabs - Domes - A. C sheets covering - G. I sheets coverings - Shingles - Slates, tiles etc

Requirements of good roofs Following are the requirements of a well planned roof; 1. It should be durable against the adverse effects of various agencies such as wind, rain, sun, etc. 2. It should grant the desirable insulation against sound and heat. 3. It should be structurally stable and sound and it should be capable of taking the loads likely to come over it. 4. It should be well-drained. 5. It should have efficient water-proofing arrangements. 6. It should be fire resistance

Classification of roofs The roofs are classified into the following three categories: 1. Pitched or sloping roofs 2. Flat or terraced roofs 3. Curved roofs 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. The choice of the type of roof will depend on the climatic conditions, shape of building, availability of materials, importance of building, etc.

Flat Roofs These roofs are nearly flat. However slight slope (not more than 10°) is given to drain out the rain water. The advantages of flat roofs are: a) The roof can be used as a terrace for playing and celebrating functions. b) At any latter stage the roof can be converted as a floor by adding another storey. c) They can suit to any shape of the building. d) Over-head water tanks and other services can be located easily. e) They can be made fire proof easily compared to pitched roof.

Flat Roofs The disadvantages of flat roofs are: a) They cannot cover large column free areas. b) Leakage problem may occur at latter date also due to development of cracks. Once leakage problem starts, it needs costly treatments. c) The dead weight of flat roofs is more. d) In places of snow fall flat roofs are to be avoided to reduce snow load. e) The initial cost of construction is more. f) Speed of construction of flat roofs is less.

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 pitched roofs are classified into; a) Single roofs b) Double or purlin roofs c) Trussed roofs

Pitched Roofs Single roofs If the span of roof is less than 5 m the following types of single roofs are used. 1. Lean-to-roof 2. Couple roof 3. Couple-close roof 4. Collar beam roof 5. Collar and scissor 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.

Lean-to roof This is the simplest form of a pitched roof. In this type one wall is carried up sufficiently higher than the another one to give the necessary slope to the roof. The rafters are suitably secured on the wall-plates and eaves boards, battens and roof covering is provided as shown. Suitability: It is generally used for sheds, out-houses attached to main buildings, verandahs, etc.

Couple roofs In this type of roof, the common rafters slope upwards from the opposite walls and they meet on a ridge piece in the middle as shown in figure The common rafters are firmly secured in position at both the ends, one end being on the ridge piece and the other on the wall plate. Suitability: This type of roof is used for span up to about 3.60 m

Coupled closed roof This roof is similar to couple roof except that the legs of common rafter is connected by a tie, preventing the spread out and overturning of walls

Suitability: This type of roofs are adopted economically up to a span of 4.80 m

Collar beam roof This roof is variation of couple close roof. The tie beam is raised and placed at a higher level. The tie beam is the known as a collar or a collar beam. A collar beam is adopted to economise the space and to increase the height of room. The collar beam is usually fixed at onethird to one-half the vertical height from the ridge

Suitability: The roof can be adopted up to a maximum span of 4.20m

Collar and scissors roof This roof is similar to the collar beam roof expect that two collar beams which are crossing each other to present an appearance of scissors are provided (e) Collar and scissors roof

Double or Purlin Roofs

If span exceeds, the cost of rafters increase 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. Figure shows a typical double or purlin roof.