Roof coveringsBCT-III

B’arch, Year-2, Semester - III

Roof is an essential component of pitched roof and is placed over the roof framework.The functions are : protection from atmospheric agencies.

The selection for roofing materials involve following considerations:1. Types of buildings,2. Type of roof framework3. Initial cost4. Maintenance requirements5. fabrication facilities6. Appearance and local tradition.7. Durability8. Material availability9. And most important; Climate.

Common roof covering materials are as follows:1. Thatch covering.2. Wood shingles3. Tiles4. Asbestos cement sheet5. Galvanised corrugated iron sheets6. Slates7. Light weight roofings.

A .Thatch covering :1. Cheapest roof covering.2. Light but highly prone to fire.3. Unstable in high winds.4. Absorbs moisture and can decay.5. Burrowing animals like rats can infest the roof.6. Smells bad when absorbs moisture.

Construction:1. Consists of bundles of reeds are straws.2. Supporting framework consists of round bamboo rafters

spaced 20-30 cm apart.3. Bamboo rafters are tied with split bamboo at right

angles.4. The reeds or straws are dipped in fire resistant solution

before tying.5. The butt ends of the reeds are placed pointing the eaves.6. Thatch is tightly tied with the framework with ropes or

twines dipped in tar.7. Thickness of thatch is 15cm minimum. Regular thickness

varies from 20-30 cm.8. The pitch is maintained at 45 degrees minimum.9. A properly constructed thatch with reeds can last 60

years and that with straw 20 years.

B. Wood shingle roofing:1. Shingles are thin slabs of wood.2. Use depends on easy availability of timber.3. Light weight.4. Susceptible to fire and termite attack.5. Length vary from 30-40 cm and width from 6-

25 cm.6. Approximately 10mm thick at butt end and

taper to 3mm or less at the head.

C. Tile roofing:Tiles is one of the oldest roofing materials and still preferred for residential structures.

One of the reasons is that they can be easily manufactured from locally available soil.Manufacturing process of tiles is very similar to those of bricks.

Some of the most commonly used are: 1. Plain or flat tiles.2. Curved or pan-tiles.3. Pot tiles or half-round country tiles.4. Spanish tiles.5. Italian or Allahabad tiles.6. Interlocking tiles.

1. Plain Tiles.

1. Mostly of clay sometimes concrete is also used in manufacture.2. Sizes 25 x 15 cm to 28 x 18cm with thickness from 9-15 mm.3. Camber of about 5-10 mm along length.4. Have stubs or nibs to hang them from battens.5. Each tile has minimum two holes for nailing them to battens

using copper or composite nails. It is not necessary to nail every tile. For very steep slopes it may be necessary to nail every tile.

6. Spacing of common rafters is 20-30 cm and batten from 4-6 cm.7. Plain tiles are regular bonded.8. For normal exposures every 4th to 5th course is nailed.

Curved or pan tiles.

1. Tiles are 33-36 cm long, 22.5-25 cm wide and 12-19 mm thick.

2. Flat longitudinally but curved transversely to flat wave or ‘s’ curve.

3. Nib is provided at the head on the underside of the trough of the wave.

4. Two opposite diagonal corners are splayed or rounded.

5. Tiles are laid with side overlaps.6. Not bonded and have continuous side

joint from eaves to ridge . 7. Sides are splayed to avoid stacking of

corners that would otherwise result in open joints.

8. Side overlap: 38-50mm and longitudinal overlap 7.5-10 cm.

Half round country tiles : Spanish tiles.

1. Form vertical columns of under-tiles and over-tiles.

2. Under tiles are placed concave side up. Over tiles are placed convex side up.

3. Diameter tapers longitudinally.

Italian or Allahabad tiles.

1. Tiles are used in pairs; flat broad bottomed undertile with convex curved over tile.2. The under tile is flat and tapered with upturned side edges or flanges. It measures 23cm at the interior and 26 cm

at the head. Length is 37 cm and flange is 4 cm.3. The head lap varies from 6.5 – 7.5 cm.4. 2.5 cm boarding covered with felt may be used.

Interlocking tiles.

Flat interlocking

Single roman

D. Asbestos cement sheets (A.C. sheets)

1. Popular for industrial buildings, factories, sheds, cinema houses, auditoriums and to an extent residential buildings.

2. They are cheap, light weight, tough, durable, water tight, fire resisting and vermin resistant.

3. Main advantage is that they are available in big sizes unlike tiles.

4. Due to this even the supporting framework is lighter and hence cheaper.

5. Do not require finishes and maintenance .6. Construction is very fast.

1. They are manufactured from asbestos fibre (15%) and portland cement.

2. Asbestos is silkty fibrous mineral found in metamorphised volcanic rocks.

3. Asbestos is used in making roofing slates, tiles and corrugated sheets.

4. In India, asbestos cement roof coverings are available as, Everest big-six, standard and trafford corrugated A.C. sheets.

Laying of A.C. sheets:1. Laid at the end opposite to the

prevailing wind.2. Spacing of purlins are designed as

per suggested overlaps and overhang.

3. Laying is always commenced from eaves.

4. When laying is done from left to right ; the first sheet is laid uncut after which all subsequent sheets should have top left corner mitred.

For all intermediate rows, sheets should have both top left and bottom right corner mitred except the first which should have only top left corner mitred. In the last i.e. the topmost row every sheet has bottom right corner mitred except the last sheet which remains uncut.

Considerations while laying A.C. sheets:1. Sheets should be laid with smooth side upwards.2. End lap and side lap should be properly maintained, general end lap is 15cm.3. Purlin spacing should be properly checked.4. The holes fro accessories should be drilled, not punched, on the crown of

the corrugations.5. Bitumen washers are provided below G.I washer.6. Nut bolts or screws should not be over tight.7. Ridge capping should be secured to ridge purlin.8. Mitred corners should be properly checked for.9. Unsupported overhangs should not exceed 30cm.

E. Galvanised Iron Corrugated sheets (G.I. sheets)

F. Slate roofing1. Slate is hard, fine grained clayey stone.2. The thickness of slate for roofing may vary from

4-8 mm. traditionally they are much thicker.3. Sizes may vary from 60 x 30 cm to 40 x 20 cm.4. Slate should be hard, tough, durable, sound

ring like sound when struck and not split when drilled.

5. Slates are laid such that they overlap those of successive courses.

6. The lap depends on pitch and exposure.7. The spacing of battens known as gauge is

determined by lap.

G. Light weight roofing.

Light weight roofing involves two types of materials:1. Materials used for sheeting e.g. Aluminium

sheets or a.c. sheets. And,2. Materials used fro decking e.g. Wood wool, Straw

board, aluminium alloy and steel decking.

1. Preferred for wide-span industrial structures to reduce weight.

2. Advantageous over conventional roof coverings in terms of weight .

3. Aluminium roof sheeting consists of aluminium alloy with small amount of manganese for strength.

4. Wood wool used fro decking is made from wood fibre woven together and bonded with cement in mould under pressure.

5. Wood wool has good acoustic and thermal insulation.

6. For roofing their thickness is generally 50 -75mm.

7. They are nailed to timber joist at 60-90 cm c/c with help clout nails.

8. Can take load upto 0.75 kN/m2.9. Aluminium alloy and steel can be pressed

to form troughed roof decking with thickness of 0.7-1.2mm.

10. Depth of corrugations vary from 25-85mm, widths varying from 450-900 mm and length upto 10m.

11. Suitable for imposed loads of 0.75 kN/M2.12. The deck is fixed to roof supports by hook

bolts.13. A felt vapour barrier is bonded with

bitumen to the top of the deck. On which fibreboard is bonded.

14. The top surface is finished with white stone chips to reflect heat

Flat Terrace RoofingFlat roof is the one which is either horizontal, or practically horizontal with slope less than 10 degrees. Even a perfectly horizontal roof has to have some slope to drain off water easily and rapidly. Flat roofs can be constructed similar to upper floors the only difference between them being the top finish, commonly called terracing to protect it from adverse effects of environment like rain, heat etc.

Advantages of Flat roof1. Can be used for playing,

gardening, and for celebrating functions.

2. Easily made fire proof.3. Avoid extra enclosure of space.4. Have better insulating properties.5. Require lesser area of roofing

material.6. They do not require false ceiling.7. Provide overall economy.8. Multistoreyed buildings can be

only made with flat terrace.9. Future expansion can be easily

achieved.

Disadvantages of flat roofs.1. Span is restricted unless intermediate unless

intermediate columns are introduced.2. Self-weight of flat roof is very high, due to

this sizes of structural members become very heavy.

3. Highly unsuitable in places of high rainfall.4. They are vulnerable high temperature

variations. Cracks develop on the surface which may lead to leaky roof.

5. It is difficult to locate and rectify a leaky roof.6. Speed of flat roof construction is much

slower.7. Initial cost of flat roof is more than pitched

roof.8. Flat roof exposes entire building to weather

agencies. Pitch roof provide protection to the building.

Types of flat roofing1. Mud-terrace roofing.2. Brick jelly or Madras terrace roofing.3. Mud-phuska terracing with tile paving.4. Lime concrete terracing.5. Lime concrete terrace with tile paving.6. Bengal terrace roofing.7. Light weight flat roofing.

Mud terrace roofing.1. Suitable where rain fall is less.2. In Punjab terracing is done on tiles

while in Madhya pradesh and Maharashtra it is done over wooden boards.

3. Terracing is done with white earth mud containing large percentage of sodium salts.

Brick jelly roofing or Madras terrace roofing.1. Wooden joists are placed on RSJ with a

furring piece in between to achieve required slope of the terrace.

2. A course of specially made brick of size 15x5x12 cm is placed on edge in lime mortar (1:1.5) laid diagonally across the joists.

3. After brick course is set, 10 cm thick layer of brick bat concrete is laid (3 parts brick bat, 1 part sand and gravel, 50% lime by volume). It is well rammed for 3 days reducing the thickness to 7.5 cm.

4. When BBLC has set 3 courses of Madras flat tiles (15x10x12 cm) are laid in lime mortar making total thickness of 50cm.

5. Finally, the top surface is plastered with three coats of lime mortar. The surface is rubbed and polished.

Brick bat koba

Mud-phuska terracing with tile paving1. This type of terracing is suitable

for hot and arid regions.2. Commonly used over RCC slab.3. A layer of hot bitumen at 1.7 kg

per sq.m 4. Layer of sand is immediately

spread over hot bitumen at rate of 0.6 cu.m for 100 sq.m of roof surface.

5. Mud-phuska is prepared from puddled clay mixed with bhusa at the rate of 8kg bhusa per cu.m of clay. A 10cm layer of mud phuska is applied over sand bitumen layer.

6. Slope of 1:40 is desirable and can be given while casting slab itself.

7. It is then plastered with 13mm coat of mud-cowdung mortar (1:3).

8. Tile bricks are laid flat on plastered surface. The joints are grouted in 1:3 cement mortar.

Lime concrete terracing with tiles.1. Commonly adopted over RCC roof.2. After cleaning the slab a layer of hot

bitumen is applied at the rate of 1.7 kg per sq.m of roof surface.

3. A lyer of coarse sand is immediately spread over the hot layer of bitumen at rate of 0.6cu.m per 100 sq.m of terrace surface.

4. 10 cm thick layer of lime concrete (2parts lime-2 parts surkhi-7 parts brick ballast of 25 mm gauge. The concrete is well beaten.

5. Two courses of flat brick tiles are laid in 1:3 cement mortar. The joints of top course are pointed with 1:3 cement mortar. The vertical joints in the two courses are broken.

Brick ballast

Bengal terrace roofing1. Adopted for timber roofs of verandahs.2. Wooden rafters are placed 30-50 cm c/c .3. Wooden battens are placed across the

rafters at 15cm c/c4. A course of flat tiles (15x8x2) soaked in

white wash is laid in lime or cement mortar over battens.

5. Two or more courses of flat tiles are laid in mortar.

6. Two or three coats of lime plaster are applied.

Light weight flat roof1. Consists of aluminium alloy and

steel decking.2. On the top of decking, felt vapour

barrier is bonded with bitumen.3. Over it fibre board is bonded for

insulation.4. This is then covered with two or

three layers of felt roofing.5. Finally, the top surface is finished

with a layer of white stone chipping spread on bitumen to reduce heat absorption.