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Department of Civil Engineering

3^rd Semester

Subject-Building material & construction technology

Topics- ROOF

ER. Sulagna Das

(Lect. In the Civil Engineering Department)

AY:2021-2022

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ROOFS

It is constructed to protect the building from weather (rain, wind, heat, snow, etc)
Roof is the uppermost part of the building

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REQUIREMENTS OF A ROOF

Durable against various natural agencies
Adequate thermal insulation
Water proof
Fire resistant
Sound insulation
Good drainage arrangement

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CLASSIFICATION OF ROOFS

Pitched or sloping

Single roof,
Double roof,
Trussed roof.

Flat or terraced

R.C.C roof,
Madras terrace roof.

Curved

Shell roof,
Dome roof.

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PITCHED ROOFS

These roofs have sloping top surface
Suitable in those areas where rainfall/snowfall is very heavy
Span: clear distance between the supports of beams, roofs or truss
Rise: vertical distance between the top of the ridge and the wall plate
Pitch: inclination of sides of a roof to the horizontal
Ridge: an apex line of a sloping roof

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PITCHED ROOFS

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TERMS - ROOF

Gable: triangular upper part of a wall formed at the end of a pitched roof
Purlin: horizontal member placed over the principal rafters to support the roof membrane.

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TERMS - ROOF

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TYPES OF PITCHED ROOFS

Single roofs
Double or purlin roofs
Trussed roof

King post truss
Queen post truss
Mansard truss
Bel fast truss
Steel truss
Composite truss

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SINGLE ROOF: 1.LEAN TO-ROOFS

Rafters slope to one side only.
Upper ends- rafters are fixed by nails to the wooden plates.
Lower ends- rafters are notched & nailed to the wooden post plate.
Battens are placed and fixed over rafters.
Roof covering materials.
Spans upto 2.5 m.

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2.COUPLE ROOF

Pair of rafters
Slopes to both sides
Upper ends- nailed to common ridge piece
Lower ends- notched & nailed to wooden wall plates.
Battens are fixed
Roof covering materials
Span upto 3.60 m.

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3.COUPLE CLOSE ROOF

Tie beam
Prevent from spreading
Spans upto 4.20 m.

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COLLAR BEAM ROOF

Span increase or heavy load- leads to bend
Prevent by raising the tie beam
1/3^rd to ½ of the vertical ht. from wall plate to the ridge.
Spans upto 5m

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DOUBLE OR PURLIN ROOFS

Rafters- rest at the ends on wall
Purlins- intermediate supports
Spans upto 5.5 m.

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TRUSSED ROOF

A number of straight members connected in the shape of triangle and forming a frame is known as truss.
Truss are provided at regular interval of about 3m along the length.
Used for the span more than 5m.

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KING-POST TRUSS

King post- central post
Struts- inclined members
Principal rafters- support the purlins
Spans- 5 to 8 m.

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QUEEN-POST TRUSS

2 vertical posts
Straining beam
2 struts
Straining sill
Spans- 8 to 12 m.

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MANSARD TRUSS

Combination of king & queen post trusses.
Upper portion- king post truss
Lower portion- queen post truss.
Upper pitch- 30 to 40 degree
Lower pitch- 60 to 70 degree.

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BELFAST TRUSS/ BOW STRING TRUSS

Thin sections of timber
Top chord – curved
30 m span – if roof covering is light
Rise not more than 1/8^th of the span

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STEEL TRUSSES

ADVANTAGE:

stronger than wooden truss
Very large span (5m to 30m)
Easy to construct
Fire-proof
More rigid
Permanent
More economical

KING-POST STEEL TRUSS

COMPOUND FINK & COMPOUND HOWE’S STEEL TRUSS

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STEEL TRUSSES…..Cont 1

DISADVANTAGE:

Likely to get rusted
Need anti corrosive paints.
Periodical maintenance required.

USES:

Building with large span
Building with no intermediate columns.
Place of heavy rainfall of snow fall.

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COMPOSITE TRUSS

Timber and steel
Tension members – steel
Compression members – timber
Light and economical

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FLAT ROOFS

It is one which is either horizontal or horizontal with slope less than 10°.

TYPES OF FLAT ROOFS:

Madras terrace roofs
Reinforced cement concrete slab roof

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MADRAS TERRACE ROOF

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