An assignment on

Pitch roof for

coastal area

The basic principles should be followed to protect the pitched roofs against uplift:

Aerodynamic roof form: • Roof pitch 30º- 40° to reduce effects of suction and uplift. • Hipped instead of gable roof. If gable, then ends tied down firmly to rest of

structure. Lean-to should be avoided. • Overhang <2’ -6” , vents in roof and masonry parapet.• RCC roof provides superior protection, but heavy in earthquake. Need for

adequately braced vertical structure.

Roof connected to structure: • Rafters at recommended spacing. • Cross-bracing in plane of roof and ceiling, and also for openings, if any.

Openings restricted in size. • Strong connections between roof and vertical structure. Metal straps,

bolts with washers on both ends instead of simple nails.

Well-fixed roof covering: • CI sheet screwed at every corrugation. Tiles fastened individually. • Use of J-hook bolts and threaded/ twisted roofing nails.

Pitched roof is very common in Bangladesh. Specially in the rural area.But our country is located in the humid tropics between the Himalayas in the north and the Bay of Bengal in the south. This geographical position provides this region with life-giving monsoons, but also exposes it to catastrophic natural calamities like floods, cyclones, and tidal surge. For that reason the traditional pitched roof is hampered .That’s why we need to know the method to secure pitched roofs.

Every sheet to be fixed to purlins with hook bolts or twisted nails at each corrugation. More frequent fixings at edges to prevent uplift.

Adequate connections should be made with nylon rope or good quality galvanized wire (instead of jute rope).

Roofing elements should be connected properly: purlin to rafter, rafter to wall plate, wall plate to posts.

Even though more expensive than lean-to (akchala) and gable (dochala) roofing, hipped roofing (chouchala) is more resistant to wind and protects gable end walls from exposure to rain and water penetration.

Fig. Basic features of wind-resistant roofing

Hipped roof This is the strongest type with all sides of the roof sloped. There are no gable ends in this roof. Instead, rafters come across diagonally from the corner and meet the ridge board a short distance from the ends of the house. These are the hip rafters. Other shorter rafters go from the wall plate to the hip rafter and are called jack rafters. After the ridge is firmly in position, the rafters are attached to fit neatly onto the wall plate.

As the corners and the roof edges are zones of higher local wind suctions and the connections of cladding/sheeting to the truss need to be designed for the increased forces.

Overhangs, patios and verandahs experience high wind pressures and should be kept short and small

• Avoid large overhangs as high wind force build up under them. • Overhangs should not be more than 18 inches at verges or eaves. • Build verandah and patio roofs as separate structures rather than extensions of the main building. • They may blow off without damaging the rest of the house.

Restrict unsupported eaves overhangs to not more than 800mm from the wall and do not notch rafters over walls by more than one-quarter of the rafters depth.

Securing the Ridge

• If the rafters are not secure, the ridge can fall apart when strong wind passes over the roof.

The ridge can be secured by using:-(i) COLLAR TIES - Timbers connecting the rafters. Nail them to the side of the rafters.

(ii) GUSSETS - Usually made of steel/plywood. This is used at the

ridge.

(iii)METAL STRAPS over the top of the rafters.

Nowadays people use metal roof structure. MS or GI roof structure can be made typically with 1½ inch x 1½ inch x /8 inch section. It is advisable to make trusses for better strength,

Rainwater Gutter • Rainwater gutters prevent creation of furrows around plinth by rain

falling down from roof eaves. • They also prevent rainwater splashing on walls. • Arsenic-free rainwater can be collected for household use by keeping

a container where the water drains down. • 4 inch diameter PVC (polyvinyl chloride, i.e. plastic) pipe can be cut

into half lengthwise using a saw. • MS (mild steel) flat bar brackets can be screwed to rafters or wall

plate to hold gutter. • If brackets prove too difficult or expensive to make, GI (galvanized

iron) wire or nylon rope can also be used for attaching gutter.

Fig. PVC pipe rainwater gutter detail and house being built with a gutter

Regular maintenance: • Should make regular checks, especially around ridge and corners. • Should replace weakened members, repair loose members. • CI sheet should be tied strongly to structural frame to resist uplift by

strong wind. To further increase wind-resistance, number of purlins should be increased near eaves, ridge and corners.

References:1. Building construction- Sushil Kumar2. Building construction- dr. B. C. Punmia, Ashok & Arun

kumar Jain3. https://www.sheltercluster.org/sites/default/files/doc

s/Cyclone-resistant-housing-BRE.pdf4. http://climatecolab.org/plans/-

/plans/contestId/1301502/planId/13200015. http://www.unisdr.org/files/11711_CycloneArchitectu

re1.pdf6. http://www.sheltercentre.org/sites/default/files/Battli

ng%20the%20Storm.pdf7. http://www.adpc.net/igo/category/ID189/doc/2013-

p74Wob-ADPC-handbook_complete-b.pdf