Reinforcing Steel Concrete Cover (nfm)

Roofing systems are exposed to the full brunt of the weather and can leak or fail prematurely if not properly designed, installed, and maintained. Leakage can be costly and adversely affect the functions within the building. Since modern roofing systems contain considerable thermal insulation, moisture intrusion lowers thermal efficiency and hinders energy conservation. Wet materials support fungus or mildew and can emit odors leading to sick buildings and occupants. There are two starting points in structural design. Further, with an occupied building, construction noises, fumes, fire hazards, and roof access all take on increased importance.

Location

cast in place

concrete type pre cast

pre stressed

Exposed to the weather

75mm

51mm

51mm

Interior beams and columns (to ties)

51mm

51mm

51mm

Interior walls and slabs

25mm

25mm

25 - 38mm

slabs on grad (wire fabric)

13mm thickness of slab but not less than 25mm

Waterfront (3m below and 3m above mean low tide)

75mm

75mm

51mm

Roofing Systems and Materials

Roofing type

cost range

kg / m2

service life

warranty

UV resistance

remarks

Built-up

asphalt, base 1-2 sheet 2 plys mineral cap

1 - 2

19.5

5 - 20

10 - 15

Good

msot prone, because of complexity to leakage and sheet moisture entrapment

Fluid applied

Acrylic elastomer

w/o foam

2-3

1.12

7 - 20

1 - 20

Good

use on slopes 0.5:12 or greater; do not use in areas with relative humidity averaging 80-85% year. Recommend use of fast setting compounds

Acrylic elast. w/l in.foam

2-3

2.24

7 - 20

1 - 20

Good

polyyethane elastomer

w/o foam

4 - 5

1.46

7 - 20

1 - 20

Fair good

moisture cured urethanes-use on slopes 0.025:12 or greater. Catalyzed urethanes - us on roof or any slope. Resistance to UV is good if top coated with aliphatic urethane

polyyethane

elast. w/l in.foam

4 - 5

2.58

7 - 20

1 - 20

Fair good

Sheet - applied

polyvinyl chloride (PVC)

4-5

1.46

10-20

5-10

poor

poor resistance to petroleum and solvents

ethylene propylene diene (EPDM)

4-5

1.7

10-30+

10-25

good

poor resistence to petroleum, most oils and plastic roof cement

chlorinated polyethylene (CPE)

4-5

1.46

5 - 15

10 max

fair

poor resistance to petroleum distillates strong oxidizers, aromatic hydrocarbons

modified bitumen

3-4

6.33

5 - 15

5 - 10

fair

poor resistance to petroleum distillates, hydrocarbons, solvents & some chemicals

fiber reinforced cement (non abestos)

3-5

19.5 - 27.3

30 - 50

30 - 50

good

use on slopes 4:12 (w/o waterproofing)

Corrugated Metal

aluminium

3-4

2.9

10

good

galvanized steel

3-4

8.8

10

good

Typical Code: Allowable Re-Roofs Over Existing Roofing

Existing Roofing

New Overlay Roofing

Built- Up

Shake

Asphalt Shingle

Tile Roof

Metal Roof

Modified Bitumen

SPF

Slate

NP

NP

NP

NP

NP

NP

NP

Built-Up

Yes

NP

Yes (1:6)

Yes (2.5:12)

Yes

Yes

Yes

Asphalt Shingle

NP

NP

Yes

Yes (2.5:12)

Yes

Yes

NP

Asphalt shingle Over Wood

NP

NP

Yes

Yes

Yes

Yes

NP

Asphalt shingle / Asphalt shingle

NP

NP

Yes

Yes

Yes

Yes

NP

Tile Roof

NP

NP

NP

NP

NP

NP

NP

Metal Roof

NP

NP

NP

NP

Yes

NP

Yes

Modified Bitumen

Yes

NP

Yes

Yes (2.5:12)

Yes

Yes

Yes