Basics in Building Construction

Basics in Building Construction 4(2+2)

Lesson 05 : Damp Proofing Treatments

Damp Proofing Treatement To Flat Roofs

Flat roofs require relatively heavier and costlier water-proofing treatment as compared with pitched or sloped roofs. The specification of material used for the purpose should be such that it performs the function of water-proofing as well as provides adequate thermal insulation. Stagnation of water on the roof is considered to be the root cause of leakage and dampness in flat roofs. This can be avoided by providing adequate roof slope and rain water pipes. In case of R.C.C. slab roofing with proper slope is considered desirable. This may be achieved either by varying the thickness of the terracing material or by constructing the roof slab with a slope, or by providing part slope in the roof slab and part in the terracing material. In addition to the slope, the size and the spacing of the rain water pipes or the outlets require due consideration for the proper drainage of the roof. In general practice, one 10 cm diameter pipe is considered suitable for every 30 sq. cm of the roof area to be drained.

In cases, where slopes for the drainage of the roof are given in the roof slab itself or in situations where thermal insulation is not important and the problem of slopes in flat roof is tackled suitably, the damp – proofing treatment for the roof may consist in laying bitumen felt directly over the surface of roof slab after painting the roof top with hot bitumen. The bitumen felt may be hessian based or fiber based. Depending upon the type of building, climate and atmospheric conditions of the site, the treatment with felts may be with four courses, six courses or eight courses. The four course treatment is recommended for moderate conditions whereas the six and eight course treatment are recommended for severe conditions.

The steps in the method of laying a four course treatment are:

1. Apply hot bitumen at the rate of 1.2 kg / m2 on the roof surface.
2. Lay, bitumen felt in single layer over the hot bonding material laid in (a) the end and side laps for the felt being not less than 100 and 75 mm. respectively.
3. Apply hot bitumen at the rate of 1.2 kg / m2 over the bitumen felt laid in (b)
4. Spread pea-sized gravel or grit at the rate of 0.008m2 per square meter over the layer of hot bitumen in (c)

In case of six courses treatment

• the first three courses are laid in a similar manner as explained in steps (a), (b) and (c) above.
• The fourth course consists of laying another layer of bitumen felt over the course of hot bitumen explained in step (c) above.
• The fifth course consists of applying hot bitumen at the rate of 1.2 kg / m2 over fourth course.
• The sixth course consists of spreading pea-sized gravel or grit at the rate of 0.008 m3 per square meter over the layer of hot bitumen laid in fifth course.