Roof Waterproofing By Brick Bat Coba

By
Er. Kaushal Kishore ,
Materials Engineer, Roorkee

INTRODUCTION
Waterproofing is a treatment of a surface or structure to prevent the passage of water under hydrostatic pressure. Waterproofing barrier system may be placed on the positive or negative side. Damp proofing is a treatment of a surface or structure to resit the passage of water in the absence of hydrostatic pressure. A damp proofing barrier system is used to perform the same functions as a waterproofing system but cannot be used to protect against water pressure. Water may be forced through building members by hydrostatic pressure, water vapour gradient, capillary action, wind-driven rain, or any combination of these. This movement is aggravated by porous concrete, cracks or structural defects, or joints that are improperly designed or installed. Leakage of water into structure may cause structural damage, and invariably cause damage to the contents of the structure.

New roofs RB or RCC slabs must be constructed specified by the designer. Roof waterproofing is a widely misunderstood subject. Often inadequate attention given during the construction of RB or RCC roof slab, wrong products used for waterproofing and generally insufficient treatment given, lead to leakage. Movement because of structural deflection, settlement, etc. and steep temperature variation being exposed, cause development of cracks in the roof slab and water start leaking from these cracks.

While constructing RCC roof slab, it should be borne in mind that the practice of using concrete which is not watertight and placing too much reliance on the waterproofing measures is not desirable. Concrete should be made watertight in itself and the waterproofing method should be looked upon as additional safety devices.

The grade of roof slab concrete shall be strictly as specified by the designer. The concrete materials should be properly proportioned, maintaining the specified maximum water, cement ratio, minimum cement content and required workability. The concrete should be admixed with a Superplasticiser.

ROOF WATERPROOFING BY BRICK BAT COBA
Roof slabs constructed either by RC or RCC needs insulation for thermal comfort and waterproofing treatment to prevent leakage of water. Both these requirements are effectively full fill by brick bat coba treatment, the details of which are being below:

All existing treatment, coatings on roof slab top is to be removed and surface cleaned by hard wire brush and washed with water. The surface should be free from any oil, grease, dust etc. Remedial measured by provided to all structural cracks. Expansions joints should be treated as per standard practice.

All non-structural cracks more than 0.5 mm wide and construction joints if any, should be cut in “V” shape, cleaned with wire brush and washed, the cracks are then filled by polymer modified cement or mortar using acrylic polymer, with addition cement slurry mix is spread upon cleaned SSD roof surface. Over this 15 mm thick cement, sand mortar, 1:4 admixed, with water proofer is laid.

BRICK BAT LAYING
On the above green mortar, a layer of brick bats, soaked over night in water, is laid, having an average thickness of about 110 mm, about 70 mm near rain water pipe and 150 mm at ridge.

The gaps between the brick bats are generally kept between 15 and 20 mm. These gaps are filled with cement sand mortar, 1:4, admixed with waterproofer. In hot and dry weather, the surface should be covered with wet gunny bags immediately after finishing. Curing should start next day and continued for 7 days.

FINISHING LAYER
The top surface is then finished smooth with 20 mm thick cement sand mortar, 1:4, admixed with a waterproofer. All liquid admixtures should be mixed with the mixing water. The surface when green is marked with 300 mm false squares. Curing is to be done by ponding.

REFERENCES
1. Kishore Kaushal, “Polymer Modified Mortars for Concrete Repair”. Civil Engineering & Construction Review, Nov. 1994, pp. 31-37.
2. Kishore Kaushal, “Polymer Adhesives Application in Concrete”, NBM & CW Dec. 98, pp. 21-26.
3. Kishore Kaushal, “Waterproofing of Buildings”, NBM & CR, Apr., 2000, pp. 50-55.
4. Kishore Kaushal, “Repair of cracks in Concrete Structures”, NBM & CW, Jul. 2000, pp. 65-68.
5. Kishore Kaushal, “Terrace & Roof Top Gardens”, NBM & CW Mar. 2001, pp. 68-72.
6. Kishore Kaushal, “Waterproofing of Old heritage Building with Ferrocement & Construction Chemicals”, NBM & CW May. 2001, pp. 54-57.
7. Kishore Kaushal, “Polymer Modified Mortars and Concrete Mix Design”, ICI Journal, Apr-Jun. 2001, pp. 39-42.
8. Kishore Kaushal, Dr. Krishan Kumar, “Waterproofing of Basements & Corrective Measures”, Proceeding of Seminar
on Structural Rehabilitation and Waterproofing. The Institution
of Engineers (India). All India Seminar 13-14 Oct. 2003,
pp. 19.1 – 19.5.
9. Kishore Kaushal, “Waterproofing of Basement”, CE % CR Dec. 2003, pp. 34-38.

We are thankful to Er. Kaushal Kishore for submitting this very useful research paper to us.

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