Components, systems and materials › Building components › Roof slab and roof waterproofing
Roof slab and roof waterproofing
What it is
The roof slab is the top reinforced concrete slab of a building; roof waterproofing is the set of layers laid on it to keep rain out, drain water quickly and reduce heat gain.
Where it is used
Flat and low-slope roofs of houses, apartments, schools, hospitals, commercial and industrial buildings.
How it works
A sloping screed sends water to outlets so it never stands for long. The membrane or coating stops water entering cracks in the slab. Insulation and reflective finishes reduce heat flow into top-floor rooms, and a protection layer shields the membrane from sun and foot traffic.
Types
- Bituminous felt and hot-applied systems
- polymer-modified bitumen sheets (APP or SBS)
- liquid-applied polyurethane, acrylic or polyurea coatings
- cementitious polymer coatings
- traditional lime-concrete or brickbat coba with a tile finish
Materials
Reinforced concrete slab, cement or lightweight screed, primer, membrane or coating, insulation boards, protection screed or tiles, outlet fittings.
The slab is designed for dead, imposed and any water tank or equipment loads. Falls of roughly 1 in 80 to 1 in 100 lead to enough outlets sized for local rainfall intensity. The membrane turns up parapets well above the finished level and tucks into a groove. Movement joints, pipe penetrations and outlets get special detailing because most leaks start there.
Installation and QA steps
- Cure the slab and let it dry; repair cracks and clean the surface.
- Lay the screed to falls and round off corners at parapets.
- Prime and apply the membrane with specified laps and upturns.
- Fit outlets with collars bonded to the membrane.
- Pond-test with water for about 48 hours and inspect the ceiling below.
- Lay protection and keep outlets clear.
- Ponding from poor falls or blocked outlets
- leaks at parapet upturns, outlets and pipe penetrations
- membrane punctured by later services or foot traffic
- thermal cracking of exposed screed
Codes cited
- National Building Code of India 2016 (India): Model code for buildings: planning, structural design, fire, construction practice and safety, building services.
Cited by number only. We do not reproduce code text, tables or figures.
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