--- title: "Below and above grade water proofing" subject: "Building Technology" chapter: 1 key: "semester_6/building_technology:waterproof-below-above-grade" source: https://www.bctengineer.com/q/semester_6/building_technology/waterproof-below-above-grade site: BCT Engineering --- # Below and above grade water proofing Building Technology · Chapter 1: Functional Requirements of Buildings - Asked in: 2078 Baisakh, 2+6 marks - Video: [Below Grade and Above Grade Waterproofing of Concrete Structure - Building Services and Repairs](https://www.youtube.com/watch?v=MxM-8s5cZws) — Ekeeda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/building_technology/waterproof-below-above-grade - Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-1 ## Question Explain below grade and above grade water proofing system. Explain the methods to moisture movement in the building. ## Short answer Below-grade waterproofing involves external positive-side membranes or internal negative-side crystalline coatings to resist groundwater pressure, with drainage. Above-grade waterproofing uses roof membranes, brick-bat coba or waterproof paints to protect against rain and sun. Moisture movement is prevented by DPC, cavity walls, sloping roofs, drainage, ventilation and integral compounds. ## Answer #### Below-grade waterproofing **Below-grade** waterproofing is applied to parts of a building below ground level (basements, foundations, retaining walls, underground tanks) that face ground water and hydrostatic pressure. - **Positive side (external)**: A membrane (bituminous coating, self-adhesive sheet, bentonite panels) is put on the earth face of the wall and under the raft, protected by a protection board or brick wall before backfilling. - **Negative side (internal)**: Cementitious or crystalline coating and plaster on the inside face. - **Integral**: Waterproof concrete with admixtures, and water bars in joints. - **Drainage**: Sub-soil drain, gravel layer, and sump. #### Above-grade waterproofing **Above-grade** waterproofing is applied to parts above the ground: exposed walls, roofs, terraces, balconies, bathrooms, parapets. These face rain and sun, but not high water pressure. - Membranes on roofs and terraces (bituminous, PVC or liquid applied) with a protective screed or tiles. - Brick-bat coba over slab with 1:4 mortar screed in slope. - Waterproof paint or silicone coating on exterior walls; cement-sand plaster with compound. - Flashings at junctions, drip moulds, and sealants at joints. #### Methods to stop moisture movement in a building 1. DPC at plinth and above ground floor level. 2. Cavity wall construction. 3. Waterproof plaster and paints; pointing. 4. Roof slope, coba, membranes, flashing, gutters and downpipes. 5. Surface and sub-soil drainage; apron around the building. 6. Ventilation and vapour barrier against condensation. 7. Integral waterproofing compounds in concrete and mortar. 8. Chemical injection or pressure grouting for old walls. 9. Tanking in basements. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does below-grade waterproofing protect against? A. Only sunlight on roofs B. Only condensation on windows C. Ground water and hydrostatic pressure on basements and foundations D. Wind-driven rain on parapets Answer: C. Below-grade systems protect parts under ground level that face ground water pressure. (Key point: Below-grade) 2. Where is the membrane put in positive-side (external) below-grade waterproofing? A. On the inner face of the wall B. On the earth face of the wall and under the raft C. Only at the roof D. Inside the concrete as an admixture only Answer: B. The membrane sits on the outside (earth face), protected by a board or brick wall before backfilling. (Key point: Positive side) 3. Which describes above-grade waterproofing? A. For roofs, terraces and exposed walls facing rain and sun, not high water pressure B. For basements under high pressure C. For underground tanks D. For retaining walls only Answer: A. Above-grade work faces rain and sun but not high water pressure. (Key point: Above-grade) 4. What is laid over the slab with a 1:4 mortar screed for roof waterproofing? A. Brick-bat coba in slope B. Gravel soling C. Sand cushion D. Bentonite panels Answer: A. Brick-bat coba with 1:4 mortar screed in slope drains the roof. 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