--- title: "Basement floor and its waterproofing" subject: "Building Technology" chapter: 1 key: "semester_6/building_technology:basement-waterproofing" source: https://www.bctengineer.com/q/semester_6/building_technology/basement-waterproofing site: BCT Engineering --- # Basement floor and its waterproofing Building Technology · Chapter 1: Functional Requirements of Buildings - Asked in: 2066 Bhadra (old course), 16 marks; 2066 Jestha (old course), 3+7 marks (asked 2 times) - Video: [Basement Waterproofing 101: Which System is Right for Your Project?](https://www.youtube.com/watch?v=CZO55iOLbFQ) — ASIRI Designs - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/building_technology/basement-waterproofing - Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-1 ## Question What is basement floor? What type of wall is appropriate for basement wall? Explain about waterproofing techniques in basement. Support your answer with neat and clean sketches. ## Short answer Basement floor is lowest floor on ground. RCC retaining wall (M25+) is appropriate. Techniques: integral waterproofing, external/internal tanking, cavity drain system, sub-soil drainage, grouting. ## Answer #### Basement floor A **basement** is the floor of a building that is partly or wholly below the ground level. Basements are used for parking, services, storage and sometimes for living. The **basement floor** is the lowest floor, usually a raft or slab resting on the ground, with a base of well-compacted hardcore and PCC. #### Type of wall appropriate for basement The wall faces earth pressure and ground water, so it must be strong and watertight. A **reinforced concrete retaining wall** (M25 or higher, 200 to 300 mm thick, with waterproofing admixture), cast monolithic with the raft, is the most suitable. Masonry walls (brick or stone in 1:4 cement mortar) are acceptable only for shallow, dry basements; the RCC wall can be designed for lateral pressure and hydrostatic pressure. #### Waterproofing techniques 1. **Integral waterproofing**: Dense concrete with low water-cement ratio (below 0.45), plasticiser or crystalline admixture, and PVC water bars in construction joints. 2. **Tanking (membrane waterproofing)** - *External (positive side) tanking*: Membrane (bituminous coating, APP/SBS sheet, or HDPE) applied on the outside face of the wall and under the raft. This is the best way because water is stopped before it enters the wall. A protection board or brick wall guards the membrane during backfilling. - *Internal (negative side) tanking*: Used when the outside is not reachable. Waterproof cement plaster with crystalline coating, or mastic asphalt, is applied inside and held by an inner loading wall. 3. **Cavity drain system**: An inner skin separated by a cavity; leaking water flows to a sump and is pumped out. 4. **Sub-soil drainage**: A perforated pipe around the footing in graded gravel, leading to a sump. 5. **Grouting**: Cement or chemical injection in cracks and joints. 6. **Surface treatment**: Cement-sand 1:3 plaster with waterproofing compound, 20 mm thick, in two coats. ```text GL ___ ___ | | backfill | | | | protection board | | | | membrane | | | | RCC wall | | |__|____________________|__| Basement floor: screed membrane PCC + hardcore sump [ ] <- drain pipe ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which wall is most suitable for a basement? A. Dry brick masonry B. RCC retaining wall, M25 or higher, cast monolithic with the raft C. Wood panel D. Thin half-brick wall Answer: B. It can be designed for earth and hydrostatic pressure; masonry is only for shallow, dry basements. (Key point: Basement wall) 2. Why is external tanking the best? A. Water is stopped before it enters the wall B. It is cheaper to inspect C. It needs no protection D. It works only from inside Answer: A. Membrane on the outer face stops water first; protection board guards it during backfill. (Key point: External tanking) 3. Internal tanking is used when A. The wall is very thick B. There is no water C. The outside is not reachable D. Outside is easily reachable Answer: C. It is applied inside and held by an inner loading wall. (Key point: Internal tanking) 4. Integral waterproofing needs a water-cement ratio A. Above 0.8 B. Exactly 1.0 C. Any value D. Below 0.45 Answer: D. A low ratio gives dense concrete, with plasticiser and PVC water bars. 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