---
title: "Positive/negative waterproofing, DPC in basement"
subject: "Building Technology"
chapter: 1
key: "semester_6/building_technology:waterproof-positive-negative-dpc"
source: https://www.bctengineer.com/q/semester_6/building_technology/waterproof-positive-negative-dpc
site: BCT Engineering
---

# Positive/negative waterproofing, DPC in basement

Building Technology · Chapter 1: Functional Requirements of Buildings

- Asked in: 2070 Bhadra, 2+6 marks
- Video: [Negative side basement waterproofing with Koster NB1 Grey (Demo)](https://www.youtube.com/watch?v=pW_PFrjTVgE) — kosterwaterproofing
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/waterproof-positive-negative-dpc
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-1

## Question

Describe positive and negative side water proofing system. Illustrate with necessary sketch for provision of DPC for basement in ordinary soil.

## Short answer

Positive-side waterproofing is on the outer face (new work); negative-side is on the inner face (repairs). For a basement DPC in ordinary soil: lay a bituminous membrane or polythene over PCC on hardcore, lap it with a bitumen-coated external wall plaster, protect with a brick wall, and provide a drain before backfill.

## Answer

#### Positive side and negative side waterproofing
**Positive side (external) waterproofing**: The membrane is applied on the outer face of the basement wall and below the raft, on the side from which water presses. Water is kept out of the structure itself. It is the most effective method and is used in new works. A protective layer (brick wall or board) is placed over the membrane before backfilling.

**Negative side (internal) waterproofing**: The waterproofing is applied on the inner face, opposite to the water side. The wall remains in contact with water, but seepage into the room is stopped. It is used when the outer face cannot be reached, such as in repairs.

| Point | Positive | Negative |
|---|---|---|
| Position | Outer face | Inner face |
| Structure | Protected from water | Exposed to water |
| Effectiveness | Very high | Lower |
| Usually used | New construction | Repair, retrofit |

#### DPC for a basement in ordinary soil
In ordinary (non-water-bearing, well-drained) soil, a basement needs a damp-proof course on the floor and walls.

1. **Floor**: The ground is compacted and a layer of hardcore or stone soling is laid, then 100 mm PCC (1:3:6), then the DPC layer (a bituminous membrane or 2 to 3 layers of polythene), then a protective cement-sand screed 40 to 50 mm thick, and then the floor slab.
2. **Wall**: The wall faces are plastered with 20 mm cement-sand 1:3 mortar with a waterproofing compound, and two coats of hot bitumen are applied on the outer face. A horizontal DPC is placed at plinth level above ground.
3. **Joint**: The floor DPC and the wall DPC are joined with a lap of at least 150 mm so that the barrier is continuous.
4. A protective brick wall is built outside the membrane, and the excavation is then backfilled, with a drain at the base.

```text
        GL ----- horizontal DPC at plinth
          |memb|wall|
          |    |    |
  floor:  screed
          membrane (DPC)  <- lap with wall membrane
          PCC 1:3:6
          hardcore
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why is positive side waterproofing more effective?
   A. It is applied inside the room
   B. Water is kept out of the structure itself
   C. It needs no protection layer
   D. It lets the wall stay in contact with water
   Answer: B. Stopping water outside protects the wall. (Key point: Positive system)

2. In the basement floor DPC, what is laid directly over the 100 mm PCC?
   A. The floor slab directly
   B. Hardcore
   C. The DPC membrane, then a protective screed
   D. Backfill
   Answer: C. Order: compacted ground, hardcore, PCC 1:3:6, DPC membrane, protective screed 40-50 mm, floor slab. (Key point: Floor DPC order)

3. How are floor DPC and wall DPC joined?
   A. Left with a gap for drainage
   B. Lapped at least 150 mm so the barrier is continuous
   C. Butted without lap
   D. Lapped 20 mm
   Answer: B. A lap of at least 150 mm keeps the barrier continuous. (Key point: Joint)

4. Why is a brick wall built outside the membrane before backfill?
   A. To increase water pressure
   B. To protect the membrane from damage
   C. To act as the DPC
   D. To replace the raft
   Answer: B. The protective wall prevents the membrane being damaged during backfilling. (Key point: Protection)

## Related in Building Technology

- [Positive and negative side water proofing](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Awaterproof-positive-negative) — Building Technology, chapter 1
- [Basement floor and its waterproofing](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Abasement-waterproofing) — Building Technology, chapter 1, asked 2×
- [Below and above grade water proofing](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Awaterproof-below-above-grade) — Building Technology, chapter 1
- [Damp-proofing versus water-proofing](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Adamp-vs-waterproofing) — Building Technology, chapter 1

## Related in other subjects

- [De Morgan; positive NAND equals negative NOR](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Ademorgan-positive-nand-negative-nor) — Digital Logic, chapter 2
- [De Morgan; negative OR equals positive AND](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Ademorgan-negative-or-positive-and) — Digital Logic, chapter 2
- [Equal positive and negative sequence impedances](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Astatic-circuit-positive-negative-equal) — Power System Analysis II, chapter 4