Moisture sources and control in basement

Moisture

Moisture is water, in liquid or vapour form, present in building materials or the air, which in excess causes dampness.

Sources of moisture in a building

1. Rising damp from the ground (capillary action).
2. Rain penetrating through walls, roofs and windows.
3. Condensation of water vapour on cold surfaces.
4. Leakage from water pipes, sanitary fittings and drain lines.
5. Moisture left in materials from construction (concrete, mortar, plaster).
6. Poor surface drainage, ground water and a high water table.
7. Hygroscopic action of salts present in bricks and sand.

Moisture control in the basement of an RCC building

A basement is below ground, so it faces ground water and hydrostatic pressure.

1. Site drainage: Surface water is diverted by sloped ground and drains. A sub-soil drain (perforated pipe in gravel) around the foundation lowers the water table.
2. Waterproof concrete: Dense M25 or higher concrete with a waterproofing admixture (crystalline or plasticiser), and water-stops at construction joints.
3. Tanking (membrane): A continuous waterproof membrane under the floor and up the walls.
   • External tanking (positive side): a membrane of bitumen or APP sheet on the earth face, protected by brick or PCC.
   • Internal tanking (negative side): a membrane and cement-sand plaster inside, held by a counter wall.
4. Floor: PCC 1:3:6 on a layer of compacted hardcore, then the membrane, and then the RCC raft or floor slab with a screed.
5. Drained cavity: An inner skin with a cavity that drains to a sump, with a pump.
6. DPC at the floor-wall junction, and at ground level in the wall above.

 Ground    |  RCC wall  |Membrane| backfill
 level --- |            |        | 
           |  Floor slab        |
 ----------+====================+---
           PCC + membrane + hardcore
           [sub-soil drain]