Dampness is one problem that is common in a building especially those located at places with high water table. If you have experienced it in a house, you will discover how unappealing it is. It often leads to many undesirable phenomenon such as bad smell, mould growth, damage of costly household properties among others. To fight dampness in the house, it is necessary to start at the construction stage. It is the requirement of building code that floors shall adequately resist the passage of moisture to the inside of the building. As concrete is permeable to moisture, it is necessary to use a damp proof membrane (DPM) under or on top of ground supported concrete floor slabs as effective barrier to moisture rising from the ground floor. The membrane should be continuous with the DPC in walls to prevent moisture rising between the edges of the concrete slab and the walls. A DPM should be impermeable to water in liquid or vapour form and should be tough enough to withstand possible damage during laying of screeds, concretes or floor finishes. The position would vary in accordance with proposed methods of construction.
Definition of terms
Damp Proof Membrane (DPM): This is usually a polyethylene sheet laid under a concrete slab to prevent the concrete from gaining moisture through capillary action. The DPM may be above or below the concrete and continuous with DPC in walls and piers (See Figure 1). The DPM above concrete should be protected by either screed or a floor finish unless it is a material that can serve as floor finish.
Damp Proof Course (DPC): This is a membrane used in walls to prevent the passage of moisture. It is necessary that the DPC is continuous with the DPM because moisture can also pass through walls. However, this is common in cavity walls. The DPC can be formed using bitumen or asphalt, polyethylene or any material that can prevent passage of moisture (See Figure 1).
Insulation: Insulation could be against noise, water, fire. In this case the insulation is against water. It is a material used to prevent the passage of moisture.
Plinth: This is the section of substructure between finished floor level and the ground surface. DPC are sometimes erroneously used in place of plinth. This should not be so. Plinth is related to level while DPC is a material whose height could be same as the level of the DPC (See Figure 1).
Relative Positions of DPM
It is not proper to place the DPM under the hardcore because the hardcore can puncture it. If it would be placed directly above the hardcore, the interstices and sharp points on the hardcore should be made even with a thin layer of sharp sand or blinding concrete (about 50 mm). Otherwise, it can be placed above the concrete.
A membrane below the concrete could be formed with a sheet of polyethylene of at least 300 µm thick with sealed joints and laid on a bed of material that will not damage the sheet.
A membrane above the concrete may either a polyethylene sheet (without a bedding material) of three coats of cold applied bitumen solution or any other moisture/vapour resisting material. It should be protected with either a screed or a floor finish unless the material can serve as floor finish. A common pattern of construction in Nigeria presently is to separate substructure block wall from superstructure block wall by oversite concrete. In this case, the DPM should be made to pass over the block wall as well because concrete alone may permit passage of moisture or vapour.
Figures 2-5 shows the different positions of DPM when insulation are used and when insulation are the not used.
