What is DPC
The damp proof course (DPC) is generally applied at basement levels, which restricts the movement of moisture through walls and floors. The selection of materials for the damp proof course and its various methods of applications in buildings is discussed.
Materials for Damp Proof Course (DPC)
Properties of Materials used for DPC
An effective damp proofing material should have the following properties;
- It should be impervious.
- It should be strong and durable and should be capable of withstanding both dead as well as live loads without damage.
- It should be dimensionally stable.
- It should be free from deliquescent salts like sulfates, chlorides, and nitrates.
Types of Materials used for Damp Proof Course
The materials commonly used to check dampness can be divided into the following three categories:
- Flexible Materials: Materials like bitumen felts (which may be hessian based or fiber/glass fiber-based), plastic sheeting (polythene sheets), etc.
- Semi-rigid Materials: Materials like mastic, asphalt, or a combination of materials or layers.
- Rigid Materials: Materials like first-class bricks, stones, slate, cement concrete, etc.
Selection of Materials for Damp Proof Course in Buildings
The choice of material to function as an effective damp proof course requires a judicious selection. It depends upon the climate and atmospheric conditions, nature of the structure, and the situation where DPC is to be provided.
Fig 1: Cross section of Damp Proof Course.
The points to be kept in view while making selection of DPC materials are briefly discussed below:
- DPC above ground level
For DPC above ground level with wall thickness generally not exceeding 40 cm, any one of the types of materials mentioned above may be used. Cement concrete is however, commonly adopted material for DPC at plinth level, 38 to 50mm thick layer of cement concrete M15 (1:2:4 mix) serves the purpose under normal conditions.
In the case of a damp and humid atmosphere, a richer mix of concrete should be used. The concrete is further made dense by adding waterproofing materials like Pudlo, Impermo, Waterlock, etc. in its ingredients during the process of mixing. It is used to apply two coats of hot bitumen over the third surface of the concrete DPC.
- DPC Material for floors, roofs etc.
For greater wall thickness or where DPC is to be laid over large areas such as floors, roofs, etc., the choice is limited to flexible materials that provide a lesser number of joints like mastic, asphalt, bitumen felts, plastic sheets, etc.
The felts, when used, should be adequately bonded to the surface with bitumen and laid with joints properly lapped and sealed.
- DPC Material for situations where differential thermal movements occur
In parapet walls and other such situations, materials like mastic, asphalt, bitumen felts, and metal (copper or lead) are recommended.
It is vital to ensure that the DPC material is flexible to avoid any damage or puncture of the material due to differential thermal movement between the material of the roof and the parapet.
- DPC material for Cavity Walls
In cavity wall construction, like cavity over the door or window should be bridged by flexible material like bitumen felt, strips or lead, etc.
Methods of Damp Proof Course Installation in Construction
The general principles to be observed while laying a damp proof course are:
- The DPC should cover the full thickness of the walls, excluding rendering.
- The mortar bed upon which the DPC is to be laid should be made level, even and free from projections. Uneven base is likely to cause damage to DPC.
- When a horizontal DPC is to be continued up a vertical face, a cement concrete fillet 75mm in radius should be provided at the junction before the treatment.
- Each DPC should be placed in correct relation to other DPC to ensure a complete and continuous barrier to the passage of water from floors, walls, or roof.