Basic facts about building materials

  • Efflock to prevent rising damp

    Photograph showing rising damp and subsequent salt attack below DPC level.

    Rising damp is a destructive phenomenon in buildings. It is common knowledge that damp very quickly will create rot in structural timbers such as bearers and joists, and therefore a DPC (damp proof course) is provided to prevent such damage. A modern DPC is typically a black embossed polythene roll that is laid out directly beneath ant-capping, or just below finished ground floor level. In the past, DPCs have been made of lead, slate, malthoid, and sometimes special bricks.

    What is not usually considered, is that rising damp can have destructive consequences on masonry below the DPC level. The standard recommended specification for brickwork below the DPC, is for mortar to be a durable cement mortar (which of course is still porous). There is a flaw in this design, over a long period, salt from the ground is carried in the rising damp, where it becomes highly concentrated in the masonry. This build-up of salt occurs in the repetitive process of moisture evaporating from the masonry and leaving the salts behind. As the concentration of salt becomes higher, the more it crystallises with each dry cycle to blow masonry apart. If the mortar is more durable than the brick, then the brick becomes the sacrificial material. (Lime mortar in old buildings acts to protect the brick or masonry units from decay, as the lime is softer and hygroscopic, acting as the sacrificial material). It is therefore incorrect to re-point an old lime mortar wall with a cement mortar. 

    Where does the salt come from? 

    Salt is present in soil. With the removal of trees and heavy vegetation to clear land for housing estates and     house building, the problem can be exacerbated with a rise of the water table resulting from less tree roots. Common garden fertilisers such as phosphate can also contribute more salt being introduced into a sub-floor masonry wall. When salt in the masonry crystallises, it can exert pressure in excess of 200MPa! The tensile strength of concrete is only about 10MPa, so you can understand how easily salt can make a wall crumble!

    So how can Efflock be used to control rising damp and salt attack? 
    By using Efflock in brick mortar for the first few courses of brickwork out of the ground, you can provide an effective damp proof course in each bed joint of the brickwork. The 10mm mortar joint containing Efflock will be permanently hydrophobic and will prevent the migration of damp up into the wall. By having Efflock in the first few courses, this will provide protection even if the soil level is altered from time to time over the years. 

    Rendered walls:
    Ordinary render is porous and is subject to salt destruction described above, also causing very obvious and unsightly blistering to paint finishes in the process. Efflock can be used in the render mix to prevent these problems occurring, and has no detrimental effect to adhesion of paint finishes.

    Photograph showing rising damp damage to painted and rendered exterior wall. (Building only 8 years old).

    For more information or advice, please contact Ben. Details can be found on our 'Contact" page.

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