Article - damp proof course injection - damp proofing questions & answers

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Chemical Damp Proof Courses - Q's and A's

 

WHAT IS A CHEMICAL DAMP PROOF COURSE USED FOR:

Chemical damp-proof courses are inserted to control the vertical passage of moisture from the ground and are almost all installed in properties where no damp-proof course exists, or it has broken down with age (not a common occurrence, contrary to popular opinion in the Trade).

For help with uses call our experts on 01626 331351.


HOW ARE CHEMICAL DAMP PROOF COURSES INSTALLED:

Chemical damp-proof courses should be installed in a position in accordance with good practice as described in BS 6576:1985, "Code of Practice for the Installation of Chemical Damp-proof Courses", a minimum of 150mm (6 inches) above external ground level, in external walls.

Additional rules apply to cavity walls, vertical damp proof courses and internal walls.

They are installed in walls by various methods depending on the particular system being employed, but the ultimate objective is to provide a water repellent or pore blocking material in a continuous horizontal band in the masonry, thereby to provide a 'barrier' to water rising from the ground.

These days Silicone Cream emulsion is used most commonly. It requires no special equipment to install, no pressure and has no chemical hazards.

For help with installation methods and rules call our experts on 01626 331351.


WHAT IS RISING DAMP

True rising damp is natural ground water rising by capillary action through porous building materials. The natural water in the ground under a building climbs up through the porous parts of the walls and floors up to the point where surface evaporation stops further climb.

For help with the causes of damp call our experts on 01626 331351.


WHY IS IT SOMETIMES REFERRED TO AS A MYTH?

Because most dampness problems in buildings are NOT natural rising dampness.

In most cases they are caused by building defects, such as:

  • water leaks inside the building - particularly in modern buildings fitted with plastic damp proof course and membrane materials

  • water leaks under the building or from next door or the road - particularly in older buildings, built prior to the use of plastics

  • 'bridging' of the damp proof course - mainly by paths, roads, steps and added structures

For help with the MYTHS call our experts on 01626 331351.


WHAT IS THE BIGGEST MYTH?

That traditional damp proof courses (slate, stone, bitumen) break down!

Buried inside the wall structure they do not significantly break down, so do not need replacement.

For help with diagnosis call our experts on 01626 331351.


HOW DOES A DAMP PROOF COURSE WORK?

The only continuous pathways through which water can rise through a wall are the mortar beds: for water to pass, say, from brick to brick it must still cross a mortar bed (bottom of figure, right). It is therefore essential that the damp-proofing material impregnates the mortar courses since these form the major pathway for the rise of water within walls. Damp-proofing the masonry units (e.g., bricks) alone is of very little value! Porous mortar and impervious/water repellent brickwork will still allow rising dampness to occur. However, if the pores in the mortar line are made water repellent or blocked then the water cannot rise since it cannot traverse the mortar beds to do so.

For help with how damp proof courses work call our experts on 01626 331351.


NEW BUILD OR NEW MORTAR - DOES IT MATTER?

In the majority of older properties the mortar is not alkaline, so that the water repellent formulations based either silicone resins, aluminium stearate, or methyl siliconate (sodium or potassium methyl siliconate) can be used. Occasionally, however, the mortar may highly alkaline such as in a recently constructed wall (e.g. where the physical damp-proof course has been omitted). This will exclude the use of the methyl siliconates since the highly alkaline conditions found, for example, in new mortars prevent the formation of the water repellent resin.

For help with types of damp proofing fluids and creams and where to use them call our experts on 01626 331351.


BRIDGING AND COVERING A DAMP PROOF COURSE

Care should be taken to ensure that the damp-proof course is not bridged by high external ground levels, blocked cavities or debris piled against the wall; ground levels should be lowered, cavities cleaned out or the area below the inserted damp-proof course might be 'tanked' internally if deemed necessary.

NOTE: Should minor bridging of an effective damp-proof course occur, for example by moderately porous plasterwork, then it is highly unlikely that the dampness would continue to rise to its original height. If the damp-proof course is effective the pathway for moisture should be limited within the wall itself. Any porous plasterwork is at the surface where evaporation would serve to restrict the flow of rising water through it.

Therefore, moisture is unlikely to pass through this relatively narrow pathway at a rate sufficient for it to reach the height of rise prior to the insertion of the damp-proof course assuming, of course, that the injected damp-proof course is moderately effective. Problems of this type together with defects in construction of the floor/wall junction usually manifest themselves at the base of the wall.

For help with 'bridging' and blocked cavities call our experts on 01626 331351.


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