| 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.
INSTALLATION:
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". 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.
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.
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.
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.
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