15 Mar What is Rising Damp?
That is the million dollar question! Basically, rising damp is the passage of groundwater rising up through a porous masonry material. The water rises through a series of inter-connecting pores called capillary action, which means the porous masonry soaks up groundwater like the wick in an oil lamp.
So if the suction of the wall is greater than the suction of the ground then water will rise, if not, then water will not rise. Think about what type of material would be more absorbent in some water? A lead brick or a sand stone brick?
In the image above you can see the salt band quite clearly. These salts are a combination of soluble nitrates and chlorides carried up from the ground through the porous concrete block work.
When the rise of the moisture reaches a level of around 1.2 metres it’ll evaporate and leave the hygroscopic salts on the wall. These salts will absorb any atmospheric moisture and create the scene you see above. A psychical or chemical damp proof course at ground level would stop the groundwater from rising to this level. In this instance the external ground levels were installed over the existing damp proof course therefore adding to the rising damp.
However, water does not automatically rise where there is no DPC present. Pore structure of the masonry will determine this factor.
The cracks and pores in brick, mortar, concrete, plaster and wood are very minute, so water can rise as a result of capillary attraction to a great height. Yet because the pores and cracks in a wall are not continuous, the process is very slow and in practice evaporation sets a limit, hence why rising damp is limited to half a metre, to one metre in height, unless evaporation is prevented.
In the picture above you can see the moisture pathways of the groundwater clearly in action. The impervious brick work is allowing moisture to travel upwards through the permeable mortar instead.
Height of the rise of water is governed by pore diameter, which means the narrower of the pores, the greater the rise.
Capillary movement is caused by surface tension of the water within the minuscule pores contained in the structure of the masonry material. Again, think lead brick pore structure and sandstone brick pore structure…which has the most microscopic pores for water to fill?
Hydrostatic pressure is where water is exerted under its own weight/gravity. It is in effect the pressure exerted by a head of water. It’ll force water out of capillaries. Think wet basement floor and walls.
How do you prevent dampness?
A damp proof course is intended to prevent moisture rising from the ground into the walls above the damp proof course level by means of capillary action. In many cases the insertion of a damp-proof course will not by itself result in dry walls.
There are other sources of dampness that you should be aware of that occurs within a property. The principal causes of these sources of dampness can be:
- Rainwater penetration caused by: –
Defective rainwater goods
Porous brick or stonework
Defective and raised pointing / mortar beds
Weathered and defective chimneys / abutting walls
Defective external plumbing / drainage pipes
- Atmospheric moisture:-
Contaminated plaster / retained hygroscopic salts in plaster attracting moisture
Lateral penetration of ground water where walls are retaining
Dampness to solid floors which lack an effective damp proof membrane
- Bridging :-
Bridging means the partial or total covering of the DPC with a porous material allowing moisture from the ground to by-pass the damp proof course…
Common examples of bridging:
- Flower beds and pathways installed over the top of existing ground levels thus raising the overall height of the ground level and bridging any existing DPC.
- Solid floors.
- Plinths and external rendering.
- Porous internal plaster and in general porous plaster finishes.
The damp conclusion
There are many pathways moisture can find its way into your home or property, unbeknown to you. Through a process of elimination, good practice and a keen eye, we can more often than not come to the conclusion on your damp quandary and give you a viable and helpful solution.
Misdiagnosis and poor practises in the damp proofing industry does occur, just like in all industries. Everyone has heard of the rogue cowboy builder or the dodgy car mechanic and that crooked banker sadly that is just the world we live in. You can’t have the good without the bad.
We are proudly good at what we do at INSITU, if just a tad modest with it! We have been successfully diagnosing damp and timber decay issues for many years and resolving them quickly and efficiently.
Our surveyors have a very enthusiastic approach to their job and to them; it’s all about the problem solving and the fault finding in a property. To confidently analyze a damp issue and put it right has to be one of the most satisfying aspects of our job. If it wasn’t, then we wouldn’t be where we are today.