Damp on a wall is a symptom and it has several different causes, and the treatments for them have almost nothing in common. Rising damp, penetrating damp and condensation all show up as a stain on a wall and all three get sold the same chemical injection by somebody, which is why so many houses have a damp course fitted and a wall that is still wet.
So the first visit is a look rather than a price. Where is it on the wall, how high does it go, is there a tide mark, what is outside that bit of the wall, what is the ground level doing, and is it worse in winter. Those answers decide the job, and sometimes the answer is that no damp proofing is needed at all.
Telling the three apart
Rising damp comes up from the ground through the wall and stops at a level, usually about a metre, often with a tide mark of salts where the water has been evaporating. It is much rarer than the number of houses treated for it suggests.
Penetrating damp comes through from outside at the point where something has failed: a cracked render, a blocked gutter running down a wall, failed pointing, a ground level that has been raised above the damp course by a new patio. It is worse after rain and it is usually patchy rather than level. Condensation appears on cold surfaces, in corners and behind furniture, is worst in winter, and comes with black mould rather than salts. Treating the third one with an injected damp course achieves nothing at all.
Where it is penetrating damp, the work is outside and it is often cheap: clear the gutter, repoint the section, cut the render back off the damp course, drop the ground level away from the wall. Do that and the wall dries on its own, and the only internal work is replastering once it has.
Where it is condensation, the answer is ventilation and heat rather than a chemical. Extract that actually vents outside, trickle vents that are open, and enough background warmth that the wall is not the coldest surface in the room. It gets said plainly even though it is not work anybody is being paid for, because selling a damp course for condensation is the oldest trick in the trade.
A genuine failed or absent damp course gets a new one, injected as a cream into a drilled course of mortar at the right height. It is a day's work on a normal wall and it does nothing on its own: the wall above it is still full of water and salts and has to be taken back and replastered before anything is decorated.
Tanking is the other answer, and it is what a cellar or a below ground room needs, because there is no damp course that will hold back water under pressure from the surrounding ground. That is a membrane or a cementitious system, taken up the wall and across the floor junction, and it is a bigger job with a different price.
The wall gets hacked back, usually to a course above the highest sign of damp, and left to dry. How long depends on the wall, and it is months rather than weeks on a solid wall that has been wet for years. Rushing that is the commonest reason a treated wall fails again.
Then it is a salt retardant backing coat rather than an ordinary one, kept off the floor at the bottom so it is not bridging, and the finish over that. Salts left in a wall come straight through ordinary plaster and show on the paint as a white bloom within months, and at that point the only fix is to take it off and do it again properly.
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It stops at a level, usually about a metre, and often leaves a tide mark of salts. If the patch is patchy, higher up, or worse after rain, it is far more likely to be coming through from outside.
Do I definitely need a damp course?
Often not. A lot of what gets treated as rising damp is a blocked gutter, a raised ground level or condensation. The first visit is a look, and if the answer is that no damp proofing is needed, that is what gets said.
How long before the wall can be replastered?
After a new damp course, months rather than weeks on a solid wall that has been wet a long time. Replastering a wall that is still drying is the commonest reason the same wall fails again.
Why does the plaster have to be a special mix?
Because the salts left in the wall travel through ordinary plaster and come out on the paint as a white bloom. A salt retardant backing coat holds them back, and it has to be kept clear of the floor so it is not bridged.
