Are your gutters overflowing and allowing water a direct route down your walls, triggering staining? If so then clean them out! The humidity that has run down the walls may manifest in salting or staining internally. This is ok, you can deal with that with a damp proof membrane.
All properties built within the last 5 decades, at least in the UK, must have a physical barrier inserted in to the brick/block work at 150mm above external ground levels. They are commonly described as a damp proof course (dpc). This form of damp proof membrane inserted into the mortar course is a highly effective barrier to deal with dampness from the surface moving up through capillary action to manifest in house within your property. Thus first it is very important to establish whether you have one.
It might not exactly become a physical buffer since there are also chemical damp proof walls which are being used on properties which weren’t built with a physical dpc or have been retrofitted to the structure if the original was defective. Evidence of this can be seen in small holes that contain recently been drilled to the mortar course and later completed, 150mm above ground level. This may have been done to deal with damp. These types of damp proof membranes are now making use of better technology from that used twenty years previously. Take note these type of products will still need skilled application and will require re-rendering up to 1m internally with a sodium retardant additive. The making process will take time and will be bringing out wet trades so you will be unable to redecorate quickly – maybe up to 6 weeks afterward. It is important to note that if your property does not have a uniform mortar course than the application of a chemical Damp proofing company in Tyne & Wear membrane layer will be extremely difficult. Be wary if someone offers you this damp proof membrane option.
Paint applied damp proof membranes are also an option. This specific is not just a matter of slapping somewhat of black bitumen paint on the wall and hoping for the best. There are plenty of “miracle” cure, paint-on options. Surprisingly enough these can actually cause more problems than were there in the beginning. Preparing is the key when applying this type of product and will be the difference in it working, not working or so that it is even worse. So be very clear to get this right and understand the amount of time needed to get the prep right. The most typical problem with this type of application is the product. If it really does deal with damp, it could form a barrier and either move damp to where you haven’t treated or push damp higher in the wall. These products also will not have to get very sympathetic to listed buildings and you may well not manage to ever get them off!
Take a look at cavity membrane systems and a damp proof membrane solution. These products are dimpled sheets which may be applied mechanically with special fixings internally or externally (stopping penetrating damp through poor pointing) and applying either a direct plaster to the mesh on the membrane or dot and dab plasterboard onto the product. The latter is very popular now as once the damp proof membrane is applied to the wall you will not be waiting for becoming dry times and you may fix the membrane over stable existing plasterwork.