Damp: Rising and Penetrating
One of the biggest building problems of any building is that of damp. There are two types: rising or penetrating with rising damp only occurring on ground floors. Internal or external walls constructed of permeable or absorbent materials such as brickwork, stone or plaster. Moisture from the ground under and around the building will rise up the walls until internal plaster finishes become saturated which leads to mould growth and the degradation of the finish. Eventually the fabric of the wall itself will start degrading too.
Typically, the height at which rising damp will be seen in the plaster finishes is usually up to 1 metre above ground floor level, with one of the most common signs being that of tide marks. Evaporation and salts from the ground are the cause. If there are no such marks then another very common sight is that of brownish/yellow damp patches or staining above the skirting board and up to around the same height.
Loosening and curled up wallpaper or peeling paper at the skirting board is another clear pointer as to the problem..
The usual solution for this kind of damp is to drill holes at regular intervals around the perimeter of the affected area (just above external ground level). A damp proof course is then applied (DPC) by means of a pressurised chemical injection into the holes. The chemical is absorbed into the masonry, forming an effective barrier to moisture traveling upwards by expanding into a line across the brickwork. The wall is then re-rendered using a waterproof rendering system
Before the affected interior can be redecorated, the decorations and plaster-work must be removed in order to get rid of the majority of hygroscopic/deliquescent salts which will have accumulated over many years of rising damp. If decorations and plaster are not removed then any salts that remain in the masonry will return and show through newly decorated surfaces. Walls take time to dry down and the lower part of the walls are always likely to remain damp because of the limitations of the chemical injection systems in terms of controlling the rising damp. So chemical injection damp-proofing is a 2 -part process: the injection to ‘control’ the rising damp, and the replastering (inside) to prevent future show through- the processes are inseparable.
We’ve had the camp-proof course installed for one of our rented flats and the interior work carried out by an individual landlord and there have been no further problems reported (fingers crossed it stays that way!) because should any further damage appear, both processes will need to be examined.
Penetrating damp is commonly caused by problems with the roof, water tanks, brickwork or windows. Also known as lateral damp, it can happen at any level and is very common with older properties that have solid walls, rather than newer properties which have cavity wall insulation. But what causes it?
- Blocked rainwater gutters, hoppers or down-pipes which overflow, causing the adjacent wall to become saturated;
- Defective concrete haunching around disused chimney pots that have been capped or sealed. This allows water to enter the unused chimney and then penetrate into the building;
- Defective flashings around chimneys, flues, soil-and-vent pipes and ventilation terminals, and at abutments of roofs to vertical walls. Whilst these defects usually cause leaks and not damp, sometimes water finds its way into the roof construction and emerges elsewhere as penetrating damp;
- External wall cavities that have been bridged by rubbish left in the cavity or by mortar droppings left on wall ties or insulation batts during construction;
- Exposure to driving rain where water penetrates the brick outer leaf which may then be led across the cavity, locally saturating the inner masonry leaf and as well as internal plaster or plasterboard finishes;
- A wall tie, which is designed not to allow moisture to cross it as it is coated with mortar when the wall is built;
- Pipework which extends through the wall;
- Window, door frames and lintels which have been poorly fitted with damp proof courses;
- Cracked brickwork;
- Defective pointing;
- Deterioration of mortar joints and water tanks.
Interior signs that point to penetrating damp are those of damaged plaster, watermarks and damp patches that won’t dry out which may also be seen to expand during heavy rain. There can also be black mould growth and the place will have that particular damp, musty smell.
It will be the freeholder or the RMC who will take action to repair any of the above exterior problems that are causing problems to the interior. But what if the problem is that of condensation? Read here.