Damp Information
Penetrating damp.
In a way all moisture from sources beyond the construction of a building be it below or above, that finds a way into the inner plaster or fabric, is if we take the word literally, a form of penetrating damp. Therefore the definition of penetrating damp in most cases refers to situations where rainwater such as that spilling onto an external brick wall because the roof gutter is blocked by leaves or a childs ball has been left stuck in the junction of the connecting down pipe so long without removal, that over- spilling water begins to soak into the immediate brickwork. see fig 8 This may seem obvious when described thus, but having seen the amazed expressions on homeowners faces when dry rot that has germinated on the internal floor timbers is proved to be caused by such a simple negligence, illustrates only too plainly the dangers of not maintaining roof gutters. In fact there are many little things that can result in penetrating damp. Take the outside hose tap. How many times have you seen a tap persistently dripping after the garden has been watered. Result. Penetrating damp coming through the wall just above the skirting board, often wrongly diagnosed as rising damp because it’s low on the wall. Another similar example that often occurs to older houses with concrete or stone sills that have been painted so many times the rain drip groove on the underside of the sill becomes blocked causing rainwater to run back under the sill and then down the wall. They also become cracked sometimes from movement and provide access for rainwater ingression.
Drains. Don’t we all take our rain water gullies against the wall, for granted, with down pipes often being a little short at the bottom so that when its raining heavily the water splashes against the outside wall. Result. Low level penetrating damp. Brick flower troughs constructed against the outside wall. Result, bridged damp proof course causing low level penetrating damp. The over-flow pipe outside the first floor bathroom or boiler cupboard, often too short to clear the building or angled too steeply so that when active, the water runs down the outside of the pipe, and back down the wall. You must have seen all that white staining on blocks of flats and nobody bothering to do anything about it. Forgive the quality of the personal artwork I have created in my endeavours to add a little colour to all this text. See figure 1. But sometimes graphics can be so much easier to comprehend.
Who would have thought that just by not using the old coal fire anymore and bricking up the fireplace could result in a damp problem. I‘m sure that most of us would not consider the fact that rainwater is capable of entering the open chimney pot to such an extent that it can actually result in penetrating damp showing up on the chimney breast. This can be made worse by a thing called salt leaching.
Damp patches are often found on the front and sides of chimneybreasts which do not seem to relate to any obvious source and are often accompanied by yellow or brown stains. These can be found both at high and low levels and are the result of the deposit of Ammonium Salts which have evolved as a by product of coal burning in years gone by. In many cases these remain undisturbed in the mortar and brickwork of the chimneystack and flue and are of no consequence if kept dry. However like chlorides (Common salt) they are hygroscopic (Absorb water) and can go into solution when in contact with water or water vapour. The sources of water available in chimneys are usually limited to the following:-
- Rainwater admission through uncapped chimney pots.
- Rising damp from the ground.
- Condensation within the stack.
- General excessive humidity within the property.
Going into solution allows the salts to become transient within the masonry as the moisture is drawn to the surfaces of the structure by external warmer temperatures, resulting in contamination of the plaster layer, as it leaves behind a cluster of concentrated or scattered salts in the plaster.
Washed out mortar joints, one of the most common causes of rainwater penetration. Again mostly akin to houses constructed during the early nineteenth century where mortar comprised of lime and sand and often some unknown ingredients. Prone areas being where the brickwork terminates at the ground and joints become washed out due to rain splash, or areas of wall higher up that are particularly exposed to harsh wind and rain from certain points of the compass.
External render covering the original brickwork does look smart when painted white or magnolia. Unfortunately often in the past, this was taken down to the ground, causing the dpc to become bridged. One way of remedying this is to cut away the render to expose the dpc and install a bell drip bead that is incorporated in the replacement render and set to a height just above the dpc line. Exposed brickwork can then be protected from rainwater splash by either applying three coats of bitumen paint such as “Synthaprufe” or to apply a single coat of waterproofed render to a height just below the dpc, thus protecting the walls without bridging it. See figure 7.
High ground levels. One very common problem that causes the dpc to become ineffective because of bridging is where ground levels have become high over the years with different successive owners having their own pathways and patios constructed around the property. As an alternative to generally lowering external patios or high ground, one way of isolating an external wall is to construct a trench, often referred to as a French Drain, that runs alongside, to a depth not less than 150mm below the existing or new dpc x150mm wide. This can then be infilled with pea shingle to help drain rainwater and minimise any hazard. These days one can purchase ready made fibre drain channels with metal grilles that fit on the top, if pea shingle is not wanted. Although these are really designed to run to a drain or soak away.
Clinging ivy Ivy or other close growing shrubs although an attractive feature often harbour water after rain and obstruct natural drying out of the brickwork which can lead to penetrating damp in time, and is best removed.
