Frequently asked questions

• Why is it necessary to re-plaster walls following a damp proof course injection?

  Dampness rising from the ground carries with it a number of impurities that are detrimental to plasterwork. These include hygroscopic nitrate and chloride salts. Once plaster has been affected by hygroscopic salts it is unlikely to dry out even after a new damp proof course has been installed.

• Is it true that some new methods of damp proofing do not require the replacing of plaster?

  Plaster is removed because of the salt damage that occurs to the plaster as a result of having rising damp. So, this is not true. Any damp proof course can be physically installed without necessarily removing plaster but it is unlikely the treated wall will dry out. Not including re-plastering work in a damp proof course installation may also affect any guarantee issued.

• Why is the new plaster following damp proof course installation affected by salt efflorescence?

  Efflorescence is the deposit of soluble salts and other contaminants on the surface of plaster during the drying process as water evaporates from the plaster. A soluble salt content in plaster of less than 0.5% is sufficient to cause efflorescence. The major source of efflorescence in new cement based plaster is calcium hydroxide resulting from the interaction of cement and lime during the hydration of Portland cement. Once on the surface of plaster the calcium hydroxide mixes with carbon dioxide in the air to form salt-calcium carbonate. The sand, cement, lime and water used in the plastering process also contain alkaline sodium and potassium sulphates which can cause efflorescence.

  Efflorescence will generally cease once the soluble salts and other contaminants have left the plaster materials. The surface of the plaster can be dry brushed or wiped with a moderately damp cloth to remove the efflorescence. Where the salts have hardened a scraper can be used or a mild proprietary acid mixture.

• I was recently told my plaster was contaminated by salts. What does this mean and why is making the walls damp?

  Salt contamination in plasterwork can occur for a number of reasons. Rising damp is usually accompanied by hygroscopic nitrate and chloride salts. Chimney breasts are often affected by ammonium salts. Plaster that contains contamination levels of salt absorbs moisture from the air to a degree that is detrimental to the plaster, causing unsightly staining.

  Walls in older properties are more susceptible to salt problems because of the materials used in construction. Should older walls be re-plastered with unsuitable plaster (gypsum based plasters) salt contamination may occur.

• Why do I get black mould on my walls during the Winter months and why do the clothes in my wardrobe go mouldy?

  Mould growth is a symptom of high humidity or condensation. Cold surfaces attract condensation, so the Winter period is the typical time of year when mould problems occur. Areas where there are cold surfaces and ventilation is restricted are particularly at risk. Wardrobes are often located against cold walls and are generally poorly ventilated. Mould will feed from any organic material including clothes.

• How long does it take for walls to dry out following a damp proof course installation and when can I redecorate?

  The old ‘rule of thumb’ for drying out periods is 1 month per inch thickness of wall. Studies conducted by the Building Research Establishment indicate a more accurate figure is 1.25 months per inch thickness of wall. Drying out times vary dependent on ventilation, humidity and temperature.

• I have been told my house has inadequate sub floor ventilation. What does this mean and can it be remedied?

  Suspended timber floors require ventilation to prevent condensation forming in the sub floor void. This is achieved through the installation of air bricks around the lower wall areas. If sub floor ventilation is inadequate then smells may rise from the sub floor in to the rooms above and floor timbers may decay. Improving sub floor ventilation requires the installation of additional sub floor air bricks.

• I recently purchased an old stone barn which I intend to convert to a holiday let. I have been told the walls are contaminated as a result of the previous use and that this might affect the plastering work. Is this correct and if so what options do I have?

  Barns often do have contaminated walls due to their previous use. It is therefore important this be taken in to account when plastering the walls. As a result, dry lined plastering systems are often used as an alternative to traditional plastering or a physical waterproof membrane is used as a contamination barrier. When using dry lining or plastic membranes it is also important that insulation values and ventilation are also considered.

• I have a damp concrete floor that I would like to cover with laminate. Is there anything I can do to make the floor dry so the laminate isn’t affected by dampness?

  One option is to replace the damp floor slab. A new damp proof membrane should be used beneath the new concrete. The floor will need to dry over several months before laminate can be applied.  Alternatively, if the damp floor is sound and sufficiently level then a waterproof coating can be applied such as epoxy resin. There are also plastic membranes specifically designed to deal with this problem.

• What is the difference between a cavity drain waterproofing system and a tanking system?

  The terms ‘tanking’ and ‘waterproofing’ are used quite loosely, so the context is important. Both are typically used for waterproofing structures below ground level such as basements. Systems are often installed on the inside of the property, making them ideal for refurbishment work. External applications include new build construction.

  A tanking system is usually taken to mean a coating applied to a wall or floor that forms a waterproof seal. Various materials are used for this including dense cement render applications.

  A cavity drain waterproofing system is usually taken to mean a barrier that channels water to a collection point where it is removed in a controlled way. This is usually achieved through a combination of drains, sump and pump.

• I have been told I have wood boring weevils in my floor joists and that the only solution is to spray the timbers with insecticide. I do not like the idea of spraying insecticides in my house. Is there an alternative treatment?

  Wood boring weevils are often wrongly thought of as woodworm and as a result insecticide treatment is recommended. Weevils are in fact quite different. One significant difference is that they can only survive in timber that has high moisture content. An alternative treatment therefore is to bring the moisture content of the timber to a normal level (around 14%). This approach may require wrapping joist ends in a waterproof membrane. In any event, timbers with high moisture content are at risk of dry rot, wet rot and woodworm attack so simply spraying them with insecticide has limited value.

• I am considering having the outside of my house sealed against penetrating dampness but I am concerned this will prevent the walls properly drying out. Am I right to be concerned?

  It is certainly a valid concern. There are however a number of waterproof coatings available that allow the treated wall to breathe (allow the passage of water vapour). These tend to be silicone based and transparent once applied.

• What is the difference between heat recovery ventilation and positive pressure ventilation?

  Heat recovery and positive pressure ventilation are both used to control condensation.

  Heat recovery ventilation uses a heat exchanger to take the warmth from the extracted air and uses it to heat the incoming (supply) air. Heat recovery units generally have a percentage figure quoted that indicates the temperature of incoming air compared to that of the extracted air. For example, a heat recovery unit with 86% HR means the incoming air is 86% of the temperature of the air extracted.

  Positive pressure ventilation typically takes air from outside or from a roof void and supplies it to a central point within a property under a small amount of pressure, thereby increasing ventilation throughout the property. As the incoming air is cold, modern positive pressure units often include a pre-heater.

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