Recommended Approach to Damp/Wet Walls

Tŷ-Mawr ‌‌ posted this on 12 Mar 2024

The general principal of treating the cause, before tackling the symptoms, should always be applied.

1. Identify the cause of damp/water ingress:

  • Starting at the top of the property, check for problems with roof tiles/slates, flashings, gutters and downpipes.
    • Blocked or leaking gutters can allow water to run down the walls, whilst missing/ineffective slates or flashings can allow water into the top of the wall.
  • External render or pointing, as well as masonry, must be in sound condition, free from cracks and defects.
    • Wind driven rain can penetrate even the finest of cracks. Cement based renders and modern acrylic paints will prevent water that has penetrated from escaping again.
  • Are surface drains working correctly?
    • Older solid walls often don’t have a damp-proof course (DPC) built in, so surface water run off must be taken away from the base of walls.
  • Is the property built into a bank, with retaining walls below ground level?
    • Where possible, it is desirable for no direct contact between walls and the soil.

2. Rectify any issues identified above before commencing internal works:

  • Repairs to the exterior of the house will prevent the re-occurrence of the cause(s) of water ingress.
    • If cement render, pointing and/or synthetic paints are identified, these should be removed to allow the wall(s) to “breathe” and dry out. Cement finishes are often harder than the masonry around them, leading to damage to the stone or brick work.
    • Timing of these works is important, since the use of breathable lime products in winter can lead to problems, and is not generally recommended.
    • Prevent surface run off water from reaching the walls by installing land drains (French drains) and ensuring gutter down pipe outlets are connected to the drainage system.
  • If the property is built into a bank, is it possible to excavate back from the wall? Land (French) drains installed well below surface ground level can help. In extreme cases consideration may need to be given to internal cavity drainage membranes.

3. Are internal wall finishes retaining moisture in the walls?

  • Cement renders, gypsum plasters and modern synthetic paints are not sufficiently vapour permeable to allow moisture trapped in a wall to escape to the atmosphere.
    • If these finishes are identified, they should be removed to aid the drying out of the wall.
    • If removal of cement & gypsum products leaves bare walls, consider applying a shelter coat of limewash to aid evaporation of moisture (this increases both the capillary action bringing moisture to the surface, and surface area for evaporation to take place).
    • Consideration could also be given to the use of industrial de-humidifiers to speed up the drying out process, but only when external repairs have been completed.

To avoid the risk of mould formation, ventilation is key.

4. Schedule repair works appropriately:

  • Start with the external faces of the walls to avoid the risk of new internal finishes being damaged before the external protection is completed.
  • If external walls are to be rendered, consider the options for external insulation to improve the thermal performance of the building.
    • Ensure the use of breathable lime mortars & renders to avoid further damage to the masonry and to allow any future possible ingress of water to escape before penetrating to the interior.
    • Paint render with a limewash or breathable paint. The paint layer acts as a sacrificial element to avoid damage to the render by the weathering process.
  • Once external weather protection is complete and internal surfaces of the walls are dry, then it is appropriate to commence internal finishes. Be aware that the drying out process in masonry can take months or even years – patience is important!
  • If you will be re-plastering external walls, consideration should be given to insulation to improve the thermal efficiency of the property.