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Dampness & Its Removal (back to index)

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You will end up at this page if you select either indoor or outdoor dampness, because often the two are related, or at least one is the cause of the other.

Dampness is a significant issue to control, in that in the short term it damages belongings, in the medium term it can lead to poor health and in the long term it can severely damage the structure of any building.

Dampness also makes restoration and new works troublesome, so do solve the existing problems before you carry out new works.

Dampness in Masonry Walls

Dampness is the main offender to both painted and unpainted walls, and can be caused by:

  • Rising damp - check to see whether the building damp course is intact or whether it needs replacement.
  • Bridged damp course - quite common is the inadvertent piling up of dirt, for garden beds or similar, that creates a water path from the ground to above the damp course. You need to clear away the offending dirt or somehow create a 100% effective water proof barrier betwen the dirt and the building (such as by using pots, rather than an open flower bed).
  • Storage - people often 'chuck' their garden waste against the shed, or worse still, a house wall. This again effectively bridges the damp course and creates a water reservoir right next to the wall. Same can be said for firewood piles and timber supplies.
  • Leaking taps or plumbing - water pipes can leak within a wall, or outside taps can drip or splash against an unsealed wall surface and create continuous dampness. Taps are easy to see and fix, but pipes inside a wall much harder. You need to convince yourself of the cause, then, if you suspect that is the case, dig out the pipe and replace it.
  • Guttering - gutters and downpipes are a common problem; suffice to say they should provide a watertight path from top to bottom, then carry water well away from the building footings. Check also that u-bend traps at the bottom of downpipes do effectively carry away the waters of a significant downpour. Blockeed traps are a regular source of dampness.
  • Poor drainage - similarly to the guttering problem, normal run-off water caused by rain must be allowed to quickly drain away from the building.
  • Sprinkler systems - quite obvious.

Also of importance is the integrity of any surface. If it is cracking, peeling or falling away from the wall, the reason(s) must be determined and following a solution, the surface must be repaired.

Quite a list, and if some are issues then you will have additional projects. How to carry out some of these is covered under Plumbing - General.

Damp Courses in Walls

An air gap exists on most (new) external walls, which are effectively two separate walls structurally linked together. The damp course is, today, a plastic strip laid upon a layer of stone or brick, below or at floor level that stops moisture migrating from the ground up the wall. Many older houses use an additive to the mortar, or even slate to achieve a damp course. Houses built on a concrete raft, that is the total house is on one continuous slab, have plastic sheeting underneath to stop water penetration, and a separate damp course above the raft.


One final issue to look at on unpainted houses is whether re-pointing is necessary. The mortar between stone and bricks can fail, due to the effects of wind, rain and ice, and if severe should be replaced. This is a job you can do yourself, but it is time consuming and you need to see someone else doing it first to get a good appreciation of what to do. Most importantly is the need to ensure a consistent mortar mix (sand, lime and cement) that is measured very accurately by volume each time to ensure minimal colour changes throughout the project.

Dampness in Timberwork and Attached Plumbing

Look also at the integrity of the house timberwork, and address issues such as:

  • Failing eaves that allow birds, insects and small animals to enter your roofspace.
  • Plumbing that pours water over areas it shouldn't, typically faulty guttering and downpipes.
  • Garden growth that penetrates the timber, or holds moisture against it.
  • Timberwork that sits out from the wall and holds debris (and hence moisture).

Filling damaged areas with wood filler is acceptable, but only when all areas of damaged timber have been fully removed. Excessive use of woodfiller is basically a way of not carrying out immediate replacement.


Once a surface is repaired and dry it needs to be sealed for refinishing. In the case of masonry, an oil based sealer is recommended, whereas in the case of timber an oil based primer is recommended.

New rendered walls should have been pre-treated prior to rendering or plastering (wet plaster) so little further treatment should be required, though it does not hurt for suspect areas to apply a good oil-based sealer, then proceed to two or three coats of emulsion.

You can batten out a wall and apply some panelling such as plasterboard or pine panelling, but this will simply trap any moisture that does penetrate and lead to a musty odour. Try and avoid this type of fix, unless you know the wall is dry.

With masonry you are generally trying to stop rising damp from escaping into the room, while with timber you are generally trying to stop the ingress of water into the timber from the masonry, typically a problem in floors, skirtings and architraves.

If you can get under or into a timber floor, it is our recommendation that you ensure a form of damp course is installed between the floor supports (vertical posts or piers of either timber or brick) and the bearers (bottom most horizontal layer of beams). This damp course is typically plastic membrane, sold by builders' merchants for this purpose.

Additionally, when renovating ensure that all underfloor vents are clear, that debris under the floor is minimal, and that air flow under the floor is unimpeded. Also remove building debris that has fallen between the sub-floor (bearers, joists and floor boards) and the wall, as this forms a moisture bridge from masonry to timber.

Favour cleaning over painting, as painting is only a cover up whilst cleaning is a preventative measure.

Please be advised: gapfilla.com does not purport to be an expert in all facets of diy, but has significant experience in almost all of the topics covered on this site. We suggest that all persons take adequate care in the translation and application of this content to their projects, particularly as projects rarely encounter identical issues and constraints. If in doubt, we suggest you seek advice from a professional tradesperson or advisor. All material copyright gapfilla.com 2005-13