External rendering is sometimes marketed as a damp solution. In some cases it is — where damp is caused by rainwater penetrating a failing external finish, replacing that finish with a modern breathable system addresses the problem at its source. But rendering is not a cure for all damp, and applying a new render over an active damp problem without understanding its cause can make things worse. This guide explains the distinction.
Types of damp and their causes
Damp in buildings has several distinct causes that require different solutions:
- Penetrating damp. Water entering the building through the external walls, typically because of a failing external finish, defective pointing, or cracks in the wall structure that allow direct water ingress. This is the type of damp that external rendering can address.
- Rising damp. Groundwater rising through the wall from below via capillary action. This requires a working damp-proof course (DPC) at low level. External rendering does not address rising damp — it cannot stop water moving upwards through the wall structure.
- Condensation. Water vapour produced inside the building condensing on cold surfaces. This is an internal ventilation and insulation problem. External render may marginally reduce it by improving the thermal performance of the wall, but it is not a condensation fix. Better ventilation and sometimes internal insulation are the correct solutions.
- Leaks from plumbing, roofing or guttering. Water entering the building from failed flashings, cracked gutters, blocked downpipes or leaking internal pipework. External render does nothing for these.
What external rendering can genuinely fix
External rendering is an effective solution for penetrating damp where the cause is a failing or permeable external finish. This is the most common form of damp in older solid-walled properties in Yorkshire.
Where the external walls have:
- Cracked or failed render that is allowing water to penetrate the wall
- Deteriorated pointing in brickwork or stonework that is no longer keeping water out
- Porous masonry that is absorbing substantial quantities of rainwater in wet weather
— then a new, correctly specified render system applied to a properly prepared substrate will address the moisture pathway at its source. A breathable silicone render that is hydrophobic externally but vapour-permeable will also allow the existing moisture in the wall structure to dry out, rather than trapping it in.
This is genuine — rendering does fix penetrating damp when penetrating damp is the diagnosis. The problem is that it is not always the correct diagnosis.
What rendering cannot fix
Rendering over an active rising damp problem creates a particularly bad outcome. Rising damp carries salts from the ground up through the wall. These salts are hydroscopic — they absorb moisture from the air — and will cause any non-breathable render or coating applied over them to blister, bubble and fail. Even a breathable render applied over active rising damp will show staining and eventual surface failure as the salts continue to migrate.
The correct sequence is to treat the cause of the damp first, allow the wall to dry out, replaster with a salt-resistant product if necessary, and only then apply any new external finish. Rendering over rising damp without treating the cause will require the work to be redone within a few years.
Similarly, condensation damp requires improved ventilation, not a new render. And damp caused by a blocked gutter or failed flashing requires the defect to be repaired — the render finish is irrelevant if water is finding another route in.
How to identify the damp type before committing to a solution
Before commissioning any damp treatment or external rendering, a correct diagnosis is essential. Key indicators:
- Pattern of the damp marks. Penetrating damp tends to appear on external walls in patches corresponding to where the external finish is failing — often under window sills, at corners or in areas of cracked render. Rising damp shows as a tide mark at low level, typically no higher than 1 metre, with associated salt deposits. Condensation appears on cold surfaces — north-facing walls, window reveals, room corners — and is often worse in winter.
- Correlation with weather. Penetrating damp appears or worsens after rain. Rising damp is continuous and not correlated with rainfall in the same way. Condensation is worst in cold weather when indoor and outdoor temperature differentials are high.
- Condition of the external wall. A visual survey of the external wall — looking for cracked render, failed pointing, gaps around frames, blocked or overflowing gutters — often identifies the route of penetrating damp quickly. An experienced rendering contractor can usually give an informed view of whether the external finish is the likely cause during a survey.
If there is genuine uncertainty about the cause of damp, an independent damp surveyor can provide a diagnosis before any remediation is decided. This is always worthwhile where the damp is significant.
