This article is related to the one discussing inappropriate pointing; only instead of pointing being the culprit, it’s sand and cement rendering this time.

This problem often afflicts properties that have solid walls, which were rendered, or re-rendered a good few years ago, often in the fifties or sixties.  However, given the right circumstances, it can also affect properties that were rendered in the recent past.

Sand and cement rendering has two distinct drawbacks: firstly, because of its relative inflexibility, it cracks; especially on southerly facing elevations where the thermal effects of summer sunshine can be quite significant, even in our UK climate.  Once this has happened, water gets into the cracks and soaks into any voids between the render and the substrate, be it brick or stone. The older the render, the more chance there is of cracking, and the bigger the voids.

Once there, the water can’t get out again, and so instead of evaporating into the open air, it evaporates into the house itself.  Result: big patches of moisture on the wall, sometimes quite a way removed from the entry point.

Secondly, it also reduces the ability of a building to evaporate the moisture that’s produced inside the house itself, by the occupants; and on northerly facing walls, or those that are shaded from the sunshine, condensation often occurs as a result.

On houses with cavity walls, neither of these problems are particularly troublesome:  if rainwater beats the render and penetrates the wall’s outer skin, the cavity allows any water to trickle down to the base of the building and thence away.

Similarly, any moisture in the internal atmosphere of the house, produced by the occupants etc, can exit the building via the internal skin and evaporate into the cavity. Condensation can still occur on walls with these orientations but it’s less likely to be a serious problem if there’s a cavity.

However, when the wall is solid, the troubles begin in earnest; and I haven’t seen many solid walls, rendered with sand and cement, that didn’t have either or both of the above problems.

Another non-starter for solid walls is to apply a ‘weatherproof’ coating to them.  These may well work when first applied but they too will eventually crack, and if they form an impervious skin, as many do, this too can develop voids; and the same symptoms as those found with sand and cement will occur.

In addition, if these coatings can stop water getting in, they’ll also stop evaporation from the inside; once again condensation is likely.

If you must render a solid stone or brick wall, you should use a lime-based render; which is easier said, or indeed written about, than done.  There aren’t many craftsmen about these days who will attempt to render with lime but if you have a property that’s suffered from cement render (see photos), you’ll probably feel it’s worthwhile trying to find one. This link will give you an insight into lime rendering but there are hundreds of web pages that will provide assistance.  The Society for the Protection of Ancient Buildings (SPAB) also cover this topic on their website.