Pebbledash, Painted Walls and Smooth Render Are Not the Same Job

From the road, three green walls look like the same problem. Up close they are three different jobs with three different things to be careful about. Here is what actually changes between them, and the one thing that does not.

Pebbledash and dry dash: all that surface area

A pebbledash wall has many times the surface area of a smooth one. Every stone has a shaded side, and between the stones are gaps that stay damp long after the face of the wall has dried in the wind.

That has two consequences. Growth establishes more readily, because there is more sheltered damp surface to establish on. And it sits deeper, so a wall that looks merely dull from the gate is often carrying a great deal more than it appears to when you put your hand on it.

It also means a pebbledash wall rarely goes back to looking new. The stones themselves have weathered over decades and the mortar between them has aged. What you are removing is the biological growth, not the years. People are sometimes surprised by that, and it is better said in advance.

Pressure is particularly poor on dash. The stones are held in a mortar bed, and a lance will take stones out of it. Once a patch has lost its stones there is no cleaning that puts them back.

Painted walls: cleaning reveals flaking, it does not cause it

A painted masonry wall is a different situation, because the finish is a coating sitting on the wall rather than the wall itself.

If that coating is sound, it cleans. If it has already lost its grip in places, which happens where water has been getting behind it, then removing the growth will show you where. Customers occasionally conclude that the cleaning lifted the paint. Almost always the paint had already failed and the algae was the only thing holding it flat.

This is worth knowing before the work rather than after, so we look for it when we quote: blistering, hairline crazing, edges lifting around sills and reveals, a hollow sound when tapped. If a wall shows a lot of that, the honest conversation is about repainting rather than cleaning, and we would rather have it early.

Smooth and thin-coat render: nowhere to hide

A modern smooth or thin-coat render is deliberately uniform, and that uniformity is the point of it. It is also what makes it unforgiving. Any variation in how the wall was treated shows, because there is no texture to break up the light.

That is why we treat a full elevation on these rather than spot-cleaning, and why we are especially firm about keeping pressure away from them. The detail on coloured finishes is in can coloured render be cleaned without patching.

Wet dash and older cement render

Older Irish houses often have a wet dash or a plain cement render, sometimes painted, sometimes not, and frequently patched at some point in their life.

These are usually the most tolerant of the lot, because they were never uniform to begin with, so a slight variation does not stand out the way it would on a thin-coat. What they do need is a proper look for loose material first. Cement render that has started to delaminate will sound hollow, and it is better found before the work starts.

What actually changes between them

Access time and dwell, mostly. A dash wall takes longer than a smooth one of the same size because there is more surface to wet and the growth is deeper in it. A painted wall takes more inspection beforehand. A thin-coat takes more care about evenness across the whole elevation.

What does not change is the method. On all four finishes the growth is killed and left to weather off, rather than being blasted off along with some of the wall. The reasoning behind that is the same in each case, and it is set out in soft washing vs pressure washing.

What to check before you call anyone

Stand close to the wall and look at the base of it, under the sills, and around any downpipe. If a wall is wetter in one strip than everywhere else, something above it is putting water there, and cleaning is treating the symptom. Gutters are the usual culprit, which is covered in why gutters need clearing in an Irish climate.

Common questions

Related services

• Softwashing for render and walls
• Gutter cleaning
• Roof cleaning