Case Study:
Render Repair on a Pebble Dashed Extension in Horsham

This is a render repair in Horsham on a poorly pebble dashed extension letting water in and trapping the moisture in. Old blown damp damaged dash removed and left to allow blocks to dry out then re- pebble dashed to match existing.

The Problem: When Render Becomes the Enemy

It might seem surprising, but the very render that is supposed to protect your home from the elements can sometimes become the source of your damp problems. That is exactly what we found on this extension in Horsham, where a poorly applied pebble dash finish had gone well past its best and was quietly letting water into the property while trapping moisture against the blockwork behind it.

The homeowner had noticed the telltale signs: damp patches appearing on internal walls, a musty smell, and a render surface that had started to feel hollow and unstable in places. To the untrained eye it might have looked like a cosmetic issue, but the reality was more serious.

Why Blown Render Is More Than Just an Eyesore

When render "blows," it means it has lost its bond with the wall behind it. This typically happens when water gets behind the render either through cracks, failing joins, or areas where the original application was less than thorough. Once water is trapped between the render and the blockwork, it has nowhere to go.

In the case of pebble dash, which is a dense, cement-based finish, breathability is already limited. Add a failing bond to the mix and you create the worst possible scenario: water gets in easily but can't get back out. In winter, that trapped moisture can freeze and expand, worsening the separation. Over time, the damp migrates inward, damaging internal plaster, causing mould, and in more serious cases, beginning to affect the structural integrity of the wall itself.

On this Horsham extension, Gavin identified areas of blown render across the affected face of the building. Tapping the surface revealed the characteristic hollow sound that signals the render has separated and a closer inspection confirmed moisture damage to the blockwork underneath.

The Right Approach: Strip, Dry, Rebuild

A common mistake with failed render is to simply patch over the problem. Applying new render over blown or damp areas without addressing what's underneath traps moisture all over again and the new work rarely bonds properly. The result is a repair that looks fine for a season or two before the same issues return.

Gavin's approach was to do the job properly. The first step was to hack off all the blown and damaged pebble dash back to the bare blockwork. This is not a quick job it requires care to avoid damaging the underlying substrate while removing all the compromised material cleanly. Once stripped, the true extent of the damp damage to the blocks became visible.

This is where patience comes in. Rather than rushing straight into re-rendering, Gavin left the exposed blockwork to dry out thoroughly. This drying period is critical. Applying new render over damp blocks is a recipe for failure, the moisture needs a route to escape, and fresh air and time are the only reliable way to achieve that before any new coat work goes on.

Matching the Existing Finish

Once the blocks had dried out sufficiently, Gavin re-rendered the extension in pebble dash to match the existing finish on the rest of the property. This is one of the more underappreciated skills in plastering and rendering work. Pebble dash varies significantly in stone size, colour, aggregate mix, and texture. Achieving a result that blends naturally with decades-old existing render takes experience and a trained eye.

Getting this right means the repair doesn't draw attention to itself. The finished result looks as though it was always there, with the new section sitting flush and consistent with the surrounding finish.

The new render also properly protects the blockwork again allowing any residual moisture to continue escaping while providing a sound, weatherproof barrier against future water ingress.

Lessons From This Job

This case study highlights a few important points worth understanding if you have an older rendered or pebble dashed property:

Blown render doesn't fix itself. What often starts as a small area of detachment will spread as water continues to get in and the freeze-thaw cycle does its work season after season. Catching it early is far cheaper than leaving it to deteriorate.
Patching over the top is a short-term fix. New render applied over compromised substrate will not bond correctly and will not solve the underlying moisture problem. A proper repair means taking it back to sound material.
Drying time matters. No amount of skill in the re-render will compensate for applying it over walls that are still damp. The drying-out period between stripping and re-rendering is an essential part of the process, not a delay.
Matching existing finishes takes skill. Pebble dash in particular is notoriously difficult to match convincingly. Getting the blend right requires experience with the material and an understanding of how it weathers and ages.

How We Can Help

At G Jones Plastering & Damp Proofing, Gavin has over 25 years of experience working with all types of render and external finishes and holds a PCA qualification in damp proofing. If you have a pebble dashed or rendered property showing signs of blown or damp-damaged render, Gavin can assess the extent of the problem, advise on the right course of action, and carry out the repair work to a high standard, matching your existing finish wherever possible.

If you're based in Horsham or the surrounding area and are concerned about render or damp on your property, get in touch with us today for a no-obligation assessment.