When should I demolish render instead of patching?

If > 30% sounds hollow, if it crumbles like sand, or if deep salt saturation exists (chronic moisture).

Which costs more: patching or full replacement?

Patching charges by day-rate (slow, detailed work). Full replacement costs skip + m² labour but gives 40+ years of life.

Repair vs Full Replacement: Technical & Financial Decision Criteria

You walk into the house you've just bought (or are about to renovate) and the walls tell their own story. A crack here, a blister there, a chunk of render missing over there. Your first instinct to keep the budget low is to tell the contractor: "Do a few patches, fill them in and let's paint."

But is that the right decision? In construction, there's a fine line between "smart economy" and "the trap of never-ending expenses." If you patch render that has already "died," next year you'll pay double. Let's look at the strict criteria that will help you choose between local repair and radical stripping (demolition).

1. The Technical Criterion: The Condition of the Substrate

Render is like skin. If the problem is superficial (a scratch), it heals. If the problem is deep in the body, a band-aid is useless. So when is stripping mandatory?

The 30% Rule: If you tap the wall (the hollow-sound test we saw in the previous article) and find that more than 30% of the total surface sounds hollow, the game is over. Having the plasterer demolish and patch 10 different spots on the same wall will create a "jigsaw" of seams that will crack at the first seismic tremor.
Extensive Damp & Salt Damage: If the wall suffers from chronic rising damp, the old render is saturated with salts (nitrates/sulphates). Even if it dries, the salts remain. Local repair will be destroyed. Full demolition is required (up to 50 cm above the tide mark) and specialist dehumidifying render must be used.
Loss of Cohesion ("Crumbles like sand"): Lightly scrape the old render with a key. If it crumbles effortlessly and falls like dry sand, the cement has lost its binding properties (either from heat damage or old age). No primer and no filler will hold this wall. It must come down.

Analogy: render = skin, scratch → band-aid, rotten skin → strip

2. The Financial Criterion: Cost per Square Metre

Many people consider stripping prohibitively expensive. But is it? Let's look at what you really pay:

The Patching Scenario:
- Labour Cost: High (due to time and detail).
- Material Cost: Low.
- Result: The wall is a "sandwich" of old and new materials. The probability of a new crack appearing within the next five years is high.
The Full Replacement Scenario (Strip & New Render):
- Labour Cost: You pay extra for demolition (breaker) days and skip hire. However, the new rendering (especially machine-applied) is very fast and charged at a clean m² rate.
- Material Cost: Higher (many bags).
- Result: A single, monolithic wall, perfectly plumb, with a 40-50 year lifespan.

Split view: left patching (day rates, seams) vs right full replacement (m², monolithic wall)

The Day-Rate Trap: Patching isn't charged per square metre - it's charged by the day. A plasterer will need 2-3 days to meticulously open 15 cracks, embed mesh, apply primers, wait for drying and sand the junctions (so the patches don't show).

Comparison: When to Choose What?

Criterion	Choose Local Repair (Patch)	Choose Full Demolition (Strip)
Damage Extent	Less than 20-30% of the wall	More than 30% of the wall
Crack Type	Hairline, superficial or around windows	Deep, diagonal or "stepped" across the wall
Substrate Sound	Solid in most areas	Widespread "hollow" sound everywhere
Moisture	Surface condensation (mould)	Deep salt saturation (rising damp)
Aesthetic Result	Acceptable (with good sanding/blending)	Perfect (Absolute flatness/plumb)

The Right Decision

The decision must always be made without sentiment, based on the unforgiving logic of engineering: If the render's "foundation" is rotten, whatever you build on top of it will fall. If you're renovating a house to sell or rent quickly, patching is the obvious choice. But if you're building the home where your family will live for the next few decades, bite the bullet, hire a skip, demolish the rotten render and build something that will stand the test of time!