Expert Painters

Cracks in Exterior Render: Cosmetic or Structural?

02 Nov

Almost every rendered house in Melbourne has cracks in it somewhere. Most of them do not matter. A few of them matter a great deal, and the difference is not how long the crack is but how wide it is, which direction it runs and whether it is still moving.

Painting over the wrong one hides a problem that gets more expensive every year.

Why Melbourne renders crack more than most

Large parts of Melbourne sit on reactive clay soil. Clay swells when it takes up water and shrinks when it dries out, and the ground under a house can move measurably between a wet winter and a dry summer. The western and northern suburbs are particularly reactive, but it is found across the city.

That movement transfers into the building, and render is brittle, so render is where you see it first. A few hairline cracks on a rendered house after a dry summer is normal behaviour, not a defect.

The cracks that are almost always cosmetic

  • Fine crazing, a web of hairline cracks across a panel, usually from the render drying too quickly when it was applied
  • Isolated hairline cracks you can barely get a fingernail into
  • Cracks that follow the line of a control joint
  • Short cracks in a render coat that do not continue into the masonry behind

These are a preparation item before painting, not a structural issue. They get cut out slightly, filled with a flexible filler and coated.

The cracks worth stopping for

  • Anything you can fit a 5 cent coin edge into, roughly 2 mm or wider
  • Diagonal cracks running from the corners of windows and doors, which is where building movement concentrates
  • Stair-step cracking that follows the mortar joints in the brickwork behind the render
  • Horizontal cracks, which are the least common and the most likely to be serious
  • Any crack that goes right through the wall and is visible inside in the same place
  • Cracks that have clearly widened since you last looked, or that are wider at one end
  • Cracks accompanied by doors or windows that have started sticking, or floors that have moved

Two or three of those together is a building movement question, not a painting question. A structural engineer or a building inspector should look at it before anyone quotes to repaint, and that advice is worth following even though it delays the job.

How to tell whether a crack is still moving

This is simple and costs nothing. Mark both ends of the crack with a pencil and write the date next to it, or bridge it with a small blob of plaster. Photograph it. Look again after a season change, so a wet period after a dry one.

If nothing has changed after six months across a weather change, the movement has settled and repair is straightforward. If the mark has opened, the cause is still active and repairing the render will simply crack again.

That six months is frustrating when you want the house painted. It is considerably less frustrating than paying twice.

What proper repair looks like

For cracks that are stable and cosmetic:

  • Cut the crack out into a shallow V so the filler has something to key into, rather than smearing filler over the top
  • Remove all loose material and dust, and dampen the edges
  • Fill with a flexible, render-compatible patching compound, not interior gap filler
  • Allow proper curing time, which is days rather than hours on a deep repair
  • Texture the patch to match the surrounding render, which is the step that decides whether the repair is visible afterwards
  • Prime the repair before topcoating, because new material absorbs differently

Where cracking is widespread but stable, a flexible or elastomeric membrane coating over the whole elevation is often the better answer than chasing individual cracks. It bridges hairline movement and keeps water out. It costs more than standard exterior paint and on the right house it is worth it.

Hollow render is a separate problem

Tap along the wall with your knuckles. Where the render has debonded from the masonry behind it, it sounds hollow and drummy rather than solid.

Debonded render will come off eventually regardless of what is painted over it, and the area has to be removed back to sound material and re-rendered. Painting a drummy wall is money spent on a surface that is already failing. Our guide to rendering costs in Melbourne covers replacement.

Cracks plus white deposits

If there are white crystalline deposits around the cracking, or the render is powdery and crumbling in the first half-metre above ground, that is moisture moving through the wall rather than movement. Different cause, different fix, and filling the crack will not address it. See our notes on efflorescence.

Paint lifting in sheets around cracks points the same way, and why exterior paint peels covers the diagnosis.

For cracking inside the house rather than on the render, our guide to plaster cracks is the one to read.

What a quote should say

A quote that lists “crack repair as required” is not a quote. It should tell you how many metres of cracking are allowed for, whether anything is being cut out or just filled, whether the render has been checked for drumminess, and what happens if more is found once work starts.

What we do

  • Inspect and categorise cracking before quoting, rather than after
  • Cut out, fill and texture-match stable cracks
  • Remove and replace debonded render
  • Apply flexible membrane systems where cracking is widespread but stable
  • Tell you plainly when a crack needs an engineer rather than a painter

We are based in Richmond and work across Melbourne, the bayside suburbs and coastal Victoria.

Call 0404 330 332. Photograph the crack with something for scale, a coin works, and we can usually tell you over the phone whether it is a painting job or not.

This is general guidance on preparing render for painting, not structural advice. Anything you are worried about should be assessed by a structural engineer.

📍 Our Service Areas Across Melbourne, the Mornington Peninsula, Surf Coast & Phillip Island

Melbourne & Bayside

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Surf Coast & Bellarine Peninsula

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