Cracked Render on Your House Exterior: What to Do Next

First, work out how serious the crack is

The width and pattern of a crack tells you a lot. Hairline cracks under about 1mm, often in a random or map-like pattern, are usually just the render shrinking as it cured or normal thermal movement. They are common on sand and cement render, which is what most older terraced and semi-detached houses around South Wales have.

Cracks wider than 2mm to 3mm, cracks that follow a straight or stepped line, or cracks that keep growing deserve a closer look. Stepped cracking that mirrors the brick or blockwork underneath can point to movement in the wall itself rather than a problem with the render alone. If you can fit a pound coin edge into the crack, get it inspected before repainting anything.

• Under 1mm and random: usually cosmetic, monitor and fill before repainting
• 1mm to 3mm: needs proper filling and sealing to keep water out
• Over 3mm, stepped, or growing: get a professional inspection first
• Hollow-sounding render around the crack: the render has likely blown and needs cutting out

Why cracked render matters more in a wet climate

Pontypridd gets well over 1,200mm of rain in a typical year, and much of it arrives on driving south-westerly winds. A crack that would stay harmless in a drier part of the country becomes a water channel here. Rain gets behind the render, cannot escape, and gradually breaks the bond with the wall. That is when render blows, meaning it sounds hollow when tapped and eventually falls away in sheets.

Trapped moisture also shows up indoors as damp patches, peeling paint or black mould on the inside face of external walls, often misdiagnosed as rising damp. Winter makes it worse: water in a crack freezes, expands, and forces the crack wider each cycle. This is why a small repair in summer often prevents a much larger one the following spring.

Repair options, from filling to full re-rendering

For stable hairline cracks, the fix is straightforward: rake out, fill with a flexible exterior filler, then coat the wall in a good quality masonry paint. On walls with lots of fine cracking, an elastomeric or high-build masonry coating bridges hairlines and stays flexible, which suits older render far better than standard smooth masonry paint.

Wider cracks need cutting back to sound material, filling with a matching mortar or render repair compound, and blending in before decorating. Where render has blown, the loose areas must be cut out and patched, and a good patch should be squared off and keyed in rather than smeared over. If more than roughly a third of an elevation is blown or cracked, patching becomes false economy and re-rendering that elevation, often in a modern silicone or monocouche render that handles movement and rain better than old sand and cement, is usually the sensible route.

What it is likely to cost

Costs depend heavily on access, wall condition and how much sound render remains, so treat these as ballpark figures rather than quotes. Filling cracks and repainting a typical three-bed semi elevation might sit in the low hundreds of pounds. Patch repairs to blown areas commonly run from around £150 to £500 or more per patch depending on size and matching. Re-rendering a full elevation on an average house is often somewhere between £2,000 and £4,500, and a whole house typically runs into five figures once scaffolding is included.

Scaffolding is a real factor on the steep, terraced streets common around Pontypridd, where access at the rear can be tight. If you are already having gable or chimney work done, it is worth dealing with render at the same time to share the scaffold cost. Always get an itemised written quote and ask what happens if more blown render is found once work starts.

When to call a professional

Call someone in if cracks are wider than 3mm, stepped, spreading, or paired with damp inside the house, or if large areas sound hollow when tapped with your knuckles. Also get advice before painting over cracks: paint hides the evidence but does not stop water, and a decorated wall that fails within a year costs more than doing it properly once.

A decent local firm will tell you honestly when a crack just needs filling and painting, and when it needs more. If you are in Pontypridd or anywhere in the South Wales valleys, we are happy to take a look and give you a straight answer either way.