Acceptable Foundation Crack Size: How to Judge and When to Repair

Quick Takeaways

• Hair‑line cracks (<0.5mm) are usually harmless shrinkage cracks.
• Cracks wider than 3mm often signal movement or structural issues.
• Local building codes (NZBC, BS8110, ASTME1155) set clear width limits for different crack types.
• Monitor any crack that widens over time, especially after heavy rain or earthquakes.
• Call a qualified inspector if a crack exceeds 5mm, follows a stair‑step pattern, or appears suddenly.

What Is a Foundation Crack?

When we talk about a foundation crack is a disruption in the concrete or masonry that makes up a building's base, caused by stresses, shrinkage, or soil movement. It’s the first sign that something beneath the floor is shifting, and the size of that crack decides whether you can ignore it or need a repair crew.

Understanding the Foundation Itself

A foundation is the structural element that transfers a building’s loads to the ground. In NewZealand most homes sit on concrete strip footings or slab‑on‑grade systems. Because the earth isn’t perfectly stable, foundations are designed to tolerate a certain amount of movement without cracking.

Types of Cracks and What They Mean

Not all cracks are created equal. Here are the most common categories:

• Hair‑line (shrinkage) cracks - usually less than 0.5mm, run randomly, and appear as the concrete cures.
• Settlement cracks - often wider than 0.5mm, run vertically or diagonally, and indicate ground settlement.
• Structural cracks - greater than 3mm, may follow a stair‑step pattern, and suggest load‑bearing elements are stressed.

Identifying the type helps you decide how urgent the repair is.

How Professionals Measure Crack Width

Accurate measurement is crucial. Inspectors typically use one of three tools:

1. Crack width gauge - a calibrated metal bar that slides into the crack.
2. Digital microscope - provides a magnified view and records the exact width to the nearest 0.1mm.
3. Tape measure with a feeler gauge - a low‑cost option for homeowners, but less precise.

Regardless of the tool, the measurement is recorded as crack width and expressed in millimetres (mm). Consistency matters-measure the same spot over several weeks to spot any growth.

Acceptable Crack Sizes - What the Standards Say

Guidelines vary by region and construction type, but three major references dominate:

• NewZealand Building Code (NZBC) - for residential slab‑on‑grade, tolerates up to 0.5mm for shrinkage and up to 2mm for settlement before action.
• British Standard BS8110 - sets a 3mm limit for cracks that may affect structural integrity.
• ASTME1155 (US) - recommends monitoring any crack wider than 5mm and repairing if it widens.

Below is a quick reference table that translates these standards into practical advice.

Factors That Influence Whether a Crack Is Acceptable

Even a 2mm crack might be fine in one scenario and alarming in another. Consider these variables:

• Soil movement - expansive clays swell when wet, creating upward pressure on the slab.
• Shrinkage cracks - normal in new concrete as it cures, usually shallow and non‑structural.
• Settlement cracks - develop when the underlying soil compacts unevenly over time.
• Load changes - additions like heavy furniture or a new garage can stress the foundation.
• Seismic activity - NewZealand’s earthquakes can trigger sudden widening.

When any of these factors are present, err on the side of caution and track the crack more frequently.

When to Call a Professional

A professional inspection is recommended if you notice any of the following:

• Crack width exceeds acceptable foundation crack limits (generally >3mm).
• Crack follows a stair‑step pattern indicating shear stress.
• The crack appears suddenly after a storm or an earthquake.
• Water is seeping through the crack.
• Multiple cracks intersect or form a network.

An engineer will assess the structural integrity, possibly recommend underpinning, pier installation, or slab replacement.

DIY Checks and Simple Repairs

If the crack falls within the “hair‑line” or “minor settlement” categories, you can manage it yourself:

1. Clean the crack with a wire brush to remove dust.
2. Apply a concrete epoxy filler designed for cracks up to 2mm.
3. Seal the area with a waterproof coating to prevent moisture intrusion.
4. Mark the ends with a permanent marker and note the date.
5. Re‑measure after 6months; if the width is unchanged, the repair was successful.

Remember, DIY fixes are only cosmetic. They don’t address underlying movement.

Quick Crack‑Assessment Checklist

• Identify the crack type (hair‑line, settlement, structural).
• Measure width with a gauge or feeler bar.
• Compare width to the table above.
• Note any external factors (soil type, recent seismic events).
• Decide: monitor, seal, or call a professional.