Freeze Thaw Action On Masonry and Concrete

Freeze-thaw damage is a common cause of external wear on brickwork, concrete, and stone, especially in parts of the UK where winter temperatures often fall below 0°C.

It can be hugely detrimental to many building materials including bricks, stone, concrete and the similar and left unchecked, can structurally affect buildings resulting in hugely expensive fixes. It’s also sometimes known as frost shattering.

In this guide we take a close look at this phenomenon and how it can be resolved quickly and easily.

What Exactly is Freeze-Thaw Action?

Freeze-thaw is a weathering process where water enters tiny cracks in masonry or concrete. When it freezes, the water expands, putting pressure on the material. Repeated freezing and thawing causes the surface to crack, crumble, and eventually break apart.

So what does this actually mean? Well in simple terms, freeze-thaw action is a common weathering process that causes damage to outdoor structures like brick walls, concrete paths, and stone surfaces across the UK. It happens when water soaks into tiny cracks and pores in these materials and then freezes as temperatures fall below zero.

When water freezes, it expands by nearly 10%. This expansion puts pressure on the surrounding material, slowly widening cracks and causing pieces to break off over time. When the ice thaws, water seeps deeper into the cracks, and the cycle repeats.

This constant freeze and thaw cycle weakens the material, leading to crumbling, flaking, or spalling, a process often called frost shattering.

This damage is especially noticeable in colder months and in areas exposed to rain and frost. Older buildings with porous materials or damaged mortar are particularly vulnerable.

Understanding freeze-thaw action is key to spotting early signs like small cracks, surface roughness, or loose bricks, which helps you act quickly to prevent costly repairs.

How Freeze-Thaw Action Affects Concrete and Masonry

Concrete, bricks, blocks, and stone all have tiny cracks and pours that fill with water when wet. When temperatures drop, this water freezes and expands, putting pressure on the cracks.

As ice expands, it widens these weak spots. When it thaws, water drains away, leaving bigger cracks. This cycle repeats, gradually breaking down the material until pieces crumble or fall off.

This freeze-thaw cycle causes significant damage to external walls. Repair usually means removing and replacing damaged bricks or sections of masonry.

Painted masonry is less prone to freeze-thaw damage, but walls must be fully repaired before repainting. See our brick replacement guide for step-by-step instructions.

Why Is Freeze-Thaw So Damaging?

Freeze-thaw may sound technical, but it’s easy to understand. Water behaves differently from most liquids when it freezes. Instead of shrinking, water expands below 4°C, increasing in volume by up to 10% as it turns to ice.

This unique trait allows ice to float, helping life survive under frozen ponds. However, this expansion also causes damage. When water trapped in cracks freezes, it pushes the cracks wider, breaking apart masonry and stone over time.

Freeze-thaw damage is worst where temperatures often hover around 0°C, like the UK, causing repeated freezing and thawing cycles that widen cracks and let in more water.

Luckily, severe damage requires high moisture levels, around 90% saturation. By keeping your walls dry and preventing water entry, you can reduce freeze-thaw damage and protect your property.

Can Insulating Walls Cause Freeze-Thaw?

Yes, but only in some cases. Insulating a wall, especially from the inside, can change how moisture moves through the building fabric. If not done correctly, this could trap moisture inside the wall, increasing the chance of it freezing in cold weather.

This is more likely to be an issue in older homes without a cavity wall, or where the exterior brickwork is exposed and already weathered.

Walls naturally allow moisture to move in and out, this is what’s known as “breathability”. When you insulate internally, you make the inner side of the wall warmer, but the outer side stays cold.

This colder exterior means any trapped moisture is more likely to freeze. Without a way for the wall to release that moisture and in these situations, freeze-thaw damage can occur faster than it normally would.

To help avoid this phenomenon from occurring, do the following:

• Check your brickwork – Make sure bricks and mortar are in good condition before insulating.
• Use breathable materials – Choose insulation systems that allow moisture to escape, such as vapour-permeable boards.
• Consider external insulation – Adding insulation to the outside of the wall keeps the bricks warm and reduces the risk of freezing.
• Get a moisture survey – A qualified surveyor can assess whether your home is suitable for insulation and advise on the safest method.

What Types of Bricks, Stone or Surfaces are More at Risk to Freeze-Thaw?

Freeze-thaw damage can quietly destroy brick, stone or rendered walls over time, especially in the damp and frosty UK climate and some materials are certainly more susceptible than others.

Older handmade or soft red bricks are particularly vulnerable. These were widely used in UK houses built before 1920 and are very porous, meaning they soak up a lot of moisture. Once wet, they freeze more easily in winter and break down faster than modern, denser bricks.

Low-fired or underburnt bricks (often seen in heritage properties) are also at high risk. They look charming, but their sponge-like texture makes them especially prone to frost damage.

In contrast, modern engineering bricks or high-fired clay bricks are more water-resistant and hold up better in freeze-thaw conditions as they are much more dense. They’re ideal in exposed areas like garden walls or foundations.

When it comes to natural stone, this can vary widely:

• Limestone and sandstone – Common in traditional UK homes, these are both porous stones and can suffer badly in freeze-thaw cycles if not properly maintained or sealed.
• Granite and slate – These denser stones are less likely to absorb water and are much more resistant to frost damage.

That said, even hard stone can suffer if joints are poorly pointed or if moisture is allowed to sit in cracks over winter.

Another surface-type that’s potential affected can be cement-based renders, when cracked or poorly applied, they are very susceptible to freeze-thaw. Water can seep behind the surface, freeze, and cause the render to blow or detach.

Traditional lime render, on the other hand, is more breathable and better at handling moisture without breaking apart.

Another point to be aware of in terms of modern surface coverings are modern external wall insulation systems must be installed with proper waterproof layers to avoid trapping water, which can freeze behind the finish and lead to rapid surface failure.

As with all things, prevention is much better than cure, so to help avoid any of the above issues, regular care and maintenance is absolutely essential, so pay close attention to the following points:

• Regular maintenance – Check for cracks or gaps in mortar and repair them before winter.
• Use breathable materials – Especially on older buildings, to let trapped moisture escape rather than freeze.
• Avoid waterproof coatings that trap moisture – Some “weatherproof paints” can worsen freeze-thaw by sealing in damp.
• Consider protective detailing – Simple things like drip edges or proper overhangs help keep walls dry.

Types of Freeze-Thaw Damage

One of the first steps to overcoming freeze-thaw issues is to firstly identify whether you actually have freeze-thaw issues and one of the best ways of confirming this is to look for signs of damage, so here follows the main visual signs of freeze-thaw:

Cracked or Spalling Bricks

One of the most common signs of freeze-thaw damage is spalling, this is where pieces of brick or stone begin to flake off or break away. This happens when water enters the surface and freezes, causing the outer layer to expand and eventually separate.

You might see bricks that look chipped, soft, or “blown out” on the face. It’s especially common in older, softer bricks found in Victorian or Edwardian properties across the UK.

Blown or Delaminated Render

On rendered surfaces, freeze-thaw can cause the finish to lift away from the wall. This is known as delamination or “blown render”.

Moisture gets trapped between the render and wall, then freezes and expands, creating bubbles or cracks.

Cement-based render is particularly prone to this if not applied properly or if hairline cracks are left untreated before winter.

Crumbling Mortar Joints

Mortar joints, the “glue” between bricks or stones, are another weak point. Over time, repeated freeze-thaw cycles can cause mortar to crack, crumble or wash out entirely.

This is known as frost decay. It reduces the structural integrity of the wall and allows more water to penetrate, accelerating further damage.

Lime mortar, common in historic UK homes, handles moisture better than cement-based mixes, but even it can degrade if neglected.

Surface Cracks and Hairlines

Not all freeze-thaw damage is dramatic. Often it starts as fine surface cracks in stone, brick or concrete. These hairline fractures can let in more water over time, leading to deeper damage if not addressed.

They’re particularly tricky because they’re easy to miss until they grow larger.

Regular inspection during seasonal changes can help catch them early.

Internal Damp or Cold Spots

In some cases, freeze-thaw damage isn’t visible from the outside. If cracks form within the wall, they can allow damp to penetrate through to the inside.

You might notice cold patches, staining or peeling paint indoors, especially on external walls during winter.

These are indirect signs that the outer wall might be compromised by freeze-thaw action.

How to Prevent and Treat Freeze-Thaw Damage in Masonry and Concrete

Preventing water from getting into your brickwork or concrete is the best way to avoid freeze-thaw damage.

Once water gets in and freezes, it expands, causing cracks, crumbling and long, term structural issues. Stopping moisture at the source is key.

Keep water away from your walls. Gutters and roof overhangs help direct rainwater away from masonry. Check they’re working properly, especially in winter.

Seal exposed surfaces. Use breathable water-repellent sealants designed for exterior brick or blockwork. These help reduce water absorption through cracks and pores.

Render vulnerable walls. Rendering adds a protective barrier. Even if the render fails over time, it’s easier to repair than replacing damaged bricks underneath.

Use outdoor-rated materials for patios and paving. Always follow manufacturer instructions, especially regarding sealing. These materials are built to resist water and frost damage.

Once freeze-thaw damage occurs, replacement is often the only fix. Damaged bricks, slabs or blocks may need to be removed and replaced. Acting early can save time and cost later on.

Helpful DIY Guides:

• How to replace a damaged brick
• Laying or relaying patio slabs
• How to lay block paving
• Repair cracks in concrete paths

How Freeze-Thaw Affects Concrete

Concrete is especially vulnerable to freeze-thaw cycles, though the effects can vary. Over time, water seeps in, freezes, expands, and breaks the surface, leading to cracks, crumbling and, eventually, complete failure. As always, prevention is always better than cure.

Common Freeze-Thaw Damage in Concrete

• Surface scaling and spalling: The top layer flakes or powders off, especially on concrete pavers. This often gets worse with road salt and grit used in icy conditions.
• D-Cracking: Deep cracks form as the aggregate inside the concrete expands. This weakens the entire slab. Using frost-resistant or smaller aggregates helps prevent this.

Choosing the Right Concrete

Most modern concrete mixes are engineered to resist frost damage. If you’re laying concrete outdoors, ask your builder’s merchant for a mix suitable for freeze-thaw conditions in the UK climate. They’ll recommend the best type for your project.

What Is Air Entrainment?

Surprisingly, some concrete includes tiny air bubbles on purpose. This is called air entrainment, and it helps absorb the expansion from freezing water. Instead of cracking, the concrete flexes slightly, reducing damage. It’s a key feature in frost-resistant concrete.

Read more about air entrainment on Wikipedia.

In summary, freeze-thaw damage, also known as frost shattering, can quickly destroy concrete once it starts. But the good news is it usually takes a high level of moisture saturation to do serious harm. That’s why good drainage, quality materials, and early action are essential.

By keeping water out and choosing the right concrete mix, you can stop freeze-thaw issues before they begin, saving you time, money, and repairs in the long run.

Freeze-thaw in any building material including bricks, blocks, stone and concrete can be an absolute nightmare and cause huge amounts of damage is allowed to persist, however if you spot any issues early and act right away, any major and lasting damage can be avoided.