What Is Frost Weathering and How Does It Damage Brickwork?

What Is Frost Weathering?

Quick answer: what is frost weathering? Frost weathering is damage caused when water inside brick, stone, render or mortar freezes and expands. Ice takes up around 9% more space than water, so it cracks the material from within. Healthy brick that can dry out properly shrugs most of it off. Brick that's kept wet by old paint or dirty, failing render is the brick that breaks.

That last point is the one that matters. Frost only does serious damage to walls that are already struggling to stay dry. So if you've been asking what is frost weathering because your bricks are flaking or your paint is bubbling, the real question is how healthy your walls are going into winter.

What is frost weathering in simple terms?

What is frost weathering? It's water getting into your walls, freezing, expanding and breaking the surface apart from the inside. You'll also hear it called freeze-thaw damage. It's the same thing that splits rocks on a hillside, just happening slowly on your house.

The freeze-thaw cycle step by step

• Water gets in. Rain soaks into the pores of the brick, mortar or render.
• The temperature drops below 0°C. Water near the surface starts to freeze.
• Ice expands. It needs roughly 9% more room than the water did.
• Tiny cracks open. The material has nowhere to give.
• It thaws and refreezes. More water runs into the cracks and the next frost opens them wider.

Why a wet wall is the real problem

A dry brick has spare room in its pores, so ice can form without cracking anything. A soaked brick has none. That's why frost damage to brickwork nearly always comes back to one thing: water that's stuck in the wall and can't get out.

Why does healthy brickwork resist frost damage?

Healthy brickwork resists frost damage because it dries out between spells of rain, so it's rarely saturated when the temperature drops. Brick isn't meant to be waterproof. It's meant to take on a bit of water and then let it go again. When that cycle works, the wall lasts for centuries.

What healthy brick looks like

• A bare, breathable face. Moisture can escape through the brick and the joints.
• An intact fired surface. The hard outer skin from the kiln is the brick's armour against weather.
• Sound mortar joints. On older homes, soft lime mortar that lets water out rather than forcing it through the brick.
• Clean surfaces. No thick algae, moss or grime holding water against the wall.

Why it's worth protecting

Brick health isn't a cosmetic thing. Healthy walls keep damp out of your home, hold onto heat better when they're dry, and save you from expensive repairs later. Unhealthy walls go the other way, and frost speeds the whole process up. A few flaky bricks this winter can be whole missing brick faces in two or three years.

How does frost weathering damage brickwork?

Frost weathering damages brickwork by blowing off the hard outer face of the brick and eroding the mortar between them. Once the face has gone, the soft clay behind it soaks up water even faster. So every winter does more damage than the last.

The three main types of frost damage

Spalling

The face of the brick flakes, pops or peels off. Sometimes it's a thin skin. Sometimes the whole front comes away.

Mortar erosion

Joints turn sandy and crumble out, which lets even more water into the wall.

Cracked bricks

Repeated deep freezing can split bricks right through, usually on chimneys, parapets and garden walls first.

Why is painted brick more likely to get frost damage in winter?

Painted brick gets more frost damage because the paint traps moisture in the wall, so the brick is still wet when the first frosts hit. This is the biggest cause of winter damage to painted brick we see. And it's completely avoidable.

Paint lets water in but not out

Masonry paint forms a film on the surface. Rain still gets in through hairline cracks, joints, sills and the top of the wall. But it can't escape at the same speed, so the brick behind stays damp for far longer than bare brick would. Our guide on how paint affects brickwork goes into this in more detail.

The autumn timing problem

• Autumn rain soaks the wall.
• The paint slows drying right down.
• The first hard frost hits a wall that's still saturated.
• The brick face freezes with water trapped right behind the paint.

"Breathable" paint doesn't solve it

Even paints sold as breathable let far less moisture through than bare brick. And they get worse with age, dirt and extra coats. See is masonry paint bad for brick and stone for the full picture.

Paint hides the damage

Here's the worst part. Spalling starts behind the paint, where you can't see it. By the time the paint bubbles and drops off, it often takes the brick face with it. Repainting over that just seals the problem in for another winter.

Can dirty or cracked render cause frost damage to your walls?

Yes, dirty or cracked render can cause frost damage because it holds water against the wall and slows down drying, just like old paint does. Render is there to protect the masonry behind it. When it's neglected, it starts doing the opposite.

How dirt and growth keep render wet

• Algae, moss and lichen act like a sponge on the surface and keep it damp for days after rain.
• Grime and pollution clog the pores of breathable renders, so moisture can't escape.
• North-facing and shaded walls rarely get the sun to dry them, so growth spreads fastest there.

What frost does to wet render

Blown render

Water behind the render freezes and pushes it away from the wall. You'll hear it as a hollow sound when you tap it.

Cracking and patches falling off

Each freeze opens cracks a little further. Eventually sections come away and leave the brick or block behind exposed.

Damp getting through to the brick

Once water is behind the render, it soaks the masonry underneath, and that's when you start seeing damp inside.

We've written more on keeping render safe in our guide on whether it's safe to clean render yourself.

What are the signs your brickwork isn't healthy before winter?

The main signs your brickwork isn't healthy are flaking brick faces, crumbling mortar, bubbling or peeling paint, green or black growth and damp patches inside. Once you know the answer to "what is frost weathering?", these are easy to spot on a quick walk round your property.

More on paint in particular in our guide to the signs your exterior paint is failing.

How does exterior paint removal protect brick from frost damage?

Exterior paint removal protects brick from frost damage by letting the wall dry out naturally again, so it isn't saturated when winter arrives. Take the paint off and you take away the thing keeping the water in. It's the single biggest step you can take for the health of a painted brick wall.

What changes once the paint is off

• The brick dries between rain showers, just as it was built to.
• Pores have room for ice, so frost stops cracking the face.
• Damp inside often eases as the wall dries out.
• You can actually see the brick and mortar, so problems get caught early.

Why the removal method matters so much

Sandblasting and high-pressure washing will get paint off. They'll also strip the fired face off the brick, which is exactly the armour frost attacks. You'd be doing frost's job for it. That's why we only use low-pressure, conservation-approved systems:

• DOFF uses superheated water up to 150°C at low pressure to soften and lift paint without abrasion. Read what is DOFF cleaning to find out more.
• ThermaTech is a hot water and steam system for stubborn coatings, set to suit the surface.
• TORC is a low-pressure vortex system for softer brick and stone.

Why repainting isn't the answer

Repainting looks tidy for a season. But every coat traps more water, and the next winter does more damage behind it. We explain why in paint removal vs repainting.

Ready to get your brickwork breathing again?

Find out more about our brick paint and masonry paint removal service or ask for a free quote.

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How does render cleaning help protect walls from frost?

Render cleaning helps protect walls from frost by removing the algae, moss and dirt that keep render damp, so it dries faster and holds less water going into winter. A clean render does its job. A dirty one slowly lets the wall behind it down.

What a professional render clean does

• Removes biological growth right down to the roots, not just the surface stain.
• Opens up the surface so breathable renders can let moisture out again.
• Slows regrowth with treatments that keep the render cleaner for longer.
• Shows up cracks and blown areas that dirt was hiding, so they can be fixed before frost gets in.

Why you shouldn't pressure wash render

High pressure can force water deep behind the render, gouge softer finishes and widen hairline cracks. That's the opposite of what you want before a frost. We use low-pressure steam and soft washing to clean render safely. 

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How can Ultra Cleaning Services help keep your brickwork healthy?

Ultra Cleaning Services helps keep your brickwork healthy by removing the paint and dirt that trap moisture, using gentle systems that protect the brick, stone and render underneath. We're exterior cleaning specialists based in Hampshire, working across the south of England on everything from Victorian terraces to listed buildings.

Why now is the time

Autumn is the best moment to act. You can still see last winter's damage, and getting walls cleaned or assessed now means they go into the frosts in better shape. Every winter you leave it, the repair bill grows.

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