Brick Quality Ratings Explained

When you buy a pallet of bricks, you'll get a spec sheet with it featuring words like F2, S2, Category II, HD and a number in N/mm². But what does it all mean? None of it means much at all unless someone tells you what it's for.

In short, those ratings and codes tell you where a brick can safely go. Put them in the wrong place and the brickwork can crumble, bloom white or push the mortar joints apart within a few winters. Get them right and the wall will outlive you.

In this guide, we're going to go over what each rating covers and when it actually affects your project.

Where Do Brick Ratings Come From?

UK clay bricks are made to BS EN 771-1 standards, which replaced the old British Standard, BS 3921, back in 2003. This industry standard doesn't hand out a single quality score but instead the brick manufacturer itself has to declare how the brick performs against a set of separate measures. The results are then published on a Declaration of Performance, or DoP, which is the document you want to look at when choosing your bricks.

Before we cover the individual codes, there are two things worth knowing. A brick can be brilliant in one respect and useless in another, so plenty of lovely facing bricks aren't fit for a coping. That's not a defect, it's just what they were built for. And "high quality brick" isn't a technical term at all. A £2.50 handmade brick and a 60p common can both be exactly right, depending on where they're going. The perceived 'quality' of a brick depends entirely on its intended use.

Frost Resistance (F0, F1 and F2)

This is the rating that catches people out most often on garden walls and extensions.

Bricks soak up water and when that water freezes it expands. If the brick you have chosen can't cope with the pressure, the face blows off. This is known as spalling in the trade.

BS EN 771-1 splits frost resistance three ways, tested to BS EN 772-1. F0 covers passive exposure, so internal walls only or brickwork fully-protected behind render or cladding. Basically, F0 bricks won't survive being wet and frozen. F1 signifies moderate exposure, so fine for normal external walling above the damp proof course where the brick doesn't sit saturated. F2 bricks are for severe exposure, which handle being saturated and frozen repeatedly, so the best quality bricks for outside.

Almost all UK external brickwork should be F2 unless you've got a specific reason to go lower. Anything that stays wet needs F2 - there's no doubt about it. Whether that's free-standing garden walls, parapets, chimneys, retaining walls, brickwork below the DPC, planters, steps, paving and even pizza ovens, you need F2-rated durable bricks.

The good news is that most quality UK facing bricks are F2 as standard. Reclaimed stock is the one to watch, since the frost performance of a decades-old brick is anybody's guess and there's no DoP to check.

Soluble Salt Content (S0, S1 and S2)

Clay in brickwork holds natural salts like sodium, potassium and magnesium. When water moves through the bricks, these salts are dissolved which can affect the structure. As such, bricks have soluble salt content ratings from S0 to S2, tested to BS EN 772-5. S0 has no limits set and is for fully protected walls only. S1 allows normal levels, with sodium and potassium combined no higher than 0.17% by mass and magnesium no higher than 0.08%. S2 is the tightest rating, with limits of 0.06% and 0.03%, respectively.

The reasons these salt ratings matter are two-fold. First, there's efflorescence, where salts get drawn to the surface as the wall dries and leave a white powdery bloom, affecting the aesthetics of the brickwork. This is usually harmless and it tends to weather off over time, though it can take a couple of seasons and can sometimes come back.

The bigger problem is that sulphates in the brick can react with the cement in your mortar, which expands and cracks the joints from the inside. This sulphate attack on an exposed wall that gets a lot of water through it will cause a repointing job at best and a rebuild at worst.

S2 cuts the risk down, so for external work, and definitely for anything that'll stay damp, always go for S2. If you're using S1 in an exposed spot, sulphate-resisting cement in the mortar can be a good answer.

Compressive Strength (N/mm²)

Compressive strength measures how much load the brick takes before it crushes and is measured in newtons per square millimetre or N/mm². Typical clay-facing bricks land somewhere around 9 to 20 N/mm², though plenty can exceed that, while engineering bricks start at 75. But for a normal house, you're rarely anywhere near the limit, because the mortar and the wall design are doing a lot of the work.

Compressive strength starts to matter more on taller structures, where NHBC guidance sets minimums by storey count, and on anything carrying a real load like a beam bearing or a pier supporting heavy gates. This generally extends to retaining walls and brickwork below ground too.

When assessing brick compressive strength, you'll also spot Category I or Category II next to the strength figure, which is all about how confident the declared number is. Category I units come with tighter quality control and a documented low probability of falling short, whereas Category II are declared without that same assurance. Structural engineers care about the difference, so if your design has been specified as Category I, the bricks must be rated as such.

Water Absorption (kg/m².min)

Brick water absorption ratings are given as a percentage of the brick's dry weight. Some spec sheets show initial rate of absorption too, in kg/m².min.

What may surprise you is that a lower absorption ratings isn't automatically better. A very dense low-absorption brick can be awkward to lay well, because the mortar doesn't grip in the same way, and a brick with almost no suction will float on the bed. Most bricklayers generally go for something in the middle.

Absorption does tell you a fair bit though. It's tied to frost performance, it affects how much water gets into the wall and it's the reason engineering bricks work as a DPC. It's also worth knowing that absorption varies hugely across normal facing bricks, because soft handmade stock might be over 20%, whereas a wirecut might be nearer 8%, but both are perfectly good bricks.

Engineering Bricks (Class A and Class B)

Class A and Class B engineering brick ratings came from BS 3921 so are technically outdated, but nobody stopped using the terms and manufacturers still print them, so they're still the quickest way to describe what you want. The classes combine ratings for water absorption and compressive strength.

Class A needs 125 N/mm² or more and water absorption of 4.5% or less. Class B is 75 N/mm² or more, absorbing no more than 7%. Class A is usually blue, fired hotter and harder, while Class B is normally red.

These engineering bricks should be used for damp proof courses, manholes, inspection chambers, foundations, retaining walls, sewer work and copings. Basically anywhere the exposure or the load is beyond what a facing brick handles.

If you're buying engineering bricks, still remember to check the F and S designations on the DoP. Class A or B tells you about the strength and absorption, not frost or salts.

What "First-Quality" Means For Bricks

This one confuses a lot of people, which is understandable. For example, 'Freshfield Lane First Quality Multi' is one of the most specified bricks in the country, but the "First Quality" bit isn't a British Standard rating. It's just a part of the product name, describing the manufacturer's own top sort from that works.

This is because most brick factories grade their output. The best-formed, most consistent units get sold as the premium line, while slight seconds, overburnt units and colour outliers get sold cheaper or go into blends.

This means a brick called 'first quality' is like to be a good brick, but the name tells you about appearance and consistency, not durability. Check the proper ratings separately.

Same logic applies at the other end. Our clearance bricks are surplus and discontinued stock rather than substandard product, and they carry the same performance data as anything else on the shelf.

Fire Resistance and Reaction To Fire

Bricks can always be rated in relation to fire, done in two different ways - resistance and reaction to fire - and we will explain this using clay bricks.

Clay bricks are classified A1 for reaction to fire, the best rating there is in this category, meaning they don't contribute to a fire. But that's a different thing from the fire resistance of a wall, which is measured in minutes and depends on the whole build-up. Resistance is affected by thickness, mortar, whether it's a single skin or a cavity and what's on either side of the wall. A brick being A1 doesn't give you a 60-minute resistant wall on its own.

If you need a specific fire rating for a party wall or a garage, that's more of a Building Regulations question for your designer rather than something you solve at brick selection.

Old Brick Ratings Still Relevant

An old British Standard, BS 3921, would combine frost and salts into just one code, and you'll still find these on older spec sheets and in the vocabulary of anyone who's been laying bricks for decades. FL and FN meant frost resistant with low or normal salts, ML and MN means moderately frost resistant and OL and ON aren't frost resistant at all. Roughly, FL maps to F2 S2 and MN maps to F1 S1.

But if someone quotes you an old designation like this, ask for the current DoP rather than guessing at the conversion.

How To Check Brick Quality Before You order

You don't need a lab to check the quality of bricks before you order them, you just need the paperwork and ten minutes spare to go over it.

Start by asking for the DoP. Every brick sold in the UK should have one, listing frost designation, salt category, compressive strength, absorption, density, dimensional tolerance and reaction to fire. While you're there, check the UKCA or CE marking, which confirms the product has been assessed properly.

Ratings won't tell you about colour, texture or how a brick sits against your existing wall, so it's always a good idea to get a brick sample before you commit. When the pack turns up, inspect it properly for consistent sizing, sharp arises, no widespread cracking and no lime blowing. Tapping one helps as well, since a well-fired brick rings and a dull thud could mean it's been underfired.

One important thing to note is that colour and texture drift slightly between production runs, which matters on a visible elevation, so order the lot in one go where you can. If you're topping up an existing job, our Brick Matching service handles the identification for you.

Matching Brick Ratings to the Job

Not Sure Which Rating You Need?

Most brick problems aren't down to bad bricks - they're down to a decent brick going in the wrong place.

Tell us what you're building and where, and our team will point you at products that suit. Our Brick Selection service does the shortlisting, Brick Matching finds you a like-for-like on existing brickwork, and the free Brick Calculator tells you how many you need.

Or just call us on 0330 122 4060 and ask.