Choosing the Right Bricks for External House Walls

Choosing bricks for an external wall is one of those decisions that looks simple on a drawing and gets complicated the moment you stand in a builder's merchant with a sample in each hand. Get it right and the wall will look good and stay put for a century or more. Get it wrong and you'll be looking at spalling faces, blown mortar and an unhappy client within a decade.

Here's what actually matters when you're specifying bricks for external house walls in the UK.

Frost Resistance Comes First

In this climate, frost is the single biggest killer of brickwork. Water gets into the face of the brick, freezes, expands by around 9 per cent, and pushes the surface off. Repeat that over a few winters and you have spalling — flakes of brick lying on the path and a face that is quietly disintegrating.

Bricks are classified for frost resistance under the harmonised European standard, and you will see them marked F0, F1 or F2:

• F0 — not frost resistant. Fine for internal work and protected locations, never for an external leaf.
• F1 — frost resistant. Suitable for normal UK external use.
• F2 — frost resistant and proven by freeze–thaw testing. The right choice for exposed sites, coastal locations, high ground and anything facing the prevailing wind.

If the wall will be soaked and frozen regularly — think west-facing elevations on high ground, or a gable end catching weather off the sea — insist on F2 and don't let anyone talk you down on price.

Strength Grades: What the Outer Leaf Actually Needs

Compressive strength is quoted in newtons per square millimetre (N/mm²), and it is often the number that gets misunderstood.

In a standard cavity wall, the external leaf is not carrying the roof — the inner leaf and any structural frame take that load. So the outer leaf usually only has to support itself plus a bit of wind loading. That means a facing brick of roughly 10–20 N/mm² is generally more than adequate, and there is little sense in paying for engineering bricks you don't need.

The exception is where the brickwork is genuinely structural: a solid nine-inch wall, a boundary wall acting as a retaining structure, a pier, or a chimney stack above roof level. That is when you move up the grades. Engineering bricks sit at the top of the tree:

• Class A — typically 70 N/mm² or above, with water absorption of 4.5 per cent or less. Dense, hard and close to waterproof. Damp proof courses and severe exposure.
• Class B — typically 50 N/mm² with absorption under 7 per cent. A good middle ground where strength and low absorption both matter.

Be aware that hard bricks need a mortar to match. A strong brick paired with a weak, over-rich mortar does not fail gracefully — it cracks. Match the mix to the brick rather than defaulting to 1:3, and use lime-based mortars with soft handmade and reclaimed bricks so the wall can move and dry out.

Water Absorption and Soluble Salts

Absorption figures tell you how thirsty a brick is, which in turn tells you how it will behave once the rain starts. Lower absorption means less water held in the fabric, less risk of frost damage and less chance of the wall staying damp for months on end.

There is also the question of active soluble salts, rated S0, S1 and S2. Salts within the brick dissolve in rainwater and migrate to the surface as the wall dries, leaving white powdery bloom — or, in persistently wet conditions, causing the face to crumble. For exposed work, look for a low salt content alongside the frost rating, and check both rather than assuming one covers the other.

Clay, Concrete and Reclaimed

All three have their place, and the right answer depends on the building, the budget and the look you are after.

• Clay bricks — fired from shale, clay or marl. The widest range of colours and textures, excellent long-term performance, and the only sensible choice where you are matching a period property. Expect variation from pack to pack; that is a feature, not a fault.
• Concrete bricks — consistent in colour and size, generally cheaper, and well suited to utility walls, boundary walls and internal leaves. They can look flat and uniform across a large elevation, and the softer mixes are best kept away from very exposed faces.
• Reclaimed bricks — the best way to blend a new extension into old brickwork. Check every batch for frost damage, salt contamination and crumbling corners, and clean them gently. Bed them in lime mortar so the new work moves in sympathy with the old.

Getting the Practical Details Right

A few habits separate a tidy job from a callback:

• Ask for a sample panel on site before the order is placed. Colour brochures and photographs lie; mortar and daylight do not.
• Order five to ten per cent extra for cutting, breakages and future repairs. Matching a batch three years later is difficult and sometimes impossible.
• Keep bricks dry and covered on site. Saturated bricks laid in freezing weather are asking for trouble.
• Check dimensional tolerance if you are working to a tight joint width — some handmade bricks run generous, and 10mm joints soon become 15mm.
• Confirm in writing that the brick meets the durability requirements for its exposure condition. Building control will want to see it.

Choose for the weather first, the structure second and the appearance third — but choose all three deliberately, and your brickwork will still be sound long after the scaffolding has gone.