Brick durability: what F2 and S2 actually mean
What the F and S designations on a clay brick actually mean, where they matter on a building, and what to put in your specification.
Almost every UK brick specification carries a designation like F2,S2. It appears on datasheets, in NBS clauses and on delivery notes, and it is usually copied forward from the last job without much thought.
It describes two entirely separate properties — and only one of them is about frost.
Freeze/thaw resistance
How the brick behaves when it is saturated and the temperature drops below zero. A physical property.
Active soluble salt content
How much soluble salt the clay itself carries, and what that can do to the mortar around it. A chemical property.
Both are declared under BS EN 771-1, the product standard for clay masonry units. A brick can be excellent at one and unremarkable at the other, which is why the two are almost always quoted together. Writing "F2" on its own leaves half the question unanswered.
Freeze/thaw resistance.
Clay brick is porous. When a brick is saturated and the temperature drops, water held in those pores expands as it freezes. If the brick cannot accommodate that expansion, the face spalls — the surface flakes away, exposing fresh clay underneath, which then absorbs more water and accelerates the process.
The UK climate does not offer much shelter. Wind-driven rain saturates masonry, and freezing follows often enough that most external brickwork is specified F2 as a matter of course.
F1 is the exception, not a cost-saving default.
| Category | Exposure |
|---|---|
| F0 | Passive — protected or internal work, never saturated in freezing conditions |
| F1 | Moderate — sheltered external work where saturation is unlikely to coincide with freezing |
| F2 | Severe — exposed external work, including areas that stay wet |
Active soluble salts.
Clay contains naturally occurring soluble salts — principally sodium, potassium and magnesium. Water moving through masonry dissolves them and carries them to the surface. Two things can follow, and they are not equally serious.
Efflorescence is a white bloom on the face as water evaporates and leaves the salt behind. It looks alarming on a new building and is almost always cosmetic, weathering off over the first year or two.
Sulfate attack is the real concern. Where masonry stays persistently wet, sulfates in solution can attack the cement in the mortar, causing it to expand and crack. That is a durability problem rather than an appearance one, and it is the reason the S rating exists.
| Category | Salt content |
|---|---|
| S0 | No requirement declared |
| S1 | Limits declared on sodium, potassium and magnesium |
| S2 | Tighter limits — the lowest salt content category |
Where durability actually matters.
Durability is not uniform across an elevation. The same brick can be entirely appropriate in a sheltered upper-storey panel and a poor choice three metres below. The parts that stay wettest carry the risk.
Below DPC
In contact with ground moisture and splashback from hard surfaces.
Retaining walls
Wet from behind, often permanently, with nowhere for water to go.
Parapets & copings
Exposed on both faces and along the top, with no protection above.
Cills & projecting detail
String courses and banding shed water onto themselves.
Freestanding walls
No roof, weather on both sides, saturated for long periods.
Chimneys
Fully exposed, and subject to sulfates carried in flue gases.
PD 6697, the UK guidance document supporting the Eurocode for masonry, sets out recommended durability designations for these situations. If you are specifying anything below DPC or above roof level, it is the document to check.
What is actually available
This reframes the question. On most schemes it is not "can I find an F2,S2 brick" — it is "is there any reason to accept less". If a brick you like is F1, that is worth a conversation about where on the building it is going.
Of 1,289 clay bricks currently in the RGB library.
What to put in your specification.
A durability requirement is only useful if it is unambiguous. A complete clause states the designation, references the standard it comes from, and says where it applies.
Facing brickwork to be clay units to BS EN 771-1, freeze/thaw resistance category F2, active soluble salt content category S2. Brickwork below DPC and to all parapets and copings to be F2,S2 throughout.
State the designation, not just a brand
Naming a product alone leaves nothing to assess a substitution against. The designation travels with the requirement.
Call out the vulnerable zones
Below DPC, parapets, retaining work. A single blanket line rarely makes clear that these need the same designation.
This guide covers selection and specification. For the underlying technical detail, the definitive sources are BS EN 771-1 for the product standard, PD 6697 for UK durability guidance, and the Brick Development Association's technical guides for design and workmanship.