The frost resistance of clay bricks is given in class F0, F1 or F2 according to DS/EN 771-1. Here is the link to the exposure classes MX1–MX5, and why driving rain decides whether a facade is MX3.1 or MX3.2.
Introduction
Not all bricks are equally frost-resistant. Clay bricks are classified into three frost classes (F0, F1, F2) according to DS/EN 771-1, and the class should correspond to the exposure the wall is subject to. Wrong stone choice gives flaking and cracked facades.
Frost classes (DS/EN 771-1)
| Class | Frost resistance | Typical application |
|---|---|---|
| F0 | Not frost-resistant | Only without frost: dry environment (MX1) or moisture without frost (MX2) |
| F1 | Moderately frost-resistant | Moisture + freeze/thaw, protected against driving rain (MX3.1) |
| F2 | Highly frost-resistant | Severe wetting + freeze/thaw: facades exposed to driving rain, freestanding walls, chimneys, plinth zone with splash water (MX3.2) |
Exposure classes MX1–MX5.
| MX class | Environment |
|---|---|
| MX1 | In a dry environment — e.g. internal masonry and the inner leaf of a cavity wall that is unlikely to become damp |
| MX2 | Moisture or wetting, but no freeze-thaw (MX2.1/MX2.2) — e.g. a laundry, an external wall sheltered by a roof overhang, masonry below the frost zone |
| MX3.1 | Moisture + freeze-thaw, protected against driving rain (as MX2.1, but with frost) |
| MX3.2 | Severe wetting + freeze-thaw, exposed to driving rain (as MX2.2, but with frost) — e.g. freestanding walls and parapets |
| MX4 | Salt-saturated air, seawater or de-icing salt — coastal areas and masonry close to roads that are salted in winter |
| MX5 | Aggressive chemical environment — e.g. soil or groundwater containing sulphates, acidic or contaminated soil, industrial air |
The classes MX1 → MX3.2 form a hierarchy of increasing moisture and frost load. MX4 and MX5 are not part of the hierarchy — they are added on top of the moisture class, e.g. «MX3.2 + MX4». Bagging (filtsning) does not protect the masonry, so the same exposure as for fair-faced masonry applies; only render provides protection. Source: DS/EN 1996-2:2007 table A.1 (Danish extract from Egernsund Wienerberger, «Eksponeringsklasser»); Mur & Tag, «Hvilken MX-klasse er hårdest» (12-07-2021).
Stone groups 1–4
Frost resistance and compressive strength are related to how solid the stone is — see stone groups 1–4 in 'Compressive strength for masonry'. Common Danish façade stone is group 1. The group is stated in the declaration and used directly in the compressive strength calculation.
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