The frost resistance of brick masonry units is specified in class F0, F1, or F2 according to DS/EN 771-1. Here is the link to the exposure classes MX1–MX5 and what a normal Danish facade requires.
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 indoors (MX1) / protected dry environment (MX2) |
| F1 | Moderately frost-resistant | Normal Danish outdoor facade with moisture and frost/thaw (MX3.1) |
| F2 | Highly frost-resistant | Severe water impact: free-standing walls, chimneys, plinth zone with splash water (MX3.2) |
Exposure classes MX1–MX5.
| MX class | Environment |
|---|---|
| MX1 | Passive — internal and not frost-exposed |
| MX2 | Protected outdoors dry (under roof without splashes) |
| MX3.1 | Outdoor, moisture + frost/thaw—normal Danish facade. |
| MX3.2 | Strong water impact + frost/thaw — plinths, freestanding walls |
| MX4 | Coastal / road salt — special salt exposure |
| MX5 | Aggressive chemical environment or substrate |
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.
Danish stone formats
The format determines both appearance and material consumption. The Danish standard format is the standard brick (Dansk NormalFormat, DNF) at 228 × 108 × 54 mm. The Flensburg brick is a narrower, lower type (typically about 35–45 mm high compared to the standard brick's 54 mm), which gives more, thinner courses and a characteristic, fine façade appearance. The course height follows from the brick height plus the bed joint:
| Format | Ca. mål L × B × H | Shift height (H + 12 mm joint) |
|---|---|---|
| Normal stone (DNF) | 228 × 108 × 54 mm | ≈ 66 mm |
| Flensburger stone | narrow, H approx. 35–45 mm | ≈ 47–57 mm |
The manufacturing method also makes a difference: soft-struck bricks are formed by pressing soft clay into moulds with water as lubricant — they get a slightly rustic surface with the characteristic 'settling edge'. Machine bricks (extruded/moulded) are more regular and at the lighter end. Massive, hand-struck and soft-struck bricks are the heavy ones.
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