Why wood actually behaves predictably in fire — charring rate 0.65 mm/min for solid coniferous timber, R/REI requirements and what DK NA:2026 changes for CLT constructions.
Introduction
Wood is a combustible material, but in large sections it behaves predictably in fire: the outer layer chars and insulates the core. This makes it possible to design load-bearing timber structures to meet fire requirements using a calculated method, instead of covering everything with gypsum.
Charring rate
Eksempel: Et 45 × 145 mm spær eksponeret på tre sider i R30. d_char = 0,65 · 30 ≈ 19,5 mm på hver eksponeret side. Resttværsnit: (45 − 2 · 19,5) mm × (145 − 19,5) mm = 6 mm × 125,5 mm — et meget lille resttværsnit, der typisk ikke holder til bærende formål. Konklusionen: spærtræ skal beklædes, hvis det skal opnå R30.
Fire classes in BR18
BR18 Ch. 5 divides buildings into fire classes BK1–4. BK1 and BK2 are the most common for single-family homes and smaller apartment buildings. For cladding in BK1–2, minimum classification K₁ 10 B-s1,d0 (classified according to EN 13501-1) is required — typically achieved with at least two layers of plaster.
| Requirement | Typical requirement | Standard/reference |
|---|---|---|
| Charring rate, solid softwood | β₀ = 0,65 mm/min | EN 1995-1-2, Tabel 3.1 (§ 3.4.2) |
| Load-bearing structure in/over basement | Min. R 60 | BR18 / EC5 |
| Floor separation | REI 60 (typical) | BR18 kap. 5 |
| Clothing BK1–2 | Min. K₁ 10 B-s1,d0 | EN 13501-1 / TRÆ 78 |
| Wood glue's reaction to fire | D-s2,d0 (untreated spruce) | EN 13501-1 |
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