Sizing provides the cross-section — but how the timber is joined determines whether the structure holds. Here are the connection types, CE requirements for nails, screws and fittings (DS/EN 14592 and 14545), and the errors that make a joint the weak link.
Introduction
A timber structure is no stronger than its connections. You can have the right cross-section in the right strength class, but if the rafter is not properly connected to the plate, or the beam is not hung correctly, the structure fails at the connection — not in the timber. Therefore, connections are a separate chapter in Eurocode 5 (DS/EN 1995-1-1, section 8), and fittings and fasteners have their own product standards and CE requirements.
Two families of connections
Connections in timber are broadly divided into two. Dowel-type fasteners transfer forces by means of a round element passing through the timber — nails, screws, bolts, dowels and clamps. Connectors (plate-type) transfer forces over a larger area — e.g. nail plates, dowel rings and tooth plates. In addition there are structural steel fittings: bent steel plates (joist hangers, angle brackets, perforated straps, storm straps) that join or anchor timber members.
| Type | Examples | Product standard |
|---|---|---|
| Dowel-shaped fasteners | Nails, screws, bolts, plugs, clamps | DS/EN 14592 |
| Connectors (connectors) | Nail plates, dowel rings, tooth plates | DS/EN 14545 |
| Building hardware | Beam shoes, angle brackets, perforated band, storm brackets | Fastener ETA / DoP |
Source: DS/EN 14592 (dowel-shaped fasteners — requirements and testing) and DS/EN 14545 (fasteners — requirements). Use in the construction is dimensioned according to DS/EN 1995-1-1 section 8 with DK NA. Building fittings are covered by the manufacturer's ETA (European Technical Assessment) and Declaration of Performance (DoP).
Nails and screws — everyday trade
Nails and screws are the most used connections. Their load-bearing capacity depends on diameter, penetration depth, wood density and whether they are loaded transversely (shear) or out of the wood (withdrawal). Two geometric requirements are critical: there must be sufficient distance from fastener to edge and end, and sufficient spacing between them — otherwise the wood splits instead of supporting the load.
- 01Edge and end distances as well as mutual distances are tabulated in EC5 section 8 and depend on the fastener's diameter and load direction — they are not free.
- 02Pre-drilling: large nails and screws in dense or dry timber are often pre-drilled to prevent splitting, especially near ends and edges.
- 03Screws absorb pull-out far better than smooth nails — therefore screws are used where the connection is pulled apart.
- 04Preferably load the connection across (shear), not in drawing out a smooth nail — a nail drawn straight out has low bearing capacity.
Minimum distances for nails (EC5 table 8.2)
EC5 angiver afstandene som multipla af sømmets diameter d, og de afhænger af vinklen α mellem kraften og fiberretningen samt af, om der forbores. Tabellen viser det enkleste tilfælde — last parallelt med fibrene (α = 0°) og uden forboring. a1 = afstand langs fibrene, a2 = afstand vinkelret på fibrene, a3,t = til belastet ende, a3,c = til ubelastet ende, a4,c = til ubelastet kant. Forboring tillader mindre afstande (fx a1 ned mod ca. 7d) — slå den konkrete værdi op i tabel 8.2.
| Distance | Multiplum (α = 0°, ikke-forboret) | Med d = 2,8 mm |
|---|---|---|
| a1 — along fibres | 10 · d | 28 mm |
| a2 — perpendicular to the fibres | 5 · d | 14 mm |
| a3,t — loaded end | 15 · d | 42 mm |
| a3,c — unloaded end | 10 · d | 28 mm |
| a4,c — unloaded edge | 5 · d | 14 mm |
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