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 gives the spacings as multiples of the nail's diameter d, and they depend on the angle α between the force and the grain direction and on whether pre-drilling is used. The table shows the simplest case — load parallel to the grain (α = 0°) and without pre-drilling. a1 = spacing along the grain, a2 = spacing perpendicular to the grain, a3,t = to loaded end, a3,c = to unloaded end, a4,c = to unloaded edge. Pre-drilling allows smaller spacings (e.g. a1 down to approx. 7d) — look up the specific value in table 8.2.
| Distance | Multiple (α = 0°, not pre-drilled) | With 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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