Fastener Spacing and Edge Distance in Timber (EC5 and NDS)

Eurocode 5 names each distance with a letter and a subscript, and the NDS uses plain-language names for the same ideas. Every distance is measured to the centre of the fastener. Edge distance is measured across the grain, end distance along it.

The force direction matters. An end or edge is loaded when the force pushes the fastener toward it, and unloaded when the force pulls away from it. In EN 1995-1-1 the angle α is measured between the force and the grain: 0° is along the grain, 90° is across it.

Wood is weakest in tension across the grain. A smooth nail pushes fibres sideways as it goes in, and the wedge acts across the grain on both sides. Two nails in the same line of fibres, close together, load the same strip of wood twice, which is why spacing along the grain (a1) is always the largest spacing in Table 8.2.

Near an end, the fibres are cut and have nothing to hold them on one side, so the end distance is larger still, and larger again when the force pushes toward the end: EN 1995-1-1 adds 5d cos α to the loaded end distance. Across the grain the force works against tension perpendicular to grain, so the loaded edge distance grows with sin α.

Density plays a part too. Table 8.2 asks for more room in timber between 420 and 500 kg/m³ than below 420, and above 500 kg/m³ Eurocode 5 says the timber should be predrilled rather than nailed directly.

Work through the questions below in order. Each one either changes which column of the table applies or tells you the simple table is the wrong tool.

The fastener spacing calculator follows the same steps, switches predrilling on where Eurocode 5 requires it and checks whether the group fits your member.

A predrilled hole removes most of the wedge, so the wood has less to resist. In Table 8.2 predrilling cuts a1 along the grain from 10d to 5d for a small nail in light softwood, and both edge distances from 5d to 3d. The NDS Commentary does the same: in-row spacing along the grain drops from 15D to 10D.

Eurocode 5 also gives a thickness test. Below t = max(7d, (13d − 30)ρk/400) the timber should be predrilled; for species especially sensitive to splitting, which the standard's note gives as fir, Douglas fir and spruce, the limit is max(14d, (13d − 30)ρk/200), unless edge distances are at least 10d (ρk ≤ 420 kg/m³) or 14d (420 to 500 kg/m³).

Hole size matters too. The NDS caps a bored hole for a nail at 90% of its diameter where G > 0.6 and 75% where G ≤ 0.6 (12.1.6.2). For laterally loaded wood screws in wood with G ≤ 0.6, the lead hole should be about 7/8 of the shank diameter for the shank and 7/8 of the root diameter for the thread (12.1.5.3).

A 45 × 95 mm C24 member is nailed with two lines of three 3.1 mm nails, not predrilled, with the force along the grain pulling toward the end. EN 338 gives C24 a characteristic density of 350 kg/m³, so the first column of Table 8.2 applies, and d is under 5 mm.

a1 = (5 + 5 × |cos 0°|) × 3.1 = 31.0 mm. a2 = 5 × 3.1 = 15.5 mm. The end is loaded, so a3,t = (10 + 5 × cos 0°) × 3.1 = 46.5 mm. Both edges get 5d = 15.5 mm because sin 0° = 0.

Across the member the two lines need 15.5 + 15.5 + 15.5 = 46.5 mm, well inside 95 mm, and the farthest nail is 108.5 mm from the end. Expression (8.18) gives 21.7 mm, so 45 mm needs no predrilling.

If the same nails carry a force across the grain (α = 90°), a1 falls to 15.5 mm but the loaded edge grows to (5 + 2 × 1) × 3.1 = 21.7 mm: the critical wood has moved from the end to the edge.

A #10 wood screw has a shank diameter of 0.190 in (4.83 mm), below the 1/4 in limit of the NDS Commentary tables. Laterally loaded wood screws need lead holes under NDS 12.1.5.3, so the prebored column for wood side members applies.

Edge distance = 2.5D = 0.475 in. End distance with the force toward the end = 10D = 1.900 in; away from the end, 5D = 0.950 in. In a row along the grain the screws sit 10D = 1.900 in apart, and in-line rows are 3D = 0.570 in apart.

On the 3-1/2 in face of a 2x4, two rows of three screws need 1.520 in of width and reach 5.700 in from the end. Coast Douglas-fir has a Wood Handbook specific gravity of 0.48, so the lead hole is about 7/8 × 0.190 = 0.166 in for the shank and 7/8 × 0.152 = 0.133 in for the thread, using the NDS Table L3 root diameter.

Under Eurocode 5 the same screw counts as a nail. Predrilled in C24, it needs a1 = 5d = 24.1 mm and a3,t = 12d = 57.9 mm, against 48.3 mm and 48.3 mm in the NDS recommendation. The EN 1995-1-1 note also lists Douglas fir as sensitive to splitting.

The table below was produced by the same functions the calculator uses. The NDS rows map in-row spacing to the along-grain column and in-line row spacing to the across-grain column for load parallel to grain.

The two codes agree on the physics: everything scales with diameter, ends need more room than edges, and predrilling helps. They differ in status and detail.

For a 3.1 mm nail without predrilling, the NDS recommendation for spacing along the grain is 15D = 46.5 mm, half as much again as the 31 mm Eurocode 5 minimum for light softwood. The NDS edge distance of 2.5D is half the Eurocode 5 value of 5d. Neither is wrong; they were calibrated differently and should not be mixed within one design.

Meeting minimum spacing makes splitting unlikely; it does not make a joint strong enough. Load capacity, the number of fasteners, group effects such as the effective number of nails in a row, and the strength of steel plates all need the design clauses of the code and an engineer's judgement.

Axially loaded screws (EN 1995-1-1 Table 8.6), screws over 6 mm, bolts and dowels, and fasteners in plywood or other panels follow other rules. Self-tapping structural screws follow their product assessment.