Timber Fastener Spacing: Nails, Screws and Bolts
How far a fixing must sit from the end of a board, from its edge, and from the next fixing along — the six distances that decide whether a timber connection holds or splits. With the actual rules from Eurocode 5 and the American NDS, the clause numbers they come from, and an honest account of where the two disagree.
Why spacing is a structural question
A nail or screw in timber does not fail the way a bolt in steel fails. Long before the fastener bends or the wood crushes under it, the timber can simply split along the grain — and a split joint carries nothing at all. Wood is many times weaker across the grain than along it, so a fixing placed too near an end, an edge, or its neighbour drives a crack instead of a load path.
That is what every number on this page is protecting against. The distances are not a rule of thumb about neatness; they are the minimum geometry that lets the connection reach the capacity the design assumed.
The six distances
Both standards describe a connection with the same six measurements, and both express every one of them as a multiple of d — the fastener diameter. Nothing scales with the size of the timber.
| Symbol | What it measures |
|---|---|
| a₁ | Spacing between fasteners within one row, measured along the grain. |
| a₂ | Spacing between rows, measured across the grain. |
| a₃,t | Distance to the loaded end — the end the force pushes the fastener towards. |
| a₃,c | Distance to the unloaded end. |
| a₄,t | Distance to the loaded edge. |
| a₄,c | Distance to the unloaded edge. |
| α | The angle between the force and the grain direction — not the angle of the fastener. |
The single most common mistake is reading α as the angle of the screw. It is the angle of the load. A screw driven straight into a face still has α = 0° if the force on the joint runs along the grain, and α = 90° if it runs across it — and the required end distance changes by a factor of three between those two cases.
Which rule book applies to you
Two families of rules cover most of the English-speaking world, and they take genuinely different approaches — not just different numbers.
| Where | Standard | Approach |
|---|---|---|
| EU, UK, Ireland, Norway | EN 1995-1-1 (Eurocode 5), Section 8 | Formulas. Every fastener type gets explicit minimum distances as a function of d, α and timber density. |
| United States | ANSI/AWC NDS, Chapter 12 | Tables for the big fasteners only. Nails and wood screws get a performance rule, not a number. |
| Australia, New Zealand | AS 1720.1 / NZS 3603 | Formula-based and close in spirit to Eurocode 5, with its own values. Not covered in detail here. |
Canada uses CSA O86, which is closer to the NDS in structure. Everywhere, the national annex or the locally adopted edition is what governs — see the note at the end.
Eurocode 5 — the nail table
EN 1995-1-1 Table 8.2. Three columns, chosen by whether you pre-drill and by the characteristic density ρk of the timber. Most construction softwood — C16, C24 spruce and pine — sits in the first column.
| No pre-drill, ρk ≤ 420 | No pre-drill, 420 < ρk ≤ 500 | Pre-drilled | |
|---|---|---|---|
| a₁ | d < 5 mm: (5 + 5|cos α|)d d ≥ 5 mm: (5 + 7|cos α|)d | (7 + 8|cos α|)d | (4 + |cos α|)d |
| a₂ | 5d | 7d | (3 + |sin α|)d |
| a₃,t | (10 + 5 cos α)d | (15 + 5 cos α)d | (7 + 5 cos α)d |
| a₃,c | 10d | 15d | 7d |
| a₄,t | d < 5 mm: (5 + 2 sin α)d d ≥ 5 mm: (5 + 5 sin α)d | d < 5 mm: (7 + 2 sin α)d d ≥ 5 mm: (7 + 5 sin α)d | d < 5 mm: (3 + 2 sin α)d d ≥ 5 mm: (3 + 4 sin α)d |
| a₄,c | 5d | 7d | 3d |
EN 1995-1-1:2004+A1:2008, Table 8.2. Density bands are characteristic density ρk in kg/m³. Pre-drilling roughly halves the required end distance — which is often the cheapest way to make a tight connection legal.
Two clauses that people miss: a connection needs at least two nails (8.3.1.1(9)), and the load-carrying capacity of a row is not simply the number of nails in it. Table 8.1 reduces a row of n nails to an effective n_ef = n^k_ef, where k_ef is 1.0 only when the spacing reaches 14d, falling to 0.85 at 10d and 0.7 at 7d. Packing fasteners closer buys less than it looks.
Screws: which table applies
Eurocode 5 has no separate spacing table for screws. Clause 8.7.1 routes them to one of the other two, and the threshold is 6 mm:
| Screw diameter | Rules that apply | Clause |
|---|---|---|
| 6 mm or less | The nail rules — Table 8.2 above | 8.7.1(5) |
| Over 6 mm | The bolt rules — Table 8.4 below | 8.7.1(4) |
Diameter means the effective diameter: the smooth shank, provided it penetrates the pointside member by at least 4d (8.7.1(2)); otherwise 1.1 × the thread root diameter (8.7.1(3)).
This is why a 6 mm and an 8 mm screw can need very different edge distances despite looking alike in the box — they cross a line in the standard.
Bolts, dowels and axial screws
| Bolts (Table 8.4) | Dowels (Table 8.5) | |
|---|---|---|
| a₁ | (4 + |cos α|)d | (3 + 2|cos α|)d |
| a₂ | 4d | 3d |
| a₃,t | max(7d, 80 mm) | max(7d, 80 mm) |
| a₃,c | 90°≤α<150°: max[(1 + 6 sin α)d, 4d] · 150°≤α<210°: 4d · 210°≤α≤270°: max[(1 + 6|sin α|)d, 4d] | 90°≤α<150°: max[a₃,t|sin α|, 3d] · 150°≤α<210°: 3d · 210°≤α≤270°: max[a₃,t|sin α|, 3d] |
| a₄,t | max[(2 + 2 sin α)d, 3d] | max[(2 + 2 sin α)d, 3d] |
| a₄,c | 3d | 3d |
Note the 80 mm floor on the loaded end distance for both — the only absolute dimension in these tables, and it governs for every bolt under about 11 mm.
Screws loaded axially — pulled out rather than sheared — have their own short table (8.6), and it applies only where the timber is at least 12d thick: a₁ = 7d, a₂ = 5d, with 10d to the end and 4d to the edge, measured to the centre of gravity of the threaded part rather than to the head.
Pre-drilling, and how deep to go
When Eurocode 5 requires pre-drilling
Any one of these triggers it:
- Characteristic density above 500 kg/m³ — most hardwoods (8.3.1.1(2)).
- Nail diameter over 6 mm (8.3.1.1(2)).
- Member thinner than max{7d, (13d − 30)·ρk/400} (8.3.1.2(6)).
- Species prone to splitting — the standard names fir, Douglas fir and spruce — thinner than max{14d, (13d − 30)·ρk/200} (8.3.1.2(7)).
The hole itself is capped: no more than 0.8d (clause 10.4.2(3)). A clearance hole is not a pre-drilled hole — drill it too big and the nail has nothing to grip.
Minimum penetration
| Fastener | Eurocode 5 | NDS |
|---|---|---|
| Smooth nail | 8d (8.3.1.2(1)) | 6D (12.1.6.5) |
| Profiled / ring-shank nail | 6d (8.3.1.2(2)) | 6D (12.1.6.5) |
| Wood screw | via the nail or bolt rules | 6D (12.1.5.6) |
| Lag screw / coach screw | via the bolt rules | 4D, excluding the tapered tip (12.1.4.6) |
Penetration is measured into the far (pointside) member for a single-shear joint — the part doing the holding, not the total length of the fastener.
Never rely on a nail in end grain. Eurocode 5 clause 8.3.1.2(3) is blunt: nails in end grain shall not be considered capable of transmitting lateral forces. There is a narrow exception at one third of the capacity — non-smooth nails, at least three of them, 10d penetration, not in service class 3 — but if a joint depends on it, redesign the joint.
The NDS takes a different view
If you come to the American standard looking for a nail spacing table, you will not find one — and that is deliberate, not an oversight.
| Fastener | Numeric placement rules? | Clause |
|---|---|---|
| Bolts | Yes — Tables 12.5.1A–D | 12.1.3.4 |
| Lag screws | Yes — Tables 12.5.1A–D | 12.1.4.7 |
| Drift bolts and pins | Yes | 12.1.7.3 |
| Wood screws | No | 12.1.5.7 |
| Nails and spikes | No | 12.1.6.6 |
For nails and wood screws the NDS says only this, in full: “Edge distances, end distances, and fastener spacings shall be sufficient to prevent splitting of the wood.” There is no formula behind it. Clause 12.5.1.1 goes further — for any dowel-type fastener under ¼ inch the geometry factor CΔ is fixed at 1.0, so the machinery that penalises tight spacing does not even engage.
American practice fills the gap prescriptively instead. Framing nailing comes from the IRC fastening schedule, Table R602.3(1) — a list of connections and the nails they take, rather than a calculation. That is what a US inspector checks against.
Where the NDS does give numbers
For bolts and lag screws, as multiples of the diameter D. The two columns are the geometry factor: CΔ = 1.0 gives the full published capacity, CΔ = 0.5 is the reduced-geometry floor.
| Table 12.5.1A — end distance | C∆ = 0.5 | C∆ = 1.0 |
|---|---|---|
| Perpendicular to grain | 2D | 4D |
| Parallel, compression (bearing away from end) | 2D | 4D |
| Parallel, tension — softwoods | 3.5D | 7D |
| Parallel, tension — hardwoods | 2.5D | 5D |
| Spacing and edge | Minimum |
|---|---|
| In a row, parallel to grain (12.5.1B) | 3D · 4D for C∆ = 1.0 |
| In a row, perpendicular to grain | 3D, or the spacing the attached members need |
| Between rows, parallel to grain (12.5.1D) | 1.5D |
| Between rows, perpendicular — ℓ/D ≤ 2 | 2.5D |
| Between rows, perpendicular — 2 < ℓ/D < 6 | (5ℓ + 10D) / 8 |
| Between rows, perpendicular — ℓ/D ≥ 6 | 5D |
| Edge, parallel to grain, ℓ/D ≤ 6 (12.5.1C) | 1.5D |
| Edge, parallel to grain, ℓ/D > 6 | 1.5D, or half the row spacing, whichever is greater |
| Edge, perpendicular — loaded edge | 4D |
| Edge, perpendicular — unloaded edge | 1.5D |
ℓ/D is the lesser of the fastener length in the main member and the total length in the side members, each divided by D. Clause 12.5.1.3 also caps the across-grain distance between the outermost fasteners at 5 inches unless the detail allows for cross-grain shrinkage.
A worked comparison
A 4 mm nail, softwood at ρk = 350 kg/m³, no pre-drilling, load running along the grain (α = 0°):
| Distance | Eurocode 5 | Works out at |
|---|---|---|
| Along the grain, a₁ | (5 + 5×1)×4 | 40 mm |
| Between rows, a₂ | 5 × 4 | 20 mm |
| To the loaded end, a₃,t | (10 + 5×1)×4 | 60 mm |
| To the unloaded end, a₃,c | 10 × 4 | 40 mm |
| To the loaded edge, a₄,t | (5 + 2×0)×4 | 20 mm |
| To the unloaded edge, a₄,c | 5 × 4 | 20 mm |
| Penetration (smooth nail) | 8 × 4 | 32 mm |
Sixty millimetres from the end of the board for a 4 mm nail, and that is the minimum. It is considerably more than most people leave by eye, which is exactly why timber ends split.
Same nail, same joint, pre-drilled: a₃,t drops to (7 + 5)×4 = 48 mm and a₁ to (4 + 1)×4 = 20 mm. When a connection will not fit, pre-drilling is usually a better answer than moving to a bigger fastener — a bigger d scales every distance up with it.
What this page is not
This is minimum geometry, not a connection design. Meeting every distance on this page tells you the timber should not split. It says nothing about whether the connection carries your load: that needs the fastener capacity, the number of shear planes, the member sizes, the load duration and service class, and the combinations your structure is actually designed for.
Anything structural needs a qualified engineer. Beams, posts, roof and floor connections, anything holding up a building or a person. Use this to understand and check the geometry — not to decide a structure is safe.
Your national annex or adopted edition governs, not this page. Eurocode 5 is published with a National Annex in every country, and several of the values here are explicitly flagged in the standard as nationally determined — the splitting-species rule in 8.3.1.2(7) among them. In the US, the edition of the NDS and the IRC your jurisdiction has adopted is the one that counts, and states amend them. Check the version that applies where you are building.
Manufactured fasteners can have their own rules. Structural screws, timber connectors and proprietary fixings are approved under an ETA or an ICC-ES evaluation report which sets its own spacings — usually tighter than the generic ones here, because the product has been tested. Where a manufacturer publishes a value, that value wins.
Figures transcribed from EN 1995-1-1:2004+A1:2008 Section 8 and ANSI/AWC NDS Chapter 12. Provided for information, without warranty. Always verify against the standard itself before relying on it.
Practical notes
Related calculators
Working out the timber itself — studs, joists, plates and the fixings that go with them.
Frequently asked questions
How far apart should screws be in timber?
It depends on the screw diameter, not the timber size. Eurocode 5 treats a screw of 6 mm or less as a nail (EN 1995-1-1 clause 8.7.1(5)), so in softwood below 420 kg/m³ without pre-drilling the minimum spacing along the grain is (5 + 5|cos α|)d, where d is the screw diameter and α is the angle between the load and the grain. For a 5 mm screw loaded along the grain that is 50 mm. Screws over 6 mm follow the bolt rules instead.
How close to the end of a piece of timber can I put a nail?
Further than you think. Under Eurocode 5 the distance to a loaded end is (10 + 5 cos α)d in softwood with no pre-drilling — 15 times the nail diameter when the load pulls straight along the grain. For a 3.1 mm nail that is about 47 mm; for a 5 mm nail about 75 mm. The unloaded end needs 10d.
Does the US code give nail spacing numbers?
No, and this surprises people. The NDS gives numeric edge, end and spacing tables only for bolts, lag screws and drift pins. For nails and wood screws it says only that distances "shall be sufficient to prevent splitting of the wood" (NDS 12.1.6.6 and 12.1.5.7). US framing gets its nailing from the prescriptive IRC schedule, Table R602.3(1), not from a spacing calculation.
When do I have to pre-drill timber?
Eurocode 5 requires pre-drilling when the timber density is above 500 kg/m³, when the nail diameter is over 6 mm, or when the member is thinner than max{7d, (13d − 30)·ρk/400}. Species prone to splitting — the standard names fir, Douglas fir and spruce — use a stricter thickness. The hole must not exceed 0.8d (clause 10.4.2(3)).
How deep does a nail need to go into the far piece?
Eurocode 5 asks for a pointside penetration of at least 8d for smooth nails and 6d for profiled or ring-shank nails (8.3.1.2). The NDS asks for 6D for nails and wood screws, and 4D for lag screws, measured into the main member.
Can I nail into the end grain of timber?
Not for anything structural. Eurocode 5 clause 8.3.1.2(3) says nails in end grain shall not be considered capable of transmitting lateral forces at all. There is a limited exception at one third of the capacity for non-smooth nails with at least three per connection and 10d penetration, but end grain is never the right place for a loaded fixing.
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