Guide
Bolt spacing, end, and edge distances in timber
Every bolt hole in our plates is placed by four distances: how far the bolt sits from the end of its timber, from the timber's edges, from the next bolt, and from the edge of the steel. The wood's distances come from the American Wood Council's National Design Specification for Wood Construction (NDS 12.5.1), and the steel's from the American Institute of Steel Construction's Specification for Structural Steel Buildings (AISC 360). This guide shows each one as our custom plate designers check it.
01 D
Counting in bolt diameters
The NDS writes each distance as a multiple of the bolt's diameter, D. A 5/8" bolt has a D of 5/8", so 3.5D is 2 3/16". The drawings on this page are to scale for a 5/8" bolt, the size our catalog plates take, in a surfaced 6x6 (5 1/2" square), with the grain running left to right.
Most distances have two values. The minimum is the least the NDS allows. The full-strength distance is where a bolt carries its full design value. In between, the NDS counts every bolt in the connection at a fraction of its value: the actual distance over the full-strength one, taken at the bolt that comes out worst. A bolt at the minimum end distance, 3.5D of 7D, counts at half.
| Bolt | Hole | Edge, and between rows (1.5D) | Spacing in a row (3D / 4D) | End (3.5D / 7D) | Steel edge (AISC) |
|---|---|---|---|---|---|
| 1/4" | 5/16" | 3/8" | 3/4" / 1" | 7/8" / 1 3/4" | 5/16" |
| 3/8" | 7/16" | 9/16" | 1 1/8" / 1 1/2" | 1 5/16" / 2 5/8" | 15/32" |
| 1/2" | 9/16" | 3/4" | 1 1/2" / 2" | 1 3/4" / 3 1/2" | 3/4" |
| 5/8" | 11/16" | 15/16" | 1 7/8" / 2 1/2" | 2 3/16" / 4 3/8" | 7/8" |
| 3/4" | 13/16" | 1 1/8" | 2 1/4" / 3" | 2 5/8" / 5 1/4" | 1" |
| 7/8" | 15/16" | 1 5/16" | 2 5/8" / 3 1/2" | 3 1/16" / 6 1/8" | 1 1/8" |
| 1" | 1 1/16" | 1 1/2" | 3" / 4" | 3 1/2" / 7" | 1 1/4" |
Pairs are the minimum and the full-strength distance. Holes are cut 1/16" larger than the bolt. The custom plate designers take bolts from 1/4" to 3/4".
02 End
End distance
End distance runs along the grain, from the bolt's center to the end of its timber. When a joint pulls a timber away from its end, each bolt bears toward the end, and the wood between can split or shear out. For that, the NDS asks for at least 3 1/2 bolt diameters, and 7 for full strength. When a timber only ever pushes into its joint, it allows less: 2 bolt diameters, or 4.
Our custom plate designers hold the larger distances at every end, whichever way the joint usually works. We don't know where a plate will go or what weather it will see. Wind can lift a porch, awning, or carport roof harder than its weight holds it down, and then a post, strut, or top chord that pushes on its joint every other day pulls on it.
03 End
Sloped ends
A strut, a top chord, or a collar tie ends in a cut that crosses the grain at an angle, so the end is nearer one side of a bolt than the other. The NDS measures a sloped end along the grain from the center half of the bolt: the middle half of its diameter, across the grain through its center. Every point of that half must be the end distance from the cut, so the point nearest the cut is the one that counts (NDS 12.5.1.2). With two rows of bolts, each row is measured on its own.
The drawing is a strut cut at 45°, its bolt 3.5D from the cut by that measure. Measured from the bolt's center instead, it would read more than it has.
04 Spacing
Spacing along a row
Bolts in a row along the grain sit at least 3D apart, center to center, and 4D apart for full strength. A designer starts each bolt size at 3", or 4D where that is more, and lets you set the spacing down to the minimum, rounded up to the next 1/8".
05 Edge
Edge distance and two rows
Edge distance runs across the grain, from the bolt's center to the timber's long edge. It is at least 1.5D. A single row runs down the timber's centerline, which in most timbers leaves well more than that.
With two rows, three distances work together: the rows are at least 1.5D apart, each row is at least 1.5D from its edge, and each row is also at least half the gap between the rows from its edge. That last rule applies when the bolt passes through more than 6 bolt diameters of wood (3 3/4" for a 5/8" bolt), which any 6x or larger timber does. Our designers don't ask how thick your timbers are, so they always apply it.
In practice, two rows can spread across at most half the timber's depth. In a 5 1/2" timber the rows can be up to 2 3/4" apart, each 1 3/8" from its edge: the quarter points, as drawn.
Under one steel plate, the outermost rows are also no more than 5" apart across the grain (NDS 12.5.1.3). Wood shrinks across the grain as it dries and steel doesn't, so bolts held farther apart than that by one plate can split the timber between them.
06 Steel
Steel around each hole
The steel needs its own edge distance, whatever the wood does: from each hole's center to the nearest edge of the plate, at least the minimum in AISC 360 Table J3.4. That is 3/4" for a 1/2" bolt, 7/8" for 5/8", 1" for 3/4", 1 1/8" for 7/8", and 1 1/4" for 1". The table starts at 1/2", so for 1/4" and 3/8" bolts we hold 1 1/4 bolt diameters.
Every hole also keeps at least 1/8" of steel to the next hole or edge, the least our shop cuts cleanly. When a leg is too narrow for its holes, the designer says so: make the leg wider, with less reveal, or use one row of bolts.
07 Designers
How the designers check them
Each custom plate designer but the plate washer's opens by asking how far bolts should sit from the ends and edges of your timbers:
- Minimum distances (recommended): every distance on this page at its minimum. This is how we lay out our own plates.
- Full-strength distances: end distance and spacing at their full-strength values: 7D from every end, and 4D along a row. Edge distance and the distance between rows are the same either way. Plates come out larger, or with fewer bolts, and will cost more.
- No minimum distances (not recommended): bolts go where you set them, and only the steel's distances are checked. Wood that close to a bolt can split. Have an engineer check the layout.
Then it measures every bolt on the timbers you describe, says which distance falls short, and won't add the plate to your cart until each one is met. Every timber that ends at the joint keeps 3.5D from its end (7D at full strength), measured from its end or its sloped cut, whether the joint pulls on it or pushes.
08 Catalog
Our catalog plates and connectors
Our catalog plates are laid out to the minimums, with 11/16" holes for 5/8" bolts, on the timbers each collection is drawn for. We check every plate at every pitch, and we don't assume a plate sits centered on its timbers: on some joints a plate meets every minimum only when set off center. Our connector configurators apply the NDS minimum end distance as the connector guide describes.
09 Engineering
What these distances don't cover
These are distances, not load ratings. A plate that meets every one of them is laid out the way the NDS lays out a bolted connection, but nothing here says what the connection can carry: that depends on your loads, your timbers' species and grade, the bolts, and the rest of the frame. The distances above are for softwood, with the load along the grain. Hardwood, loads across the grain, glued laminated timber, and lag screws have rules of their own. If your project needs rated capacities, or a permit calls for an engineer's stamp, have your engineer review the plate and its bolts.