Structural Lags & Bolts

Q: Can I use lag screws in end grain?

You can, at a 25% capacity penalty (Ceg = 0.75), but it is a poor detail and it is worth designing around. Ordinary wood screws in end grain are assigned zero withdrawal capacity by the standard, which is as clear a warning as it gets.

A screw holds because its threads grip wood fibres. A bolt holds because a steel shank clamps two pieces together. Nearly every heavy-duty joint that racks loose in a year got those two mechanisms confused, and nearly every one that stays tight for decades matched the fastener to how the load actually arrives. Every number below is traceable to the NDS or DCA 6, cited inline so you can check it.

Read this first. The values on this page are reference design values taken from published standards, reproduced here for education and relative comparison. They already include the safety margins the standard applies, but they still have to be adjusted for your conditions (moisture, temperature, load duration) before they mean anything about a real joint. For decks, ledgers, guards, stair stringers, and anything else where a failure hurts somebody, follow your local building code and the fastener manufacturer's published technical data. When the two disagree, the code wins.

When to use a bolt instead of a screw

A screw is a one-way device. Its threads cut into wood, and every pound it resists is carried by the thin band of wood fibres wrapped around those threads. A through-bolt is a clamp. It passes through clearance holes in both pieces, and tightening the nut squeezes the joint together. The load rides on a solid steel shank in shear plus the friction between the clamped faces, so wood fibres never have to hold threads at all.

That difference buys you three things. Strength: a steel shank in shear is stronger than any wood-thread engagement of the same diameter. Serviceability: a bolted joint can be re-tightened when the wood shrinks in its first winter, and taken apart without destroying anything. Predictability: bolt strength comes from the grade stamped on the head, not from how cleanly the threads happened to bite.

The catch is access. A through-bolt needs a hole through the whole assembly and a wrench on both sides, or a square-neck carriage bolt that holds itself. When you can only reach one face, which is the usual case when you are fastening to a wall stud, a beam, or the inside of a bench leg, the lag screw is the compromise: a bolt-style head and shank, with screw threads doing the holding.

This guide starts where the load is heavy enough that a screw is the wrong answer. For the lighter end of the same decision, see nails vs. screws. For the raw numbers behind every fastener we cover, see the technical specifications tables.

How a lag screw actually carries load

Almost every mistake people make with lag screws comes from not knowing which of two completely different jobs the screw is doing. The standards treat them as separate problems with separate numbers, and so should you.

Withdrawal is the load pulling the screw straight out along its own axis, resisted only by the threads gripping side grain. This is a shelf bracket hanging off a wall, or a swing hook. Lateral load, which engineers call shear, is the load trying to slide the two pieces past each other, resisted by the shank bearing sideways against the wood. This is a ledger board carrying joists, or a bench apron resisting racking. A bolt adds a third mechanism, clamp friction, that neither screw type has.

Figure 1

Three ways a fastener resists load

1 Withdrawal

Threads grip side grain

Only the wood around the threads resists. Weakest of the three.

2 Lateral (shear)

Shank bears sideways

Wood crushes against the shank at the shear plane (marked in red).

3 Clamp (bolt only)

Friction carries load first

Washers spread the clamp.

Why it matters

A lag screw in withdrawal is the weakest of the three, and it is also the one people reach for most often because it needs access to only one side. If the load pulls straight out and it matters, use a through-bolt.

Carriage bolt vs lag bolt vs through-bolt

These three cover almost every heavy-duty fastening job in a home shop. The honest way to choose between them is to start with how many sides you can reach, not with how strong each one is.

Structural lag and bolt comparison
Fastener	Access needed	What carries the load	Washers	Typical uses	Grades / standards
Lag screw (lag bolt)	One side only	Threads in withdrawal; shank in shear	Flat washer under the head. The hex head will crush into the wood without one.	Deck ledgers, bench legs to aprons, mounting heavy items to studs	ANSI/ASME B18.2.1; commonly low-carbon steel, ungraded
Carriage bolt	Both sides for the hole; wrench on the nut side only	Steel shank in shear plus clamp friction	Flat washer under the nut only. The square neck locks the domed head into the wood.	Workbench frames, swing sets, gates, knock-down furniture	ASTM A307 / SAE Grade 2 common; Grade 5 available
Hex through-bolt	Both sides, wrench on both	Steel shank in shear plus clamp friction	Flat washers under both the head and the nut	Bed rails, bench trestles, timber connections, anything taken apart repeatedly	SAE J429 Grade 2, 5, 8; ASTM A307; stainless ASTM F593

A lag screw is often sold as a "lag bolt." It is not a bolt: it has no nut and it holds with threads, so it is governed by the withdrawal and lateral rules below, not by bolt rules.

Lag screw pilot hole size chart

A lag screw needs two holes, not one, and getting this wrong is the most common way people split a board or snap a screw. The standard is explicit about both (NDS 2018, 12.1.4.2):

The clearance hole takes the smooth, unthreaded shank. It is drilled at the full shank diameter, only as deep as the unthreaded portion is long. Its job is to let the shank pass freely so the threads can pull the two pieces together instead of jacking them apart.
The lead hole takes the threads. It is drilled smaller, sized as a percentage of the shank diameter that depends on how dense the wood is, and at least as deep as the threaded portion is long.

Figure 2

Anatomy of a correctly drilled lag hole

Symbols

Dimension symbols follow NDS Appendix Table L2 (S = unthreaded body length, T = thread length, E = tapered tip length). Withdrawal capacity is calculated on T − E, never on the full screw length.

The lead hole percentage is set by the wood's specific gravity, and the standard gives three bands. Most published pilot-hole charts collapse this into "softwood" and "hardwood," which silently mis-serves Southern Pine and the denser Douglas Fir grades. The table below keeps all three bands separate.

Lag screw pilot hole sizes by shank diameter and wood density
Shank dia. D	Clearance hole	Lead hole, by specific gravity band (NDS 12.1.4.2)
Shank dia. D	Clearance hole	G ≤ 0.50 SPF, hem-fir, cedar · 40–70%	0.50 < G ≤ 0.60 Southern pine, red maple · 60–75%	G > 0.60 Oak, hickory, beech · 65–85%
1/4"	1/4"	5/32"	5/32"	3/16"
5/16"	5/16"	3/16"	7/32"	1/4"
3/8"	3/8"	7/32"	1/4"	9/32"
7/16"	7/16"	9/32"	5/16"	11/32"
1/2"	1/2"	5/16"	11/32"	3/8"
5/8"	5/8"	3/8"	7/16"	1/2"
3/4"	3/4"	1/2"	9/16"	5/8"

Sizes are the nearest common drill bit inside each NDS percentage band. The standard notes that the larger percentile in each range applies to larger-diameter screws. Exception: lag screws 3/8" and smaller loaded primarily in withdrawal in wood with G ≤ 0.50 do not require a lead hole at all (NDS 12.1.4.3), provided edge and end distances are sufficient to prevent splitting.

Installation rules that are actually in the standard

Three of these get ignored constantly, and all three are written down:

Turn it, do not drive it. The threaded portion must be inserted by turning with a wrench, not by driving with a hammer (NDS 12.1.4.4). Hammering a lag screw destroys the thread engagement you are relying on.
Lubricant is allowed and free. Soap or another wood-compatible lubricant on the threads or in the lead hole costs you nothing in capacity: the standard anticipates no reduction in design values for using it (NDS 12.1.4.5). On a 1/2" lag in dense stock it is the difference between a clean pull-up and a snapped head.
Washers are not optional. A hex head bearing directly on wood crushes the fibres under it and loses clamp as it embeds. For bolts, the standard requires a standard cut washer or equivalent metal plate between the wood and the head, and between the wood and the nut (NDS 12.1.3.3).

How deep does a lag screw need to go?

There is a hard floor, and it is easy to remember. The minimum penetration into the main member, not counting the tapered tip, is four times the shank diameter (NDS 12.1.4.6).

p_min = 4D Minimum penetration into the main member, excluding the tapered tip E

Figure 3

The 4D minimum penetration rule

Minimum penetration into the main member (4D)

Bars drawn to true scale — 4 × shank diameter

This is a floor, not a target. It is the depth below which the published lateral design values stop being valid at all.

Withdrawal capacity keeps rising in direct proportion to thread depth, so deeper is genuinely stronger — right up until the tip breaks through the far face.

In practice

A 1/2" lag screw that only bites 1-1/2" into the main member is below the 4D floor, no matter how long the screw is or how thick the piece it passes through.

How much weight can a lag screw hold?

This is the most-asked question about lag screws and the one most often answered with a made-up number. The standard gives a real one. Withdrawal capacity is calculated per inch of thread penetration, and it depends on exactly two things: how dense the wood is, and how fat the screw is.

W = 1800 · G^3/2 · D^3/4 NDS Eq. 12.2-1 — reference withdrawal design value, lb per inch of thread penetration into side grain

Two things about that formula are worth internalising. Capacity scales with G^3/2, so wood density matters more than proportionally: white oak holds about 2.3 times what spruce-pine-fir holds, at every diameter. And it scales with only D^3/4, so doubling the screw diameter buys you about 68% more grip, not 100%. Depth is the cheaper lever than diameter.

Figure 4

Withdrawal capacity by species and diameter

lb / in of thread penetration

Spruce-Pine-Fir G 0.42 Douglas Fir-Larch G 0.50 Southern Pine G 0.55 White Oak G 0.73

Source

Reference withdrawal design values from NDS 2018 Table 12.2A, in pounds per inch of thread penetration. Multiply by your actual thread penetration to get a load in pounds. Species gravities from NDS Table 12.3.3A.

Worked example — 1/2" lag into a Southern Pine beam

1 Look up the species. Southern Pine has an assigned specific gravity of G = 0.55 (NDS Table 12.3.3A).

2 Read the unit value. At G = 0.55 and D = 1/2", Table 12.2A gives W = 437 lb per inch of thread penetration. The formula agrees: 1800 × 0.55^1.5 × 0.5^0.75 = 437.

3 Measure real thread penetration. A 6" lag has 3-1/2" of thread (T), of which 5/16" is tapered tip (E). Usable penetration is 3-3/16", and only the part inside the main member counts.

4 Multiply. 437 × 3.1875 = 1,393 lb.

Roughly 1,390 lb allowable withdrawal for that one screw, before adjustment factors. Wet service, elevated temperature, and load duration all modify it (NDS Table 11.3.1), and every one of those adjustments in an outdoor application pushes the number down.

These are allowable values, not breaking strength. A reference design value already has the standard's safety margin built into it, so it sits well below the load at which the connection would actually let go in a test. Do not "add a safety factor" on top and then wonder why your bracket needs nine screws, and do not read the tabulated number as the failure load.

Never hang a lag screw off end grain

Threads cut into end grain are biting into the ends of hollow fibres, like pushing a screw into a bundle of drinking straws. There is very little for the threads to grip. The standards handle this bluntly with a penalty factor:

End grain penalty, C_eg
Fastener and loading	Factor	What it means
Lag screw, withdrawal from end grain	0.75	Allowed, but you lose a quarter of the capacity (NDS 12.5.2.1)
Wood screw, withdrawal from end grain	0.00	Not permitted. The standard assigns it zero capacity (NDS 12.2.2.3)
Dowel-type fastener in end grain, lateral	0.67	A third of the lateral capacity is gone (NDS 12.5.2.2)
Toe-nailed connection, withdrawal	0.67	NDS 12.5.4.1; the wet service factor does not apply

A wood screw into end grain is assigned C_eg = 0. That is the standard's way of saying the connection does not exist. If your design has a screw pulling out of end grain, redesign it — add a cleat, a bracket, or a through-bolt.

Bolt spacing, edge distance, and end distance in wood

Fastener capacity assumes the wood around the fastener stays intact. Put a bolt too close to an end and the load shears out a plug of wood before the bolt ever reaches its rated value. These minimums are not rules of thumb; they are tabulated, they are all expressed as multiples of the bolt diameter D, and they differ depending on which way the load runs.

Figure 5

Bolt layout geometry

Plan view — D = bolt diameter

minimum maximum limit

Applies to

Values shown are the C_Δ = 1.0 case for softwood loaded parallel to grain in tension — the geometry that earns the full tabulated capacity.

Minimum geometry for bolts and lag screws (NDS Tables 12.5.1A–D)
Dimension	Loading	Reduced C_Δ = 0.5	Full C_Δ = 1.0	At 1/2" dia.
End distance	Parallel, tension (softwood)	3.5D	7D	3-1/2"
End distance	Parallel, tension (hardwood)	2.5D	5D	2-1/2"
End distance	Parallel, compression	2D	4D	2"
End distance	Perpendicular to grain	2D	4D	2"
Spacing in a row	Parallel to grain	3D	4D	2"
Spacing in a row	Perpendicular to grain	3D	per attached member	1-1/2"
Edge distance	Parallel, ℓ/D ≤ 6	1.5D		3/4"
Edge distance	Perpendicular, loaded edge	4D		2"
Edge distance	Perpendicular, unloaded edge	1.5D		3/4"
Between rows	Parallel to grain	1.5D		3/4"
Between rows	Perpendicular, ℓ/D ≥ 6	5D		2-1/2"

C_Δ is the geometry factor. Meet the C_Δ = 1.0 column and the connection earns its full tabulated capacity. Fall between the two columns and you multiply capacity by actual ÷ minimum-for-1.0. Fall below the C_Δ = 0.5 column and the tabulated values do not apply at all. ℓ/D is fastener length in the member divided by diameter.

The 5-inch rule almost nobody knows. The perpendicular-to-grain distance between the outermost fasteners in a connection must not exceed 5 inches unless you detail specifically for it (NDS 12.5.1.3). This is not about splitting under load. It is about cross-grain shrinkage: a wide board loses more width than a steel plate loses length, and a rigid row of bolts spanning too much width will split the board as it dries, with no load on it at all. If you are bolting a steel bracket across the full width of a 2x10, this rule is the one you are breaking.

Lag screws loaded only in withdrawal

If a lag screw carries no lateral load at all, the geometry relaxes to a simpler set: edge distance 1.5D, end distance 4D, and spacing 4D (NDS Table 12.5.1E).

Deck ledger fastener spacing (DCA 6)

The deck ledger is where lag screws stop being a shop question and become a life-safety one. Ledger failure is one of the most common causes of catastrophic deck collapse, and it is almost always a fastener problem: nails instead of lags, lags into siding instead of the band joist, or lags spaced by eye.

The prescriptive guide gives an exact schedule. Spacing tightens as the joist span grows, because a longer joist delivers more load per foot of ledger. If you are sizing the deck itself, our deck calculator works the joist and beam spans to the same DCA 6 rules, and the deck framing standards page carries the surrounding provisions.

On-centre fastener spacing, deck ledger to 1-1/2" solid-sawn band joist (DCA 6 Table 5)
Fastener	6' and less	6'1"–8'	8'1"–10'	10'1"–12'	12'1"–14'	14'1"–16'	16'1"–18'
1/2" lag screw	30"	23"	18"	15"	13"	11"	10"
1/2" through-bolt	36"	36"	34"	29"	24"	21"	19"

Joist span headings are the span the deck joists carry. Assumes 40 psf live + 10 psf dead load, ledger of Southern Pine, Douglas Fir-Larch or Hem-Fir, and maximum 15/32" sheathing. Spacings differ for LVL rim joists — see DCA 6 Table 5 in full. Note the gap: at a 12-foot joist span the through-bolt is permitted at nearly twice the spacing of the lag screw. That is the clamp mechanism earning its keep.

Figure 6

Ledger fastener placement

Elevation — stagger, do not align

Section — the tip must break through

Drilling

Drill 1/2" through the ledger and sheathing, then 5/16" into the band joist — drilling the band joist at the full 1/2" removes the thread grip that is carrying your deck. Placement and stagger per DCA 6 Figure 19; staggering keeps two fasteners off the same grain line, so one drying split cannot unzip the whole connection.

Three ledger rules that are non-negotiable. Lead anchors are prohibited outright. Every lag screw and bolt requires a washer. And the tip of the lag screw must fully extend beyond the inside face of the band joist — if you cannot see it poking through from the basement side, the screw is too short. Drill a 1/2" hole through the ledger but only a 5/16" hole into the band joist; drilling the band joist at full 1/2" removes the thread grip that is carrying your deck.

Galvanized vs stainless for pressure-treated lumber

Modern copper-based preservatives (ACQ, copper azole, ACZA) replaced the old CCA formulations, and they are considerably more aggressive toward steel. An electroplated zinc fastener that was fine in 1995 lumber can be visibly rusting inside a season in modern treated stock. The corrosion requirements are specific, and "galvanized" on the box is not enough information.

Figure 7

What may touch what in treated lumber

Correct

One material throughout

Same coating on the screw and the connector. Nothing in the joint is more noble than anything else, so nothing gets sacrificed.

Wrong

Mixed metals in one joint

The stainless screw is the noble metal. The zinc on the connector gives up its coating to protect it, and the connector fails before the screw does.

Prohibited

Aluminium on copper-treated

Copper-based preservatives attack aluminium directly. Flashing has to be stainless, copper, or a product coated and rated for treated wood.

The rule

DCA 6 states it directly: fasteners and connectors shall be of the same corrosion-resistant material, and aluminium shall not be used in direct contact with ACQ, copper azole, or ACZA-treated lumber. Moisture carries copper ions to the fastener, which makes the wood itself part of the circuit — and the less noble metal gives up its coating.

Corrosion requirements for fasteners in preservative-treated wood (IRC R317.3, per DCA 6)
Situation	Requirement	Standard
Screws, bolts, washers, nuts, nails in treated wood	Hot-dipped zinc-coated galvanized steel, stainless steel, silicon bronze, or copper	—
Hot-dipped galvanized, 3/8" diameter and smaller	Class D coating	ASTM A153
Hot-dipped galvanized, over 3/8" diameter	Class C coating	ASTM A153
Mechanically deposited zinc (not nails or timber rivets)	Class 55 minimum coating weight	ASTM B695
Connectors galvanized before fabrication	G-185 coating	ASTM A653
Connectors galvanized after fabrication	Hot-dip galvanizing	ASTM A123
Salt water, or within 300 ft of a salt shoreline	Stainless steel only	Grade 304 or 316

Two rules travel with this table. Fasteners and connectors shall be of the same corrosion-resistant material — mixing a stainless screw with a galvanized hanger sets up a galvanic cell that eats the zinc. And aluminium must not contact copper-treated lumber at all (DCA 6 note 11), which rules out aluminium flashing directly against an ACQ ledger.

The 300-foot rule catches people out. It is measured from the shoreline, not from the water's edge at low tide, and it applies to coastal air exposure, not just splash. If you are inside it, 304 or 316 stainless is the requirement and hot-dipped galvanized will not satisfy an inspector.

Choosing at a glance

Access decides this more often than strength does. Work down the questions in order and you will land on the right fastener without doing any arithmetic.

Figure 8

Which fastener, in four questions

Why it matters

The reconsider branch is not pedantry: a lag screw in pure withdrawal is the one arrangement on this chart that can fail suddenly, with no visible warning first. And when two answers both work, pick the one that can be re-tightened — wood moves for years after it is built.

And once you have chosen

The flowchart gets you to a fastener. These are the details that decide whether it holds.

Can you reach both sides?

Use a through-bolt. It is the strongest of the three, it can be re-tightened as the wood moves, and it is the only one that adds clamp friction. Washers under head and nut.

Only one side, serious load?

Use a lag screw. Drill both holes, respect the 4D penetration floor, turn it with a wrench, and put a washer under the head.

Coming apart someday?

Use a carriage bolt or a through-bolt. Lag screws lose grip each time they are backed out and re-driven into the same hole.

Load pulls straight out?

Prefer a through-bolt. Withdrawal is the weakest mode a lag screw has, and it is the mode most likely to fail without warning.

Common questions

Is a lag bolt the same as a lag screw?

Same fastener, and "lag bolt" is the more common name in stores. It is technically a screw: it has no nut, and it holds with threads biting into wood rather than by clamping. That distinction matters because the design rules that apply to it are the screw rules (withdrawal, lead holes, 4D penetration), not the bolt rules.

What size pilot hole for a 1/2" lag screw?

Two holes. A 1/2" clearance hole through the piece being attached, as deep as the unthreaded shank, and then a lead hole into the main member: 5/16" in spruce-pine-fir, hem-fir or cedar, 11/32" in southern pine, and 3/8" in oak or other dense hardwood. For a deck ledger specifically, DCA 6 names the sizes directly: 1/2" through the ledger, 5/16" into the band joist.

Do I really need a washer under a lag screw?

Yes. Without one the hex head crushes into the wood as you tighten, so the clamp you thought you applied disappears into a dimple. On deck ledgers a washer is mandatory under DCA 6, not merely advisable.

Can I use lag screws in end grain?

You can, at a 25% capacity penalty (C_eg = 0.75), but it is a poor detail and it is worth designing around. Ordinary wood screws in end grain are assigned zero withdrawal capacity by the standard, which is as clear a warning as it gets.

Why did my lag screw split the board?

Almost always one of three causes: no lead hole, a lead hole drilled too small for the density of the wood, or too little end or edge distance. A bolt or lag needs 1.5D from the edge and up to 7D from the end of a board loaded in tension. On a 1/2" fastener that is 3/4" and 3-1/2" respectively.

Can I reuse bolts and lag screws?

Bolts, yes. The steel is undamaged by being unbolted, so inspect the threads, replace anything galled or corroded, and reuse it. Lag screws in the same hole, no. Every time threads are backed out and re-driven they tear the fibres they were gripping, and the second bite is into a hole that is now oversized. If a joint has been apart and needs to go back together and stay, move to a fresh location, step up a diameter, or convert it to a through-bolt.

Should I use a lag screw or a structural screw?

A structural screw, if the manufacturer publishes an evaluation report for it. Proprietary structural screws are not covered by the NDS tables on this page; they carry their own tested values in an ICC-ES evaluation report, and that report governs their use. In exchange you usually get no lead hole, a hex or star drive that installs with an impact driver, and far less labour per fastener. Lag screws remain the right answer when you need a generic, standards-covered fastener, when no evaluation report is available for the condition, or when an inspector wants to see the prescriptive DCA 6 detail.

What grade of bolt should I use for wood?

For most wood connections, ordinary ASTM A307 or SAE Grade 2 is enough, and upgrading rarely buys what people expect. The NDS yield model shows why: in most geometries the connection yields by the wood crushing against the bolt, not by the bolt bending. The commentary notes that bending yield strength for bolts is roughly the average of tensile yield and tensile ultimate, which spans about 48,000 to 140,000 psi across SAE J429 grades, yet 45,000 psi is treated as reasonable for many commonly available bolts. Grade matters most when the bolt is loaded in tension along its axis rather than in shear.

Why did my lag screw snap while driving it?

Usually a lead hole that was too small for the density of the wood, and often an impact driver finishing what a wrench should have. In dense stock a lead hole at the bottom of the range asks the screw to cut its own thread through material that will not yield, and torque climbs until the shank fails at the root diameter. Drill to the correct band, use soap or a wood-compatible lubricant, and turn the screw with a wrench rather than hammering or hammering-by-impact.

How tight is tight enough?

Snug, then stop. DCA 6 puts it as "thoroughly tightened but not over-tightened to avoid wood damage." Once the washer stops moving and the faces are drawn together, extra torque only crushes fibres under the washer and reduces the clamp you were trying to build.

Sources

NDS 2018 — AWC National Design Specification for Wood Construction. Chapter 12 (dowel-type fasteners): 12.1.3 bolts, 12.1.4 lag screws, 12.2.1 and Table 12.2A withdrawal design values, Table 12.3.3A assigned specific gravities, Tables 12.5.1A–E geometry, 12.5.2 end grain factor, Appendix Table L2 lag screw dimensions.
DCA 6 (2015) — AWC Prescriptive Residential Wood Deck Construction Guide. Minimum requirements note 7 (corrosion), Table 5 (ledger fastener spacing), Figures 19 and 20 (placement and lag screw requirements).
IRC 2021 — R317.3 fastener corrosion requirements for preservative-treated wood.
Fastener dimensional standards: ANSI/ASME B18.2.1 (bolts and lag screws), B18.6.1 (wood screws), ASTM A307, SAE J429, ASTM F593 (stainless).

Local amendments vary, and your building department has the final say. Where a manufacturer publishes an evaluation report for a specific structural screw, that report governs for that product.