Hex Lag Screws
ASME B18.2.1 · No. 10 – ⅞″The standard lag bolt, with a full-diameter body, hex head and gimlet point. NDS design values are based on this screw.
ASME B18.2.1 & DIN 571 Lag Screws: Manufacturer & Exporter
A lag bolt, properly a lag screw, is a heavy wood screw with a hex head. Its coarse, sharp thread cuts its own mating thread in timber, and it is turned into a pre-drilled lead hole with a wrench. There is no nut, so it fastens where the far side of the timber cannot be reached.
Exotic Alloy manufactures lag screws in Mumbai, India. Inch hex lag screws follow ASME B18.2.1, the standard the US National Design Specification (NDS) for Wood Construction is based on, and metric screws follow DIN 571. We supply timber builders, deck and dock contractors, pallet and crate makers, and fastener distributors in India, and export to the USA, Canada, Europe and the Gulf.
Ready Stock, Custom Lengths, Fast Delivery
Common ¼″ to ½″ lag screws in hot-dip galvanized steel and SS 304 / 316 ship from stock. Long lengths to 16″, the larger sizes, DIN 571 metric sizes and special alloys are made to order.
No. 10 – ⅞″ | 4 – 20 mm
Gimlet point
Hex or square head
HDG & stainless
Bulk & retail packs
Lag bolt at a glance
- Head
- Hexagon (or square), driven with a wrench or socket
- Thread
- Coarse, sharp wood thread; ½″ has 6 threads per inch
- Point
- Gimlet (cone) point that starts in the lead hole
- Standards
- ASME B18.2.1 (inch), DIN 571 (metric)
- Design
- AWC NDS withdrawal and lateral values
- Related
- Carriage bolts, washers, lag shields
Lag Bolt Specification
The range we manufacture and supply.
StandardsASME B18.2.1 hex and square lag screws; DIN 571 hexagon head wood screws (12/2016); chemistry to ASTM A307 Grade A (steel) or ASTM F593 (stainless)
Size rangeInch No. 10, ¼″ to ⅞″, lengths 1″ to 16″; metric 4 to 20 mm, lengths 16 to 200 mm; longer and larger to order
ThreadCoarse lag (wood) thread, for example 10 tpi for ¼″, 6 tpi for ½″, 4½ tpi for ¾″; minimum length L/2 + 0.50″ or 6″, whichever is shorter
Head & pointHex head (square head on request); gimlet point; full-diameter body as standard, reduced body on request
MaterialsLow-carbon steel AISI 1006–1022; stainless 18-8, 304, 316; silicon bronze, duplex and nickel alloys to order
FinishPlain; zinc plated to ASTM F1941; hot-dip galvanized to ASTM A153 or F2329; passivated stainless
PropertiesB18.2.1 sets no strength grade for lag screws. Steel lag screws run Rockwell B70–B100; stainless strength comes from cold work
CertificationMill certificate with heat number; EN 10204 3.1 with chemistry, hardness, coating thickness and dimensions; PMI for stainless
MarkingManufacturer's mark on the head; 304 or 316 on stainless; no grade marking (none is specified)
PackingBulk cartons, retail boxes or buckets, with labels showing size, length, finish, quantity and heat number; private labelling for distributors
Types of Lag Bolts We Manufacture
DIN 571 Hexagon Head Wood Screws
DIN 571 · 4 – 20 mmThe metric lag screw for timber work in Europe, the Middle East and Asia. DIN 571 covers non-structural use. Load-bearing timber structures in Europe need screws with a product approval.
Square Head Lag Screws
ASME B18.2.1 · traditional patternThe original lag screw head, still used for restoration work, rustic timber and pole-line hardware. See also square head bolts.
Hex Washer Head Lag Screws
Fully threaded · ¼″ & ⅕″ · low carbon or Grade 5A hex head with an integral washer face and threads to the head, for ledger, pallet and crate work where a separate washer is unwanted.
Galvanized & Stainless Lag Bolts
ASTM A153 / F2329 HDG · SS 304 · SS 316For pressure-treated lumber, decks, docks, piers and coastal buildings. Stainless 316 is used for marine and pool work.
Special Alloy Lag Screws
Silicon bronze · duplex · MonelFor boat building, marine timber and chemical plant timber structures, made to order to the B18.2.1 or DIN 571 pattern.
Lag screw, carriage bolt or structural screw?
| Feature | Lag screw | Carriage bolt | Proprietary structural screw |
|---|---|---|---|
| Access needed | One side only | Both sides (nut) | One side only |
| Pre-drilling | Clearance + lead hole | Through hole | Usually none (self-tapping) |
| Design basis | AWC NDS (B18.2.1 lag screws) | AWC NDS (bolts) | Product evaluation report |
| Installation | Wrench or impact driver | Spanner on nut | Drill-driver |
| Best for | Ledgers, posts, heavy timber, equipment on wood | Through-bolted joints, rails | Fast structural screwing per its report |
Lag Bolt Dimensions: ASME B18.2.1 & DIN 571
Inch dimensions from ASME B18.2.1 hex lag screw tables, metric from DIN 571 (12/2016).
| Size | Threads / in | Body E max / min, in | Across flats F basic (max / min), in | Across corners G max / min, in | Head height H basic (max / min), in | Shoulder S min, in |
|---|---|---|---|---|---|---|
| No. 10 | 11 | 0.199 / 0.178 | 9/32″ (0.281 / 0.271) | 0.323 / 0.309 | 1/8″ (0.140 / 0.110) | 0.094 |
| 1/4″ | 10 | 0.260 / 0.237 | 7/16″ (0.438 / 0.425) | 0.505 / 0.484 | 11/64″ (0.188 / 0.150) | 0.094 |
| 5/16″ | 9 | 0.324 / 0.298 | 1/2″ (0.500 / 0.484) | 0.577 / 0.552 | 7/32″ (0.235 / 0.195) | 0.125 |
| 3/8″ | 7 | 0.388 / 0.360 | 9/16″ (0.562 / 0.544) | 0.650 / 0.620 | 1/4″ (0.268 / 0.226) | 0.125 |
| 7/16″ | 7 | 0.452 / 0.421 | 5/8″ (0.625 / 0.603) | 0.722 / 0.687 | 19/64″ (0.316 / 0.272) | 0.156 |
| 1/2″ | 6 | 0.515 / 0.482 | 3/4″ (0.750 / 0.725) | 0.866 / 0.826 | 11/32″ (0.364 / 0.302) | 0.156 |
| 5/8″ | 5 | 0.642 / 0.605 | 15/16″ (0.938 / 0.906) | 1.083 / 1.033 | 27/64″ (0.444 / 0.378) | 0.312 |
| 3/4″ | 4½ | 0.768 / 0.729 | 1-1/8″ (1.125 / 1.088) | 1.299 / 1.240 | 1/2″ (0.524 / 0.455) | 0.375 |
| 7/8″ | 4 | 0.895 / 0.852 | 1-5/16″ (1.312 / 1.269) | 1.516 / 1.447 | 37/64″ (0.604 / 0.531) | 0.375 |
Dimensions in inches. Length tolerance: ±0.12″ up to 6″ long and ±0.25″ over 6″ for ½″ and smaller; ±0.25″ for larger sizes.
| Nominal length L in | Minimum thread length T L/2 + 0.50 or 6 in, in | Unthreaded length at most, in |
|---|---|---|
| 1 | threaded as close to the head as practicable | – |
| 1.5 | 1.25 | 0.25 |
| 2 | 1.50 | 0.50 |
| 2.5 | 1.75 | 0.75 |
| 3 | 2.00 | 1.00 |
| 3.5 | 2.25 | 1.25 |
| 4 | 2.50 | 1.50 |
| 5 | 3.00 | 2.00 |
| 6 | 3.50 | 2.50 |
| 8 | 4.50 | 3.50 |
| 10 | 5.50 | 4.50 |
| 12 | 6.00 | 6.00 |
| 14 | 6.00 | 8.00 |
| 16 | 6.00 | 10.00 |
B18.2.1 sets only the minimum thread length; body length is not a controlled dimension. Thread may run further toward the head.
| Size d mm | d min mm | Across flats s mm | Head height k mm | Across corners e min, mm | Nearest inch size for reference |
|---|---|---|---|---|---|
| 4 | 3.52 | 7 | 2.8 | 7.5 | – |
| 5 | 4.52 | 8 | 3.5 | 8.63 | – |
| 6 | 5.52 | 10 | 4 | 10.89 | 1/4″ (6.35 mm) |
| 7 | 6.42 | 12 | 5 | 13.07 | – |
| 8 | 7.42 | 13 | 5.5 | 14.2 | 5/16″ (7.94 mm) |
| 10 | 9.42 | 17 | 7 | 18.72 | 3/8″ (9.53 mm) |
| 12 | 11.3 | 19 | 8 | 20.88 | 1/2″ (12.7 mm) |
| 16 | 15.3 | 24 | 10 | 26.17 | 5/8″ (15.9 mm) |
| 20 | 19.3 | 30 | 13 | 32.95 | 3/4″ (19.1 mm) |
Dimensions in mm. Metric and inch lag screws are not interchangeable; the nearest inch size is given only to help with conversions.
Lag Screw Pilot Hole Sizes & Installation
A lag screw needs two holes: a clearance hole for the plain shank and a smaller lead hole for the thread. The NDS rules below are what the published design values assume.
Lead hole and minimum penetration by size
| Size | Shank clearance hole = D, in | Lead hole, G ≤ 0.50 40–70 % of D, in | Lead hole, 0.50 < G ≤ 0.60 60–75 % of D, in | Lead hole, G > 0.60 65–85 % of D, in | Min. penetration 4D, excl. tip, in |
|---|---|---|---|---|---|
| 1/4″ | 0.250 | 0.100 – 0.175 | 0.150 – 0.188 | 0.163 – 0.212 | 1.00 |
| 5/16″ | 0.312 | 0.125 – 0.219 | 0.188 – 0.234 | 0.203 – 0.266 | 1.25 |
| 3/8″ | 0.375 | 0.150 – 0.262 | 0.225 – 0.281 | 0.244 – 0.319 | 1.50 |
| 7/16″ | 0.438 | 0.175 – 0.306 | 0.263 – 0.328 | 0.284 – 0.372 | 1.75 |
| 1/2″ | 0.500 | 0.200 – 0.350 | 0.300 – 0.375 | 0.325 – 0.425 | 2.00 |
| 5/8″ | 0.625 | 0.250 – 0.438 | 0.375 – 0.469 | 0.406 – 0.531 | 2.50 |
| 3/4″ | 0.750 | 0.300 – 0.525 | 0.450 – 0.562 | 0.488 – 0.637 | 3.00 |
| 7/8″ | 0.875 | 0.350 – 0.612 | 0.525 – 0.656 | 0.569 – 0.744 | 3.50 |
Our calculation from the NDS percentages of the shank diameter D; use the larger end of the range for larger screws. G is the specific gravity of the wood (see the withdrawal section). The NDS lets 3/8″ and smaller lag screws loaded mainly in withdrawal go in without a lead hole in wood with G ≤ 0.50, if edge distance, end distance and spacing prevent splitting.
Do
- Put a flat washer under the head on timber so the head does not crush the fibres
- Use soap or another lubricant on the screw or in the hole
- Turn the screw in with a wrench or impact driver until the washer just bears
- Use galvanized or stainless lag screws in treated or wet timber
Don't
- Drive a lag screw with a hammer: it destroys the thread grip
- Skip the lead hole in dense hardwood: the screw can twist off or split the wood
- Over-tighten until the head sinks into the wood
- Rely on lag screws loaded in withdrawal from end grain without design checks
Lag Screw Withdrawal Values & Calculator
How hard a lag screw holds depends far more on the wood than on the steel. The NDS gives the reference withdrawal value per inch of thread in side grain as W = 1800 G1.5 D0.75 (lb/in).
Withdrawal calculator
Reference values before NDS adjustment factors (load duration, wet service, temperature, group action). Thread penetration excludes the tapered tip. Check the steel tensile capacity at the thread root as well.
Result for 1/2″ in Douglas Fir-Larch
378
lb per inch of thread
1,134
lb reference W × p
(5.04 kN)
Lead hole for thread:0.200 – 0.350 in
Clearance hole for shank:0.500 in
Minimum penetration (4D):2.00 in (51 mm)
Reference withdrawal values W, lb per inch of thread penetration (side grain)
| Size | Spruce-Pine-Fir G = 0.42 | Hem-Fir G = 0.43 | Douglas Fir-Larch G = 0.50 | Southern Pine G = 0.55 | Red Oak G = 0.67 | White Oak G = 0.73 |
|---|---|---|---|---|---|---|
| 1/4″ | 173 | 179 | 225 | 260 | 349 | 397 |
| 5/16″ | 205 | 212 | 266 | 307 | 413 | 469 |
| 3/8″ | 235 | 243 | 305 | 352 | 473 | 538 |
| 7/16″ | 264 | 273 | 342 | 395 | 531 | 604 |
| 1/2″ | 291 | 302 | 378 | 437 | 587 | 668 |
| 5/8″ | 344 | 357 | 447 | 516 | 694 | 789 |
| 3/4″ | 395 | 409 | 513 | 592 | 796 | 905 |
| 7/8″ | 443 | 459 | 576 | 664 | 893 | 1016 |
Computed with the NDS equation; the figures agree with the published NDS table (for example 305 lb/in for ⅜″ and 378 lb/in for ½″ at G 0.50). Specific gravities are the NDS values for the species combinations named. Multiply by 0.75 for end grain.
Lag Bolt Grades, Coatings & Corrosion Protection
For lag screws, the finish usually matters more than the steel grade. Most lag screw failures in service come from corrosion in wet or treated timber.
| Material / grade | Basis | Properties | Use |
|---|---|---|---|
| Low-carbon steel | ASTM A307 Grade A chemistry; AISI 1006–1022 | Rockwell B70–B100; no specified tensile | Standard lag screws, dry interior timber |
| Grade 5 steel | SAE J429 Grade 5 | 120 ksi min tensile (to 1″) | Fully threaded hex washer lag screws, by order |
| Stainless 18-8 / 304 | ASTM F593 chemistry | Strength from cold work | Exterior, decks, food plants |
| Stainless 316 | ASTM F593 chemistry | Strength from cold work | Coastal, marine, pools, treated lumber |
Finishes
| Finish | Specification | Coating | Use |
|---|---|---|---|
| Zinc clear (trivalent) | ASTM F1941 Fe/Zn 3AN or 5AN | 3 or 5 µm | Dry interior only |
| Zinc yellow | ASTM F1941 Fe/Zn 3C or 5C | 3 or 5 µm | Dry interior only |
| Hot-dip galvanized | ASTM A153 or F2329 | 43 µm (⅜″ and under), 50 µm (over ⅜″) | Treated lumber, exterior timber, decks |
| Passivated stainless | ASTM A967 / A380 | – | 304 / 316 lag screws |
Treated lumber eats thin zinc. Modern copper-based preservatives corrode electroplated zinc quickly. The International Residential Code (R317.3) requires fasteners in preservative-treated wood to be hot-dip galvanized, stainless steel, silicon bronze or copper, with limited exceptions. Zinc-plated lag screws belong indoors in dry timber only.
Lag Bolt Materials
Every grade links to its datasheet on this site.
Special alloy lag screws: minimum properties of the bar we make them from
| Alloy | UNS | Condition supplied | Tensile min MPa | Yield 0.2 % min MPa | Elongation % | Why it is chosen for hex bolts |
|---|---|---|---|---|---|---|
| Inconel 625 | N06625 | Solution annealed (Grade 1) | 827 | 414 | 30 | Seawater, sour gas, cryogenic to 980 °C oxidation; subsea flange bolting |
| Inconel 718 | N07718 | Solution treated + aged (AMS 5662) | 1240 | 1035 | 12 | Highest strength to 650 °C; turbine and wellhead bolting |
| Monel 400 | N04400 | Hot-finished annealed | 517 | 207 | 35 | Seawater, hydrofluoric acid, no chloride SCC; marine and valve bolting |
| Monel K-500 | N05500 | Age hardened (AMS 4676) | 960 | 690 | 18 | Monel 400 corrosion resistance with three times the yield; pump shafts, subsea |
| Hastelloy C-276 | N10276 | Solution annealed (ASTM B574) | 690 | 283 | 40 | Wet chlorine, hot acids, mixed oxidising / reducing media; chemical reactors |
| Incoloy 825 | N08825 | Annealed (ASTM B425) | 586 | 241 | 30 | Sulphuric and phosphoric acid, sour gas (NACE MR0175); pickling plant |
| Duplex 2205 | S32205 | Solution annealed | 655 | 450 | 25 | Twice the yield of 316 with chloride SCC resistance; offshore topsides |
| Super Duplex 2507 | S32750 | Solution annealed | 750 | 550 | 15 | PREN > 40; seawater and desalination bolting |
| Titanium Grade 5 | R56400 | Annealed (ASTM B348) | 895 | 828 | 10 | Half the weight of steel, immune to seawater; aerospace and chemical |
| A-286 | S66286 | Solution treated + aged | — | — | — | Iron-base superalloy for 650 °C bolting; on request with certified values |
Bar minimums from the linked datasheets for the supply condition shown. In timber, the holding power still comes from the wood; the alloy is chosen for corrosion resistance.
How Our Lag Bolts Are Made
1
Wire & bar
Low-carbon or stainless wire received by heat with mill certificates.
2
Cold heading
Hex or square head formed on multi-station headers; large sizes are hot forged.
3
Pointing
The gimlet point is formed by pinch-pointing or cutting before threading.
4
Thread rolling
The deep, coarse lag thread is rolled to the point, so the thread starts cutting as soon as the tip enters the lead hole.
5
Finishing
Hot-dip galvanizing with spinning to keep the thread clean, zinc plating, or passivation for stainless.
6
Inspection
Head, body and thread length gauged, coating thickness measured and hardness checked per lot.
Where Our Lag Bolts Are Used
Deck ledgers & framing
Fixing ledgers, posts and beams where the far side cannot be reached for a nut.
Docks, piers & boardwalks
Stainless 316 and galvanized lag screws in wet and marine timber.
Timber frames & log homes
Long lag screws for heavy timber connections and log walls.
Pallets, crates & skids
Holding machinery and heavy loads to timber skids for transport.
Utility poles & hardware
Fixing brackets, insulators and hardware to wooden poles and cross-arms.
Masonry with lag shields
With a lag shield, fixing fixtures and equipment to concrete and brick.
Lag Bolt Inspection & Certification
| Check | Method | Applied to |
|---|---|---|
| Chemistry / PMI | Mill certificate; XRF PMI | Every heat; all stainless lots |
| Head & body | Across flats, corners, head height, body diameter | Every lot to ASME B18.2.1 or DIN 571 |
| Thread length | Against L/2 + 0.50″ (max 6″) | Every lot |
| Hardness | Rockwell B | Every lot |
| Coating | Magnetic thickness gauge; ASTM A153 / F2329 or F1941 limits | Every coated lot |
| Drive test | Driven into a lead hole in timber | Sample per lot, on request |
Every shipment includes
- Mill certificate with heat number
- Dimensional and thread length report
- Coating thickness report
- Packing list by size and length
On request
- EN 10204 3.1 certificate
- Salt spray test report
- Third-party inspection
Lag Bolt Questions, Answered
They are the same fastener. ASME B18.2.1 and the NDS call it a lag screw; in the trade it is often called a lag bolt because of its size and hex head. It has a coarse, sharp wood thread and a gimlet point, is turned into a pre-drilled lead hole with a wrench, and needs no nut. The metric equivalent is the DIN 571 hexagon head wood screw.
Drill two holes. The clearance hole for the plain shank equals the shank diameter and is as deep as the unthreaded length. The lead hole for the thread depends on wood density: 40 to 70 percent of the shank diameter for specific gravity up to 0.50, 60 to 75 percent from 0.50 to 0.60, and 65 to 85 percent above 0.60, using the larger percentage for larger screws. For a 1/2 inch lag screw in Douglas fir-larch (G 0.50) that is 0.200 to 0.350 inch.
The NDS reference withdrawal value is W = 1800 G^1.5 D^0.75 pounds per inch of thread penetration in side grain, not counting the tapered tip. A 1/2 inch lag screw in Douglas fir-larch (G 0.50) gives 378 pounds per inch, so 3 inches of thread penetration gives 1,134 pounds reference value. End grain values are 75 percent of this, and all NDS adjustment factors must still be applied.
The NDS sets a minimum penetration of four times the screw diameter into the main member, not counting the tapered tip; a 1/2 inch lag screw therefore needs at least 2 inches. Lateral design values for penetrations between the minimum and the full value are reduced, so check the NDS edition your design follows.
ASME B18.2.1 sets a minimum thread length of half the nominal length plus 0.50 inch, or 6 inches, whichever is shorter. A 4 inch lag screw has at least 2.50 inches of thread. Screws too short for the formula are threaded as close to the head as practicable. Body length is not controlled by the standard.
Use hot-dip galvanized lag screws to ASTM A153 or F2329, or stainless steel 304 or 316. The International Residential Code (R317.3) requires fasteners in preservative-treated wood to be hot-dip galvanized, stainless steel, silicon bronze or copper, with limited exceptions. Stainless 316 is preferred near the sea and for pool and dock work. Do not mix stainless screws with galvanized hardware in wet service.
Not directly. In concrete, brick or block a lag screw is used with a lag shield, an expansion anchor sleeve that is set in a drilled hole and expands as the lag screw is turned in. The holding power then depends on the anchor and the base material, not on the wood values on this page.
ASME B18.2.1 lag screws have no specified mechanical properties. Steel lag screws are made to the chemistry of ASTM A307 Grade A, typically AISI 1006 to 1022, and stainless ones to the chemistry of ASTM F593 304 or 316. NDS design values are based on the wood, not the steel grade, but the designer should also check the tensile capacity of the screw at the root of the thread.