Lag Bolts
Manufacturer
India · Mumbai
Anatomy of a Lag Bolt — The Wood Itself Becomes the Nut
A lag bolt (also called a lag screw or coach screw) is fundamentally a wood screw scaled up to structural size — with an external hex or square head instead of a slotted or Phillips drive, allowing it to be torqued to a specified installation force using a wrench or socket rather than a screwdriver. The thread is deep, coarse, and sharply pitched — designed to displace and grip wood fibre as it is driven, rather than engaging a machined nut thread the way a standard bolt does.
The gimlet point — a sharpened, slightly tapered tip — allows the lag bolt to self-start and begin cutting its own thread path into the wood as it is driven, provided a correctly sized pilot hole has first been drilled. This is the critical installation detail: a pilot hole is always required, sized smaller than the lag bolt’s major thread diameter (to preserve full thread engagement in the wood) but large enough to prevent the wood from splitting as the coarse thread displaces fibre on its way in. Pilot hole diameter is typically 60–70% of the lag bolt’s nominal diameter for the threaded portion, and matching the shank diameter for any unthreaded shank length.
Holding power comes entirely from the mechanical interlock between the thread flanks and the surrounding compressed wood fibre — there is no separate nut, and none is needed. This is why lag bolts are specified wherever a heavy connection must be made directly into solid timber without access to the opposite face to fit a nut: deck ledger boards against a house frame, heavy timber beam connections, and any structural wood-to-wood or wood-to-masonry connection where a through-bolt with a nut is not practical.
Why Lag Bolts — Structural Wood Connections Without a Nut
Structural-Grade Holding Power in Solid Wood
Lag bolts provide significantly higher withdrawal and shear resistance than standard wood screws because of their larger diameter, deeper thread engagement, and the ability to be torqued to a specified installation value with a wrench. Structural engineering design values for lag screw connections are published in timber design codes (IS 883, NDS in the US) based on wood species, lag bolt diameter, and embedment depth — allowing calculated structural connections rather than rule-of-thumb fastening.
No Access Required to the Opposite Face
Because the wood fibre itself provides the clamping reaction (no nut is needed on the far side), lag bolts can be installed into solid timber, a beam, or a post where the opposite face is inaccessible, embedded in another structure, or simply too far away to reach with a standard through-bolt and nut. This single property is why lag bolts are the default for deck ledger boards (fastened directly into a house’s structural frame from outside) and heavy beam-to-post connections.
Removable and Re-Drivable — Unlike Nails or Adhesive
A correctly installed lag bolt can be unscrewed and removed for disassembly, structural inspection, or component replacement — unlike nailed connections (which must be pried apart, often damaging the timber) or adhesive/epoxy connections (which are effectively permanent). For structures that may require future maintenance access, heavy timber lag bolt connections offer serviceability that other high-strength wood fastening methods do not.
Pre-Drilled Pilot Hole Prevents Wood Splitting
The gimlet point allows self-starting into a correctly sized pilot hole without requiring a full pre-tapped thread — but the pilot hole itself is essential engineering practice, not an optional step. Installing a lag bolt without any pilot hole risks splitting the wood along the grain as the coarse thread displaces fibre faster than the wood can compress, particularly near the end grain or close to the edge of a timber member.
Hex Head vs Square Head Lag Bolts
Both drive types are externally wrenched — the choice depends on regional convention, available tooling, and whether a washer face is required under the head.
Hex Head Lag Bolt
Standard hexagon head driven by an open-end spanner, ring spanner, or socket — the dominant lag bolt head style in current construction and engineering practice worldwide. Compatible with the same wrench and socket sets used for every other hex-head fastener on a job site, requiring no dedicated tooling.
Square Head Lag Bolt (Coach Screw)
Square head driven by an adjustable spanner or a dedicated square-socket tool — the traditional “coach screw” head form historically used in coach-building and heavy timber carpentry, still specified for heritage restoration work and in regions where square-head coach screws remain the local convention for heavy timber fastening.
Lag Bolt Material & Corrosion Protection Options
Material and finish selection depends on exposure — outdoor decking and treated lumber require corrosion-resistant coatings or stainless steel to prevent both fastener corrosion and chemical reaction with wood preservative treatments.
Lag Bolt Configurations & Related Timber Hardware
Standard hex and square head lag bolts, plus the washers used beneath every lag bolt head. M6–M24.
Hex Head Lag Bolt
Standard hexagon head, coarse gimlet-point thread. M6–M24, standard and custom lengths. The default lag bolt specification for structural timber connections, deck ledger boards, heavy beam-to-post fastening, and general heavy carpentry requiring wrench-driven installation.
Square Head Coach Screw
Traditional square head, coarse gimlet-point thread. M8–M20. For heritage restoration, coach-building, and regional markets specifying the traditional coach screw head form. Same holding-power thread geometry as the hex head variant.
Washers
Plain flat washers to DIN 125 / IS 2016, sized for lag bolt shank diameter. Always specified beneath the lag bolt head to distribute clamping load over a wider bearing area on the wood surface — critical because wood is significantly softer than the steel or stainless head, and an unwashered head will crush into the surface fibre over time.
Custom Lag Bolts
Non-standard lengths, thread diameters outside the standard range, and drawing-specific configurations in any available material and head style. Contact sales@akbarfasteners.in with head type, diameter, length, and grade requirement.
Lag Bolt Pilot Hole & Size Chart — M6 to M24
Pilot hole diameter must match wood density — softwoods (pine, fir) can use a smaller pilot hole than hardwoods (oak, teak) at the same lag bolt diameter, since denser wood offers more resistance to the coarse thread displacing fibre.
| Thread Size | Softwood Pilot Hole (mm) | Hardwood Pilot Hole (mm) | Hex Across Flats (mm) |
|---|---|---|---|
| M6 | 3.5 | 4.2 | 10 |
| M8 | 4.8 | 5.6 | 13 |
| M10 | 6.0 | 7.0 | 17 |
| M12 | 7.2 | 8.4 | 19 |
| M14 | 8.5 | 9.8 | 22 |
| M16 | 9.8 | 11.2 | 24 |
| M20 | 12.2 | 14.0 | 30 |
| M24 | 14.6 | 16.8 | 36 |
Values shown are general reference guidance. Always confirm pilot hole diameter against the specific wood species and structural design calculation for load-bearing connections.
Lag Bolts Application Areas
Structural timber construction, decking, heavy carpentry, and pole-line hardware — wherever a wrench-driven wood screw provides the correct fastening method.
Structural Timber Framing
heavy timber joints
Decking & Ledger Boards
SS 316L coastal decking
Utility Pole Hardware
pole-line fitting fastening
Heavy Carpentry
heritage restoration work
Agricultural Structures
livestock enclosure timber
Playground & Outdoor Structures
pergola & gazebo framing
Bridge & Boardwalk Timber
coastal boardwalk hardware
Fencing & Rail Systems
equestrian fencing hardware
Lag Bolt Standards Coverage
Same ISO 9001:2015 quality system as our complete fastener range. Thread geometry and gimlet point profile verified against the applicable standard on every production lot.
Complete Documentation Package
Grade / Standard Stated
Tensile · Hardness
Thread & Point Profile
Conformance
ASTM A153 (if HDG)
Related Bolt Types & Materials
Lag bolts for direct wood-fibre grip. Carriage bolts for through-bolted wood connections with a nut. J-bolts for concrete foundation anchoring. SS 316L for marine and coastal timber applications.
Carriage Bolts
For through-bolted wood connections where access to the opposite face is available and a nut can be fitted — the self-locking square neck prevents rotation while a nut on the far side provides the clamping reaction, contrasted with the lag bolt’s nut-free single-face installation.
View Carriage Bolts →J-Bolts
For anchoring a wood post or structure into a concrete foundation — J-bolts are cast into the concrete before pour, then the timber base plate is bolted to the projecting threaded end, connecting the lag-bolted timber structure above to its concrete foundation below.
View J-Bolts →SS 316L Fasteners
For lag bolt applications in marine, coastal, or treated-timber environments where carbon steel and standard zinc plating are insufficient — SS 316L lag bolts for coastal decking, boardwalks, and ACQ-treated lumber where copper-based preservatives accelerate corrosion of lesser materials.
View SS 316L →Full Bolt Range
Complete bolt manufacturing capability at Akbar Fasteners — lag bolts, carriage bolts, J-bolts, hex, flange, and all specialty head forms across carbon steel through exotic alloy, all from a single Mumbai facility.
View Bolt Range →Need Lag Bolts /
Coach Screws?
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Export: +91 8696 32 4646 · Support: +91 800 9315 333 · export@akbarfasteners.in