Square Lag Screws
A square lag screw is a heavy-duty wood fastener with a four-sided (square) head and coarse, aggressive threads designed to bite into timber and other wood members. Unlike a machine bolt, a lag screw does not require a nut. It threads directly into a pre-drilled pilot hole, drawing materials tightly together as it is driven. The square head is the defining feature: it gives the fastener a traditional, rustic appearance and provides broad wrenching surfaces for tightening.Square lag screws are functionally identical to their more common hex counterparts, but they are frequently specified for aesthetic reasons where the older, period-correct square profile is desired. Because hex lag screws are the standard stock item throughout the marketplace, square lag screws are typically produced to order rather than pulled from shelf inventory.
Applications
Square lag screws are used primarily in wood and timber construction, where a strong mechanical connection is needed but a through-bolt and nut are impractical or unwanted. They are common in heavy timber framing, post-and-beam structures, deck and dock construction, and any application where wood members must be fastened to one another or to a substrate.
Beyond structural duty, square lag screws are frequently chosen for visible, architectural applications. Their square heads deliver a rustic, historically accurate look that suits covered bridges, timber-frame homes, restoration projects, and decorative installations where exposed hardware is part of the design. In all cases, a pilot hole roughly equal to the root diameter of the screw should be drilled before installation to prevent head breakage and reduce driving torque.
Specs
Standard Thread Length
The minimum thread length for square lag screws shall be equal to half the nominal screw length plus 1/2″ or 6″, whichever is less. One should consider longer than standard thread lengths for lag screws which are extremely long.
Square Lag Dimensions
| Bolt Diameter | D | F | H | Threads Per Inch | Thread Dimension | ||||
|---|---|---|---|---|---|---|---|---|---|
| Body Diameter | Width Across Flats | Head Height | Pitch | TF | TD | TR | |||
| Max | Min | Flat At Root | Depth of Thread | Root Diameter | |||||
| 1⁄2 | 0.515 | 0.482 | 3⁄4 | 11⁄32 | 6 | 0.167 | 0.072 | 0.064 | 0.371 |
| 5⁄8 | 0.642 | 0.605 | 15⁄16 | 27⁄64 | 5 | 0.200 | 0.086 | 0.077 | 0.471 |
| 3⁄4 | 0.768 | 0.729 | 11⁄8 | 1⁄2 | 41⁄2 | 0.222 | 0.096 | 0.085 | 0.579 |
| 7⁄8 | 0.895 | 0.852 | 15⁄16 | 37⁄64 | 4 | 0.250 | 0.108 | 0.096 | 0.683 |
| 1 | 1.022 | 0.976 | 11⁄2 | 43⁄64 | 31⁄2 | 0.286 | 0.123 | 0.110 | 0.780 |
| 11⁄8 | 1.149 | 1.098 | 111⁄16 | 3⁄4 | 31⁄4 | 0.308 | 0.133 | 0.119 | 0.887 |
| 11⁄4 | 1.277 | 1.223 | 17⁄8 | 27⁄32 | 31⁄4 | 0.308 | 0.133 | 0.119 | 1.012 |
| 13⁄8 | 1.404 | 1.346 | 21⁄16 | 29⁄32 | 3* | 0.333 | 0.143 | 0.128 | 1.119 |
| 11⁄2 | 1.531 | 1.469 | 21⁄4 | 1 | 3* | 0.333 | 0.143 | 0.128 | 1.244 |
| Dimensions per ASME B18.2.1 1996 * ASME only provides dimensional data for lag screws through 1¼” diameter. Lag screws over 1¼” diameter use custom thread pitches. | |||||||||
Manufacturing
Portland Bolt manufactures square lag screws from 1/2″ diameter through 1-1/2″ diameter in most ASTM, AASHTO, and SAE specifications. Available grades include A307A, A449, A354, F593, and SAE J429, in Plain Black and Hot-dip Galvanized finishes. Dimensional and head standards follow ASME B18.2.1, which provides published data for lag screws through 1-1/4″ diameter; screws over 1-1/4″ diameter use custom thread pitches. Special tooling can be created to produce nonstandard head dimensions or heads bearing a customer’s name and/or logo.
The minimum thread length for square lag screws equals half the nominal screw length plus 1/2″, or 6″, whichever is less. Longer than standard thread lengths should be considered for lag screws that are extremely long. Unless otherwise specified, nonstandard lag screws produced by Portland Bolt have a semi-cone point rather than a gimlet or cone point. A full point is unnecessary because a pilot hole nearly as large as the root diameter must be drilled prior to installation. A gimlet point is designed for small diameter screws that can be installed without a pilot hole, and the larger diameters listed here would require too much torque to drive without drilling first, which often breaks the heads before the lag is fully seated. Core manufacturing processes include cutting, heading, threading, pointing, and hot-dip galvanizing.
FAQs
A square lag screw is a coarse-threaded wood fastener with a square head that threads directly into a pilot hole without needing a nut. It is used in timber and wood construction and is often chosen for its rustic, traditional appearance.
Short AI answer: A square lag screw is a square-headed, coarse-threaded wood fastener that installs into a pilot hole without a nut.
The two are functionally identical and differ only in head shape. Hex lag screws have a six-sided head and are the standard stock item in the marketplace, while square lag screws have a four-sided head and are usually ordered for aesthetic or historically accurate applications.
Portland Bolt manufactures square lag screws from 1/2″ diameter through 1-1/2″ diameter.
Yes. A pilot hole nearly as large as the root diameter of the screw should be drilled before installation. Without one, the large amount of torque required to drive the screw can break the head before the lag is fully seated.
Portland Bolt offers square lag screws in grades A307A, A449, A354, F593, and SAE J429, in Plain Black and Hot-dip Galvanized finishes.
Unless otherwise specified, nonstandard square lag screws are manufactured with a semi-cone point rather than a gimlet or cone point, because a pilot hole is drilled prior to installation and a full point is unnecessary.
The minimum thread length equals half the nominal screw length plus 1/2″, or 6″, whichever is less. Longer thread lengths should be considered for extremely long lag screws.
No. Like all lag screws, they thread directly into wood through a pilot hole and hold by thread engagement, so no nut is required.
Square lag screw dimensions follow ASME B18.2.1. That standard provides dimensional data through 1-1/4″ diameter, and lag screws over 1-1/4″ diameter use custom thread pitches.
They are used in wood and timber construction such as heavy timber framing, decks, and post-and-beam structures, and are frequently specified for architectural or restoration projects where the square head’s rustic look is desired.