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Lag Screws

Lag screws are heavy-duty wood screws that usually have a hex head, which allows for higher installation torque using a wrench, and are available in larger sizes and longer lengths. Common applications include wood to wood and metal to wood fastening, attaching objects to wood, and affixing items to concrete and masonry when combined with lag expansion shields. Although lateral strength is lower than a comparable bolt and nut, lag screws are useful when only one side of the joint is accessible, the presence of a nut is unacceptable or through-bolt length is excessive. They are also known as: 

lag bolts, hex lag bolts, coach screws.

 Lag screws are a type of thread-forming screw because mating threads are automatically created during installation—a through-hole and nut are not required.

Lag screws have widely spaced, coarse threads (10 threads per inch for a 1/4" diameter screw, decreasing to just 3 1/4 threads per inch for a 1 1/4" lag screw). Threads are standard right-hand. Short lengths are fully threaded while longer lengths have a the minimum threaded length that is one-half the screw length plus 1/2", or 6", whichever is shorter. A gimlet point allows them to easily start threading into wood: "A gimlet point is a threaded cone point usually having a point angle of 45 to 50 degrees."

Typically, lag screw sizes range from 1/4" to 3/4" in diameter, and lengths span from about 1" to 16". Length is measured from under the head to the threaded tip of the screw.

Head sizes vary with the size of the screw. Since lag screws are externally wrenched, sufficient space around the head must be allowed for wrench clearance. Table 1 lists head width across flats, which is wrench size, and across corners, head height and a listing of threads per inch.

The most prevalent materials for lag screws are steel and stainless steel.

Common finishes for steel are zinc plating and hot dip galvanizing. Zinc offers moderate corrosion resistance and is the most popular and least expensive commercial plating. Hot dip galvanized lag screws have a thick coating of zinc that protects against corrosion in harsh environments. When corrosion is of concern, stainless steel is a better choice. Hot dip galvanized and stainless steel are usually recommended if the screws (less than 1/2" in diameter) will be used with pressure preservative treated wood such as "ACQ" (Alkaline Copper Quaternary)—check local building codes and contact your lumber supplier for recommendations.

A stepped pilot hole is recommended for lag screws just like wood screws. The pilot hole for the body (the unthreaded section of the shank adjacent to the head) should match the screw size (for example, use a 3/8" hole for a 3/8" screw). The pilot hole for the threaded section, though, is based on screw size and wood density, as suggested in Table 2 (see a table of Species Groups for Sawn Lumber, which lists the species in each group). Undersized pilot holes may cause the screw to break during installation. If the hole is oversized, withdrawal resistance may be compromised. If you drill into an untreated area of treated lumber, saturate drilled holes with a preservative to prevent the migration of decay fungi (American Wood Protection Association [AWPA] Standard M4: Standard for the Care of Preservative-Treated Wood Products provides options for field treatment). Beeswax or paraffin should be used to lubricate lag screws during installation and although bar soap is an accepted alternative, glycerin-based products may speed up the screw corrosion process due to the hygroscopic nature of glycerin, which causes it to absorb moisture (be sure to remove any exposed lubricating residue before applying a wood finish).

To distribute clamping force over a larger area and minimize compression of the wood, always use a large diameter washer under the head of the lag screw.

It is advisable to match materials and finishes of screws and washers (for example, use a hot dip galvanized washer with a hot dip galvanized screw).

Ultimate tensile strength of a lag screw is said to be developed with a thread penetration of about seven times the screw diameter in denser species (specific gravity greater than 0.61) and 10 to 12 times the screw diameter in less dense species (specific gravity less than 0.42). (The table, Species Groups for Sawn Lumber, includes specific gravity for each of the species.)

The preferred fasteners for fastening heavy wood members in structures are lag screws and machine bolts. However, a machine bolt with washers under the head and nut, will provide increased rigidity and higher load-carrying capacity than a lag screw.

Refer to American Society of Mechanical Engineers Standard ASME B18.2.1, Square and Hex Bolts and Screws, for specifications relating to lag screws.

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