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Lag bolts are used to fasten wood to wood and metal to wood, attach objects to wood, and affix items to concrete and masonry when combined with lag expansion shields. They are heavy-duty wood screws that are available in larger sizes—both in terms of diameter and length—and usually have a hex head to facilitate higher installation torque using a wrench. While lateral strength is lower than a comparable bolt and nut, lag bolts are useful when access to one side of the joint is limited, the appearance of a nut is undesirable or through-bolt length is excessive. Lag bolts are also known as:
hex lag bolts, lag screws, coach screws.
Being a thread-forming-type screw, they create their own mating threads during installation—no nut is needed so a through-hole is not required.
Lag bolts have coarse, widely spaced threads (10 threads per inch for a 1/4" bolt, declining to just 3 1/4 threads per inch for a 1 1/4" lag bolt); threading is standard right-hand. Short lengths are fully threaded. The minimum threaded length for longer lengths is one-half the bolt length plus 1/2", or 6", whichever is shorter. A gimlet point allows them to start threading into wood easily: "A gimlet point is a threaded cone point usually having a point angle of 45 to 50 degrees."
Typically, lag bolt sizes range from 1/4" to 3/4" in diameter, and lengths span from about 1" to 16". Although 7/16" is detailed in the ASME standard, that size is not commonly available. Measure length from under the head to the threaded end of the bolt.
The size of the head varies with the size of the lag bolt. Since the bolt is 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.
Steel and stainless steel are the most prevalent materials for lag bolts.
Common finishes for steel are zinc plating and hot dip galvanizing. Zinc, the most popular and least expensive commercial plating, offers moderate corrosion resistance. Hot dip galvanized is a thick coating of zinc that protects against corrosion in harsh environments. Stainless steel, though, is a better choice when corrosion is of concern. Hot dip galvanized and stainless steel are usually recommended if the bolts (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.
Similar to wood screws, a stepped pilot hole is recommended for lag bolts. The pilot hole for the body (the unthreaded section of the shank adjacent to the head) should match the bolt size (3/8" hole for a 3/8" bolt, for example). The pilot hole for the threaded section varies with bolt 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). If the pilot hole is too small, the bolt may break during installation. If the hole is too large, withdrawal resistance may be compromised. Saturate drilled holes with a preservative to prevent the migration of decay fungi if you drill into an untreated area of treated lumber. Use soap or beeswax to lubricate lag bolts during installation (be sure to remove any exposed residue before applying a wood finish).
Always use a large diameter washer under the head to distribute clamping force over a larger area and minimize compression of the wood.
It is advisable to match materials and finishes of bolts and washers (for example, use a hot dip galvanized washer with a hot dip galvanized bolt).
Ultimate tensile strength of a lag bolt is said to be developed with a thread penetration of about seven times the bolt diameter in denser species (specific gravity greater than 0.61) and 10 to 12 times the bolt 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.)
Lag and machine bolts are the preferred fasteners for fastening heavy wood members in structures. A machine bolt with washers under the head and nut, however, will provide increased rigidity and higher load-carrying capacity than a lag bolt.
Refer to American Society of Mechanical Engineers Standard ASME B18.2.1, Square and Hex Bolts and Screws, for specifications relating to lag bolts.
Lag Bolt Dimensions
|3/4||0.7500||4 1/2||1 1/8||1 17/64||1/2|
Lag Bolt Pilot Hole Sizes
|Bolt Size||Pilot Hole Size|
Groups III & IV
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