Lag bolts, or lag screws, are deep-reaching fasteners generally used to fasten heavy lumber and other materials that bear an intense load. Lag “bolt” is a bit confusing because they don’t require a nut but are more like large screws in nature.
You will find lag bolts used throughout wood construction, from wood frames to load-bearing beams to picnic table legs. You tighten lag bolts down with a wrench that easily fits onto the hex head, not a standard screwdriver. The name lag screw comes from their original use in securing barrel staves, also known as lags. Lag bolts are much larger than standard screws because of their need to support greater loads.
Using Lag Bolts
To install a lag bolt requires a drilled pilot hole; the pilot hole needs to be equal to or slightly smaller than the root diameter of the bolt. The length and diameter of the bolt generally determine the load it is expected hold. A rough formula:
- A 1/4 inch bolt needs a 3/16 inch hole for hardwood, 3/32 inch hole for softwood.
- A 3/8 inch bolt needs a 1/4 inch hole for hardwood, 11/64 inch hole for softwood.
- A 5/16 bolt needs a 7/32 inch hole for hardwood, 9/64 inch hole for softwood.
The thicker and longer the bolt, the more weight it can bear. Weight bearing is also a function of the bolt metal, such as stainless steel, zinc-plated steel, and galvanized steel.
- 1/4 inch bolts hold up to 272 lbs. per inch of thread.
- 3/8 inch bolts hold up to 200 lbs.
- 5/16 in bolts hold up to 100 lbs.
The bolt’s teeth are designed to achieve optimal purchase in the wood. Installed securely, the bolts will provide a stable base to hold the material together for many, many years. Lag bolts aren’t substitutes for regular screws; their larger size can damage smaller, thinner pieces of wood. Measure and weigh accordingly before picking what size bolt will work best.
Square Vs. Hex Heads
The vast majority of lag bolts feature hexagonal heads, which are easily fitted with a socket wrench. Alternatively, a square head lag bolt, also known as a square head lag bolt or coach screw, provides a vintage look for any project. This fastener style was originally hand-forged by blacksmiths until the Industrial Revolution when the hex head became ubiquitous. The square head is now sought out by blacksmiths, woodworkers, furniture builders, restoration specialists, and other makers looking to give their project an authentic finishing touch. These are best installed with an 8 point socket.
Bolt Materials
Lag bolts are machined from hard carbon steel and are typically installed out of sight below finished surfaces. However, for projects where the work is exposed, back oxide or stainless steel is the designer’s way to go to complement and accent the overall construction. These unique lag bolts offer finishes like aged and oil-rubbed bronze, wrought iron, gunmetal, or black oxide.
Your project may call for a decorative bolt head, which will perform well as long as the shaft and threads aren’t inferior. Hammered bolts have a pock-marked “freshly smithed” look for a rustic appeal or smooth bolts that are machine polished. In projects that emulate a show of strength, a working bolt is typically the accent, and lag bolts are some of the hardest working bolts.