Lag screws (or lag bolts) are used in applications that require intense load support. They have coarse threads and a nut to hold them in place.
They are usually manufactured from steel and are often finished with zinc or hot dip galvanized to resist corrosion. They are also available in stainless steel for increased strength and resistance to wear.
- Diameter
Lag screws are heavy duty fasteners used in wood construction to hold together lumber and other materials that bear a significant load. They have the sharp cutting threads of a wood screw, but are much larger in diameter to handle heavier loads. They also have a head that’s either hex or square-shaped and can be driven in place with a wrench.
Like bolts, lag screws have mating threads that form automatically during installation, and so do not require a nut to secure them in place. For safety reasons, however, it is generally recommended that a pilot hole be pre-drilled for all lag bolt sizes prior to installing them.
Using the chart below, find your lag screw size by measuring the outer diameter of the threads. For maximum strength, it’s important that the lag screw have a thread length at least twice as long as the thickness of the material you are attaching to prevent the screw from pulling out or “walking”.
- Threads
Lag screws have coarse threading that extends throughout their length, unlike normal wood screws that only thread as they enter the application. This helps them support more intense loads than wood screws, which is why they’re preferred in heavy carpentry applications.
Lag screw sizes are listed in fractional and decimal inches with their nominal threads per inch also noted. In addition, the head width across flats and across corners, as well as the head height, are listed for each lag screw size.
The first step in determining a lag screw size is to determine how thick the materials you are fastening together are. Next, use the chart to find the corresponding lag screw size in the column that lines up with those dimensions. This will ensure that the lag screw has sufficient length to penetrate both materials and bind securely.
- Length
Often used in construction and woodworking, lag screws (also called hex lag bolts or coach screws) are heavy-duty fasteners designed to be driven into thicker materials. They are typically used for fastening wood-to-wood joints. They differ from other screws in that they have coarse threads and a square or hex head, making them easy to install using a wrench or ratchet.
When choosing a lag screw size, it is important to consider the material you will be working with as well as the load strength needed. The length of the head should be at least twice as long as the thickness of the material you’re attaching to. This ensures a strong hold. Using the wrong length could weaken your joints and cause the screw to loosen over time.
- Head
Lag screws (sometimes referred to as lag bolts) are large wood screws with hex heads that can be turned by a wrench. They have more coarse threads than standard screw threads and are typically used for heavier loads.
To install lag screws, first make sure the materials being joined are aligned and clamped together. Then, pre-drill a pilot hole using a drill bit that’s slightly smaller in diameter than the lag screw’s overall diameter.
Lag screws have one of three types of points: cone, semi-cone or gimlet. Portland Bolt uses semi-cone points when manufacturing lag screws to ensure that the head is easy to insert into the pilot hole. The chart below shows the head height and width across flats and corners of a lag screw’s head in inches.
- Material
Lag screws are one of the toughest non-industrial fasteners on the market. They are used to fasten dense timbers together and can be found in a number of heavy carpentry applications. They differ from regular wood screws and self-drilling screws in that they have coarse threads that do not cover the entire length of the screw and require a pilot hole to be drilled before installation.
They are typically made of steel or stainless steel and can be hot-dip galvanized to accommodate any project requirements. They are also available in silicon bronze for corrosive or marine applications.
The hex head on lag screws is formed by heat treatment, a process called upsetter forging. This creates the correct hex shape, markings and head dimensions for easy recognition. They are a bit thicker than a normal wood screw and have a pointed tip rather than the blunt end seen on most bolts.