
Lag screws are heavy-duty fasteners with sharp threading that covers only a part of the bolt, allowing for adjustments to be made as needed. They are typically used for wood-to-wood and metal-to-wood fastening, and are available in a range of sizes and lengths. When selecting a lag screw for a project, it is important to refer to a lag screw size chart to ensure the correct screw size is used. This involves identifying the various parts of the lag screw, including the head, the threadless part of the bolt, the helical structure, and the distance between adjacent threads. By understanding these components, users can accurately measure and select the appropriate lag screw size for their specific application.
Characteristics | Values |
---|---|
Typical Diameter Range | 1/4" to 3/4" |
Length Range | 1" to 16" |
Length Measurement | From under the head to the threaded tip of the screw |
Head Shape | Hex or Square |
Thread Type | Coarse, Right-Hand |
Thread Count | 10 threads per inch for a 1/4" diameter screw |
Thread Count | 3 1/4 threads per inch for a 1 1/4" lag screw |
Minimum Thread Length | One-half the nominal screw length plus 1/2" or 6", whichever is less |
Thread Length for Longer Screws | One-half the screw length plus 1/2" or 6", whichever is shorter |
Point Type | Gimlet Point |
Point Angle | 45 to 50 degrees |
Materials | Steel, Stainless Steel |
Common Finishes | Zinc Plating, Hot-Dip Galvanizing |
Hex lag screws
Lag screws have a lower lateral strength than bolts and nuts, but they are useful when only one side of the joint is accessible, or when the presence of a nut is unacceptable or undesirable. Hex lag screws have a deeper and sharper thread than national coarse thread screws, as they are designed to drive into wood.
The most common materials for lag screws are steel and stainless steel. Steel screws are often zinc-plated or hot-dip galvanised, offering moderate corrosion resistance. Hot-dip galvanised screws are coated in zinc to protect against corrosion in harsh environments. Stainless steel is a better choice when corrosion is a concern.
Lag screws have widely spaced, coarse threads. For example, a 1/4" diameter screw will have 10 threads per inch, while a 1 1/4" screw will have just 3 1/4 threads per inch. The length of the screw is measured from under the head to the threaded tip. Typically, lag screw sizes range from 1/4" to 3/4" in diameter, and lengths span from about 1" to 16".
When installing a lag screw, it is important to use a pilot hole to guide the screw. The pilot hole should be slightly smaller than the screw size, and a large diameter washer should be used under the head of the screw to distribute clamping force and minimise wood compression.
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Thread-forming screws
Lag Screws
Lag screws are heavy-duty wood screws that usually have a hex head, allowing for higher installation torque using a wrench. They are commonly used for wood-to-wood and metal-to-wood fastening, and are available in larger sizes and longer lengths. Lag screws have widely spaced, coarse threads, with 10 threads per inch for a 1/4" diameter screw, decreasing to 3 1/4 threads per inch for a 1 1/4" lag screw. The length of a lag screw is measured from under the head to the threaded tip of the screw.
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Pilot holes
When using lag screws, it is important to first drill a pilot hole to guide the screw and prevent damage to the material or the screw itself. A pilot hole is a small hole drilled into a construction material to guide a larger drill or the insertion of a screw.
Drilling a pilot hole before using a lag screw is essential, especially when the screw is being installed near the edge of a piece of wood or into a thin piece of wood. This will prevent the wood from splitting. Pilot holes are not necessary when installing screws in very soft materials like gypsum board (drywall).
To drill a pilot hole, first select the right drill bit. For lag 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 is based on screw size and wood density. If the pilot hole is too small, the core of the screw may bind and lead to the screw breaking. If the hole is too large, withdrawal resistance may be compromised.
After measuring and marking the location of the pilot hole, install the drill bit in the chuck (the part of the drill that holds the bit in place). Tighten the chuck completely, ensuring the bit is centred, and turn the drill/driver's setting to 'drill'. Wear eye protection when using a drill/driver.
When drilling the pilot hole, set the drill/driver direction to forward and grip the drill with your forefinger on the trigger. Align the drill perpendicular to the surface, using a speed square as a guide. Start slowly, gently pulling the trigger, and then squeeze harder to accelerate.
Once the pilot hole is drilled, the screw can be installed. Fit the drill with a screwdriver bit and position the screw's tip into the pilot hole. Drive the screw in, being careful to angle it to follow the path of the hole.
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Ultimate tensile strength
Lag screws are heavy-duty wood screws used for fastening heavy lumber and other materials that must bear intense loads. They are also used for 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.
The ultimate tensile strength of a lag screw is 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). This ensures that the threads don't strip at the tensile strength limit of the material.
The thread penetration depth required to achieve ultimate tensile strength depends on the specific gravity of the material being fastened. For denser species with a specific gravity greater than 0.61, the thread penetration should be about seven times the screw diameter. For less dense species with a specific gravity less than 0.42, the thread penetration should be increased to 10 to 12 times the screw diameter.
The pilot hole for the body of the lag screw should match the screw size perfectly. For example, a 3/8-inch screw requires a 3/8-inch hole. The pilot hole for the threaded section, however, is based on both screw size and wood density. Undersized pilot holes may cause the screw to break during installation, while oversized holes may compromise withdrawal resistance.
To ensure the safety and security of your project, it is crucial to select the appropriate lag screw size and achieve the required thread penetration for the specific material being fastened.
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Head sizes
The head is the top part of a lag screw, which is either hex or square-shaped. Head sizes vary according to the size of the screw. For instance, a larger screw will have a larger head. The head size must also allow for sufficient space for wrench clearance.
The head size across flats, or wrench size, is the distance between two opposing flat surfaces on the head of a screw. This is also known as the head width across flats. The head width across corners measures the distance between two opposing corners on the head of a screw.
The basic, minimum, and maximum head heights are given in decimals. The head height is the length from the top of the head to the bottom of the head.
When selecting a lag screw, it's important to consider the head size in relation to the size of the screw and the amount of space needed for wrench clearance.
In addition to head size, other important dimensions of a lag screw include the thread dimension and the length of the screw. The thread dimension refers to the size of the threads that run around the cylinder of the screw, and the length is measured from under the head to the threaded tip.
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Frequently asked questions
Lag screws are heavy-duty construction fasteners with sharp threading that covers only a part of the bolt, allowing for adjustments as needed. They are generally used for wood-to-wood and metal-to-wood fastening, but can also be used for affixing items to concrete and masonry when combined with lag expansion shields.
Lag screws typically range from 1/4" to 3/4" in diameter, with lengths spanning from 1" to 16". The length is measured from under the head to the threaded tip of the screw.
It is important to refer to a lag screw size chart to ensure you are using the correct screw size for your project. Measure the width of the material you are fastening and select a lag screw that is slightly shorter. For example, if you are fastening two pieces of 2-inch thick wood, use a lag screw that is at least 4 inches long.
The most prevalent materials for lag screws are steel and stainless steel. Common finishes for steel include zinc plating and hot dip galvanizing, while stainless steel is recommended when corrosion is a concern.