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Driving a lag bolt into wood is a job an impact driver can handle, but the right accessories matter. A hex-shank socket may be all you need for a small screw in softwood; a larger structural screw in dense lumber calls for a pilot hole, a properly sized socket, and a bit that can tolerate repeated impacts. The aim is to seat the fastener without stripping its head, snapping the bit, or splitting the wood.
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Check the fastener before buying accessories
“Lag bolt” often refers to a traditional hex-head lag screw with coarse wood threads. Many newer structural wood screws use similar threads but have a different head, such as a washer head or Torx drive. Match the accessory to the actual head: a socket for a hex head, or the specified driver bit for a Torx or other recess. A socket will not help with a recessed drive.
Check the package for the fastener’s diameter, length, drive type, and pilot-hole guidance. For a traditional lag screw, the pilot hole through the threaded portion is generally smaller than the screw’s outside thread diameter; the clearance hole through any unthreaded portion is larger. Wood species and screw dimensions affect the correct sizes, so use the manufacturer’s chart when available rather than guessing.
Accessories compared
| Accessory | Best for | Main trade-off |
|---|---|---|
| Impact-rated hex-shank socket adapter | Occasional hex-head lags | Affordable, but sockets can wobble and an adapter adds length |
| Impact-rated nut driver | Frequent work with one hex-head size | Compact and convenient, but limited to its fixed size |
| Impact-rated Torx or other matching driver bit | Structural screws with recessed drives | Good engagement when correctly matched; wrong size can strip the recess |
| Wood twist or brad-point drill bit | Pilot and clearance holes | Requires a separate drill, but offers better control than forcing the screw |
Choose a socket or driver bit that fits
For traditional hex-head lag screws, the simple option is an impact-rated hex-shank socket adapter with a socket that fits the head snugly. A loose socket rocks on the hex corners, rounds them off, and can slip when torque rises. Use a six-point socket where possible; it bears on the flats more securely than a twelve-point socket. Avoid chrome-plated hand sockets on an impact driver unless the manufacturer specifically rates them for impact use. They can crack under repeated hammering.
If you drive the same size hex-head fastener often, an impact-rated nut driver set is quicker and usually shorter than a socket-and-adapter combination. The fixed sizes are convenient, but less versatile. For recessed-drive structural screws, buy the exact impact-rated driver bit type and size specified for the screw. A bit that is worn or only nearly the right size can cam out, damaging the recess and making the screw difficult to remove.
Use the right drill bits for pilot holes
A pilot hole reduces the force needed to drive a lag screw and lowers the chance of splitting wood, especially near an end or edge. For dense hardwood, a large screw, or a fastener close to an edge, skipping the pilot hole is a common cause of splits, broken bits, and stalled drivers. A clearance hole in the top piece also helps when the screw must clamp two pieces together; without one, the threads can catch both pieces and leave a gap.
A set of wood drill bits for pilot and clearance holes is useful if you work with different screw sizes. Brad-point bits start neatly and help keep the hole centered; ordinary twist bits are cheaper and work fine for occasional, non-visible holes. Keep the hole straight and deep enough for the threaded length that enters the wood. If you need several holes at the same depth, a depth stop or tape marker makes repeat work easier.
Drive steadily, not at full throttle
Start the screw by hand for a turn or two, then seat the bit firmly in the drive. Begin at a moderate trigger speed and keep the driver aligned with the screw. Let the impact mechanism work rather than leaning hard on the tool. As the head nears the surface, ease off: an impact driver can bury a head, crush soft wood, or twist off a fastener quickly once the screw is nearly seated.
Longer fasteners and hardwood create more resistance, but that does not mean you should keep hammering through a stall. Stop if the driver struggles, the bit begins to slip, or the screw stops advancing. Back it out, check for a wrong or shallow pilot hole, and inspect the drive for damage. A fresh bit is cheaper than a ruined screw or a broken fastener below the surface.
When a drill is the better tool
A cordless drill is often easier to control for making pilot holes and may be the better choice for a small number of short lag screws in softwood. An impact driver is useful when the screws are larger, the material is dense, or repeated driving would strain a drill. Neither tool replaces a correctly sized hole or a well-fitting accessory. If the fastener manufacturer specifies a torque or installation method, follow that guidance, and stop when the head is seated rather than trying to gain extra tightness by driving harder.