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A cordless drill can drive a lag screw into wood, but only if you drill a pilot hole and keep the screw straight. Driving a large lag screw into solid lumber without a pilot can stall a drill, snap the screw, or split the wood. For a few fasteners, a standard drill may be enough; for larger screws or repeated work, an impact driver is often easier to control.
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Check the material and fastener
Lag screws, also called lag bolts, are intended for wood. They need solid lumber behind the surface—not drywall, plaster, or a hollow wall by itself. If you’re fastening into masonry, use a suitable masonry anchor or a purpose-made concrete screw instead. For a hollow wall, choose an anchor rated for the load and wall material.
Check the screw’s diameter, length, and required washer. The screw should pass through the attached part and enter sound wood for the embedment specified by the fastener maker or project plan. Avoid assuming that a longer screw is automatically stronger: it can hit a pipe, wire, or the far side of a thin member. Before drilling into a wall, check for utilities and confirm what is behind the surface.
Choose a drill and bit
Use a sharp bit sized for the screw’s pilot hole. The right size depends on the screw’s diameter and the wood: hardwood generally needs a larger pilot than softwood. Consult the screw manufacturer’s pilot-hole chart when available. As a rough guide, the pilot should match the screw’s solid core diameter, not the diameter across its threads. Too small makes driving hard and raises the risk of splitting; too large weakens the threads’ grip.
A regular cordless drill-driver works well for occasional lag screws, especially smaller ones, provided it has enough torque and a sound chuck. Start in low gear, use a fully charged battery, and grip the drill with both hands. A cordless impact driver can make larger screws easier to drive, but its hammering action can overdrive or break a fastener if you keep the trigger down. Neither tool replaces the pilot hole.
For the pilot hole, choose a bit long enough for the intended embedment. A drill stop or a piece of tape on the bit helps mark depth, though tape is only a visual guide. If the screw’s smooth shank passes through the attached piece, that section may need a clearance hole so the threads pull the joint tight rather than binding in both pieces.
Prepare and drill
Mark the center of the fastener and clamp the parts together if possible. Keep the drill square to the surface; a crooked pilot makes the screw track off course and can leave the washer sitting unevenly. Drill at a steady speed without forcing the bit. Pull the bit out occasionally to clear chips, especially in a deep hole. Stop if the bit binds, overheats, or meets an unexpected obstruction.
When fastening close to a board end or edge, splitting is more likely. Follow the project or fastener guidance for minimum edge and end distances. If the wood is dry, dense, or prone to splitting, use the recommended pilot size and avoid placing the screw too close to another fastener.
Drive the lag screw
Fit a socket or hex driver that fully engages the screw head. A socket gives better contact than a loose-fitting bit and reduces the chance of rounding the head. Set a drill-driver to low gear and a moderate clutch setting, if available. Start the screw by hand for a few turns, then drive slowly while keeping the tool aligned. Increase the clutch setting only as needed.
With an impact driver, use short trigger bursts as the screw nears the surface. Stop when the washer is seated firmly; crushing wood fibers or burying the washer can weaken the connection. If the screw stalls, do not keep leaning on the tool. Back it out, check for a misaligned hole or packed chips, and verify the pilot diameter. Lubricating the threads with a small amount of wax can reduce driving effort, but don’t use oil or grease unless the fastener instructions allow it.
If the head twists off, the wood splits, or the screw spins without tightening, stop and assess the damage. A broken screw may require careful extraction; a stripped hole may need a different fastening plan, such as relocating the fastener or repairing the wood. Simply installing a larger screw in the same damaged hole is not a reliable fix.
Which tool fits the job?
| Tool | Best fit | Trade-off |
|---|---|---|
| Cordless drill-driver | A few small or medium lag screws, with a pilot hole | More control, but may stall on large screws |
| Impact driver | Repeated driving or larger screws where access is tight | Fast, but easier to overdrive or damage a screw |
| Ratchet and socket | Final tightening or a small number of fasteners | Slow, but gives direct feel and needs no battery |
If you already own a drill-driver and are installing only a few screws, there is little reason to buy another tool. Buy an impact driver when the workload, screw size, or awkward access justifies it—not as a substitute for correct drilling. For hex-head screws, a hex socket set can be a more useful purchase than a higher-powered drill.
Check the finished joint
Confirm that the screw is seated, the washer is flat, and the attached parts are drawn together without a gap. The fastener should feel secure, but don’t judge structural capacity by how hard it was to drive. Load requirements, wood condition, screw dimensions, spacing, and embedment all matter. For a railing, deck connection, overhead support, or other safety-critical job, follow the applicable plans and fastener specifications rather than estimating by feel.