How to Use an Impact Driver: Long Screws, No Stripping

How to Use an Impact Driver: Driving Long Screws and Lag Bolts Without Stripping Them

If you have ever watched a long deck screw stop dead halfway into a stud, or heard the sickening snap of a lag bolt shearing off inside a post, you know the moment an impact driver earns its place in the workshop. A drill-driver can do a lot of jobs, but once fasteners get long, thick, or stubborn, the impact driver is the tool that finishes the work without a fight. The catch is that it is also easy to misuse. Used carelessly, it will strip screw heads, snap bits, and split wood just as fast as it drives.

This guide covers how an impact driver actually works, when to reach for it instead of a drill, and the techniques that keep your fasteners intact and your projects solid.

How an Impact Driver Is Different From a Drill

A regular drill-driver spins a chuck in one smooth, continuous motion. When resistance builds up, the motor strains, your wrist twists, and the bit usually cams out of the screw head. An impact driver works differently. Inside the tool, a spring-loaded hammer mechanism delivers thousands of small rotational impacts per minute on top of the spinning motion. Each impact is a short, sharp twist that breaks the screw free and drives it a fraction further.

Three practical differences follow from that design:

No kickback into your wrist. Because the impacts are internal and rotational, the tool does not try to twist out of your hand the way a drill does under load. You can drive a 150 mm structural screw one-handed without bracing yourself against the torque.

It uses a hex quick-release chuck, not a keyless chuck. Bits slide in and lock with a collar pull. That also means you cannot use ordinary round-shank drill bits in it — you need hex-shank impact-rated bits.

It is louder, and it is supposed to be. The rattling hammer sound is the mechanism working. If your impact driver goes quiet under load, something is wrong — usually the bit has slipped or the screw has bottomed out.

The one thing an impact driver is not good at is delicate work. It has no clutch like a drill-driver, so it will keep hammering until you let go of the trigger. That is why fine screws, small brass hardware, and anything you need to stop at an exact depth belong to the drill, not the impact driver.

When to Reach for the Impact Driver

Reach for it whenever the fastener is long, the material is dense, or the job involves a lot of screws:

Deck screws and structural screws (75 mm and longer) into studs, joists, and posts
Lag bolts and coach screws for framing, pergolas, and heavy brackets
Long screws into hardwood, where a drill would stall or cam out
Removing rusted, painted-over, or seized screws — the impacts break corrosion loose
Repetitive fastening like decking, fencing, and subfloor work, where speed and low fatigue matter

Stick with the drill-driver for pilot drilling, small screws under 40 mm, drilling holes, and any job where you need precise depth control, such as driving screws into hinges or mounting cabinet hardware.

Bits: The One Thing You Should Not Cheap Out On

The single most common cause of stripped screws with an impact driver is a worn or wrong bit. Standard drill bits and cheap screwdriver bits are not built for the hammer mechanism — they flex, twist, and shatter under impact loads.

Use impact-rated bits. They are made from tougher steel with a torsion zone — a slightly narrower section that flexes a fraction under impact instead of snapping. They cost a little more and last many times longer.

Match the bit to the screw head exactly. A PH2 bit in a PZ2 screw, or a slightly worn Torx bit in a Torx screw, will cam out no matter how good the tool is. Torx (star) and square (Robertson) heads hold up far better under impact driving than Phillips heads, which were designed to cam out at high torque. If you are buying screws for a big project, choose Torx-head screws and you will strip far fewer of them.

Replace bits early. The moment a bit looks rounded at the edges or starts slipping, throw it away. A fresh bit costs pennies; a stripped screw in the middle of a deck board costs you an hour.

Pilot Holes: When You Still Need Them

One of the great advantages of an impact driver is that it often lets you skip pilot holes in softwood. But “often” is not “always”:

Hardwood always gets a pilot hole, or at least a clearance hole near the end of the board. Dense timber splits under impact driving just as it does under a drill.
Any screw within 50 mm of the end of a board should be piloted, or the end grain will split and the screw will lose its grip.
Lag bolts over 8 mm diameter should always be piloted, with the pilot hole roughly 60 to 70 percent of the bolt’s shank diameter.
Near edges and in plywood, a pilot hole prevents the layers from delaminating and bulging.

For the pilot hole itself, use your drill-driver, not the impact driver. Drilling is the drill’s job.

Technique: Driving Long Screws Cleanly

Start slow. Squeeze the trigger gently to seat the screw and get the first few threads biting straight. If the screw starts at an angle, stop, back it out, and restart. A screw that enters crooked will never straighten itself out.
Keep steady, straight pressure. Push the tool firmly along the axis of the screw. Side pressure is what causes cam-out and stripped heads, especially with Phillips screws. Let the impacts do the work — you do not need to lean your whole body weight into it.
Let the impacts work, don’t fight them. As the screw meets resistance, the hammer mechanism kicks in and the sound changes to a rattle. That is normal. Hold your position and keep the trigger squeezed. The screw will keep advancing in small steps.
Feather the trigger near the end. As the screw head approaches the surface, ease off the trigger in short bursts. This is the moment most stripped heads and overdriven screws happen. With a little practice, you can stop a screw head exactly flush without burying it.
Stop at flush — or just below. For decking and framing, flush is right. For structural screws, follow the manufacturer’s guidance: some are designed to sit slightly countersunk, others must not be overdriven or they lose holding power.

Lag Bolts: The Heavy-Duty Routine

Lag bolts deserve a slower, more deliberate routine than screws:

Mark and drill a pilot hole through both pieces, sized to about two-thirds of the bolt shank.
Add a washer under the bolt head. Without one, the head will crush into the wood and you will lose clamping force — and you may never get the bolt out again.
Drive with a hex socket, not a screwdriver bit. A 6-point impact-rated socket grips the bolt head properly and will not round it off.
Apply a little wax or soap to the threads on long bolts. It sounds old-fashioned, but it noticeably reduces the torque needed and the risk of snapping the bolt in hardwood.
If the bolt stops turning, do not just hold the trigger down harder. Back it out a quarter turn, clear any debris from the hole, and drive again. A bolt that snaps inside a post is a miserable repair.

Removing Seized and Stripped Screws

The impact driver is also your best tool for getting stubborn screws out:

For rusted screws, start with a drop of penetrating oil and a few minutes of patience. Then set the driver to reverse and use short trigger bursts — the impacts break the rust bond without rounding the head.
For a screw that is already partly stripped, press hard, use a fresh bit one size up if it will fit, and go slow. The impacts often bite where a drill would just spin.
For a fully rounded head, switch to a screw extractor set or cut a slot with a rotary tool and use a flat bit. The impact driver has done what it can at that point.

Common Mistakes to Avoid

Using the impact driver for everything. Small screws, brass fittings, and plastic anchors will crack or strip under impact force. Keep the drill-driver within reach.
Skipping the right bit. A worn bit is the fastest route to a stripped head. Replace bits the moment they show wear.
Overdriving screws. Burying a screw head deep into the wood weakens the joint and looks terrible. Feather the trigger at the end.
Driving without a pilot hole in hardwood. The impact driver is powerful, not magic. Dense wood still splits.
Forgetting hearing protection. Impact drivers are loud, especially indoors. Ear protection is not optional for extended use.

Which Impact Driver to Buy

If you are buying your first one, the practical checklist is short. An 18-volt (or 20-volt max, depending on the brand’s labelling) model covers nearly every workshop job. Look for a brushless motor — it runs cooler, lasts longer, and delivers more power from the same battery. Variable speed with a responsive trigger matters more than the maximum torque number on the box; most home workshop jobs never need the headline torque figure. Finally, buy into the battery platform you already own. A bare tool on your existing batteries is almost always better value than a new kit in a different brand.

The Bottom Line

An impact driver is not a replacement for a drill — it is the partner that takes over when fasteners get serious. Use impact-rated bits, match the bit to the screw, pilot-hole hardwood and lag bolts, start slow, and feather the trigger at the end. Do those five things and the tool will drive screws you used to dread, without stripping them, snapping them, or splitting your work.