I know these tools have tons of uses on all kinds of sites, but the first thing I thought when I saw the new generation of cordless impact drivers was that these could be the ultimate deck-building tools. Manufacturers make big power and speed claims for these little tools so I had to check them out.

I tested the 9.6-volt Hitachi WH9DM and Makita 6990D; Hitachi's 12-volt WH12DM; the 14.4-volt Makita BTD150 and Milwaukee 9081-20 models; Panasonic's 15.6-volt EY6535NQKW; and Milwaukee's big 18-volt 9079-20 driver.

First, I evaluated their screw-driving power and speed using 3-inch deck screws. Then I drove 1/2" x 6" lag screws and tightened nuts on 1/2-inch carriage bolts. I also drove Tapcon screws into a block wall and solid concrete. Then I took a run at driving screws into metal studs.

Screw driving. Not surprisingly, there was no problem driving 3-inch screws into treated pine other than the noise. Impact drivers make a racket. The 12- and 14.4-volt models drove screws about as fast as my 15.6-volt drill/driver and faster than my 12-volt drill/driver on high-speed. The 9.6-volters weren't far behind. All the impact drivers (except Milwaukee's 18-volt tool, which is designed for heavier work) had plenty of power left once the screws seated, while my drills' power tailed off. All the batteries have ample run time for lots of screw driving.

Next, I used each driver on some light-gauge steel framing with wood-to-steel and steel-to-steel connections. The tools performed a lot like my cordless drills on high speed again, except they're a lot noisier.

Pushing hard on an impact driver will only give you sore hands; whether I pushed hard or just firmly guided the tools, the screws sunk at the same speed. But be sure to make good contact between the driver bit and the screw head, because it's easy to ream out the screw head.

Lags. In the shop, I piloted a 3/8-inch hole in a double stack of 2-by and sandwiched a piece of 1-by pine in there to simulate siding or sheathing for a 1/2" x 6" galvanized lag screw.

The 14.4-volt tools and Panasonic's 15.6-volt model took between 20 and 30 seconds to drive the big screws. It was so easy that I added another thickness of 2-by. It took longer (about 45 seconds) but the tools seated the screws securely. This test was a cinch for the 18-volt Milwaukee tool, which easily sunk and removed screws. I was happy to discover that as the screw head bottomed out, there was no sudden reaction torque like you get with a corded drill.

I then took the tools to a deck job so I could try them in old, hard framing. I drilled the same 3/8-inch pilot hole through the new PT ledger, 1/2-inch OSB, and into the old SPF rim joist. This time, the little tools took longer (an average of 30 to 60 seconds) and worked a bit harder to drive a lag, but they seated the screw enough to squeeze water out of the PT.

Among the compact tools, the 14.4 Milwaukee takes the power title -- it even drove one lag in 15 seconds -- but averaged around 30 seconds. The 12-volt Hitachi and 14.4-volt Makita run second and third, respectively, at around 33 seconds. Panasonic's 15.6-volt tool is close behind at 45 seconds. However, no tool could compete with the 18-volt Milwaukee for lag-driving strength. Its shape and mass are similar to corded impact drivers and it easily sunk screws in about 13 seconds -- even in solid 6-by.

Nut driving. Tightening carriage bolts on decks can be maddening. They always seem to end up right next to a joist or wedged in a corner. A corded impact driver is bulky and a ratchet can't always turn. All the cordless impact drivers, though, were perfect for this task.

They're so small they get into tight spaces easily and have no problem crunching the head of the carriage bolt right into the lumber, which keeps the connection tight as treated stock dries and shrinks. At 14-plus inches (with socket), the big Milwaukee tool was more unruly to handle and a bit long for working between joists.

Concrete. Finally, I used the tools to drive Tapcon screws into a block wall. The drivers effortlessly snugged a piece of 2-by to it. Then I drove them into solid concrete, which didn't work as well. With all of the tools, some of the screws sank and some didn't. However, the 12-volt Hitachi had enough power to break two screw heads off.

Conclusion. These little tools pack more driving wallop than a drill/driver of equal -- or even greater -- voltage. I'd rather do my screw and nut tightening with these than with my 1/2-inch corded drill or a corded impact driver. I'd pull them out first for driving lags in deck boards, too, as well as for other tasks.

Although I've heard some carpenters swear by these little guys for everything from hanging doors to driving screws in plaster, I'm not ready to give up my quiet cordless drill/driver yet. Cordless impact drivers are loud and you don't need their impressive speed and power for every screw-driving job.

Also, with the exception of the 18-volt Milwaukee, you can't drill holes with them. Milwaukee promises impressive drilling power with the 9079-20: It has an adapter that accepts special bits. The company claims you can easily drill a 1-inch hole through solid material like 6x6 or other larger timbers. If the smaller tools could drill holes, they could take one more job away from my cordless drill.

I'd happily add one to my cordless fleet, though. Despite their $250-plus price tags, I liked all the tools I tested. The 9.6-volt tools, the Milwaukee 14.4- and 18-volters, and Panasonic's 15.6-volt model come with two batteries, which would sway me at the checkout line. And, the Panasonic is the only tool that's both a drill/driver and impact driver -- a tremendous feature.

The Hitachi 12-volt is very comfortable to use and has a great combination of big power and compact feel, but the 14.4 Milwaukee has all that, too. The 18-volt Milwaukee is a different kind of tool and I'd seriously consider replacing my corded impact driver with this battery-powered bulldog.

Sources of Supply

Hitachi Power Tools
800-598-6657
www.hitachi.com/powertools

Makita USA
800-462-5482
www.makitatools.com

Milwaukee Electric Tool Corp.
877-729-3878
www.mil-electric-tool.com

Panasonic
800-338-0552
www.panasonic.com/cordlesstools

Spec out Cordless Impact Drivers on ebuild, the Professional's Guide to Building Products(TM).