How to Drive Metal Roof Screws With a Screw Gun or Impact Driver
Straight, flush and snug. A washer squeezed just enough seals the hole. Too tight crushes it, too loose leaves a gap.
- Intermediate
- 10 minutes to set up and test
- Updated
The short answer
Drive metal roof screws with a variable-speed screw gun that has a depth-setting clutch, or an impact driver used with a light touch. Fit the correct bit or magnetic hex socket, hold the tool square to the panel, and stop when the sealing washer is compressed to the edge of the metal washer without bulging. Test on scrap, and recheck the depth as the day warms or cools.
On an exposed-fastener metal roof, every screw is a hole in the roof that has to be sealed. The seal is a rubber washer under a metal cap, and it only works when it is compressed the right amount. Too loose and the washer does not seal. Too tight and the rubber is squeezed out and cracks in the sun. A screw that is driven crooked, over-tightened or stripped is a leak waiting to happen. Because a roof can have thousands of screws, consistent driving matters more than speed.
This guide covers choosing between a screw gun and an impact driver, setting the depth, using the right bit or socket, driving the screws straight and handling the common problems. For the fastener choice itself, see metal roof screws, and for the panel system, how to install exposed fastener metal roofing. For cutting panels first, see how to cut metal roofing with shears and nibblers.
Tools
- Variable-speed screw gun with a depth-setting nose
- Impact driver with an adjustable clutch or torque setting
- Magnetic hex driver sockets, commonly 1/4 inch or 5/16 inch
- Bit holder and spare bits
- Scrap panel and scrap framing for testing
- Chalk line
- Safety glasses
Materials
- Metal roofing screws with sealing washers, of the length and type the panel supplier specifies
- Closure strips and butyl tape where the system uses them
- Touch-up paint for scratches
- Replacement screws
Safety first
- Wear safety glasses. Steel chips and screws that snap can fly off the panel.
- Keep your footing secure and stay tied off. A screw gun pulls you off balance when a screw strips or jumps, and painted metal panels are slippery.
- Keep your free hand clear of the screw and the panel edge. Sheet metal edges cut, and a driver that slips can go through a glove.
- Sweep up all shavings and dropped screws. They rust and stain the panel and cut feet.
Step by Step
Step 1: Choose the right driver
A screw gun turns at a lower speed and has a depth-sensing nose that disengages the bit when the screw reaches the set depth. This gives consistent results. An impact driver delivers fast rotation and hammering torque, which drives screws quickly but can over-drive them in an instant. Some impact drivers have torque or speed settings, and they work well if you release the trigger as the washer seats. Choose the tool you can control.
Battery tools are common on the roof. Cord tools need a long, rated cord kept off the panels.
Step 2: Match the screw, bit and socket
Most metal roof screws have a hex head, and a snug magnetic socket holds the screw and keeps the driver aligned. A worn or loose socket wobbles, damages the head and marks the paint. Use the size that matches the head, commonly 1/4 inch or 5/16 inch. Replace worn sockets. Screw length must reach the framing or purlins by the amount the manufacturer specifies, and the screw must have the right thread for wood or steel.
Step 3: Test and set the depth on scrap
A properly driven screw has its washer compressed slightly so the rubber just shows a small, even bulge at the edge of the cap, without being crushed or cracked. The screw head sits square and the panel is held tight against the framing. If the rubber is squeezed out, reduce the depth or torque. If there is a gap, increase it. Check on the same type of framing and the same thickness as the roof.
Tip: Keep a scrap with a correct screw to compare against throughout the day.
Step 4: Hold the tool square and drive straight
A screw driven at an angle leaves the washer compressed on one side and open on the other. Keep the tool in line with the screw, and use both hands on the tool where you can. Start slowly to seat the screw, and then speed up. Stop at the set depth or the moment the washer seats. Do not keep the trigger pressed after the screw is seated, which strips the thread or spins the washer.
Step 5: Place screws in the right location
Exposed fastener panels usually call for screws in the flat of the panel next to the rib or on the rib itself, depending on the profile, with a defined spacing along each purlin. Mark lines with chalk to keep the pattern straight. Extra screws at the eaves, rakes and ridge are common. See how to install exposed fastener metal roofing for the pattern.
Step 6: Deal with stripped, crooked and overdriven screws
Back out the faulty screw and look at the washer. A cracked or crushed washer is not a seal, so use a new screw. If the hole in the framing has stripped, move the screw a short distance and seal the old hole with compatible sealant, or use a slightly larger screw if the manufacturer allows. Do not leave a bad screw in place and count on the others.
Step 7: Clean up and recheck as conditions change
Steel shavings rust into the paint and leave stains. Sweep them off or use a magnetic sweeper. The rubber in washers is stiffer in cold weather and softer in the heat, and the depth may need a small change through the day. Check a few screws in the morning and again in the afternoon. Also check the torque on any screw that was driven early in the day.
Screw Gun Versus Impact Driver
- Screw gun: lower speed, depth-sensing nose, best consistency over thousands of screws.
- Impact driver: fast and compact, good with self-drilling screws into steel, but easy to overdrive.
- Drill with a clutch: workable when it has an adjustable clutch and a magnetic socket, but slower.
What a Good Screw Looks Like
The screw is straight, the head square to the panel and the washer evenly compressed, with a slight bulge of rubber around the cap but no cracking or extrusion. The screw is firmly in the framing and does not spin. The panel sits tight without a dimple or a dent around the head. Anything else is a screw to replace.
Common Mistakes
- Overdriving. It crushes the washer, cracks the rubber and dimples the panel.
- Underdriving. The washer never seals, and the panel can move.
- Driving at an angle. The washer is compressed on one side only.
- Worn sockets. They wobble, round the head and mark the paint.
- Leaving shavings on the panels. They rust and stain.
- Using the wrong screw length or type for the framing.
When to Call a Professional
If you find many leaking screw holes on an existing metal roof, backed-out screws, or rust stains around fasteners, have a roofer inspect it. Some fasteners can be replaced, but widespread problems may mean the panels have moved or the framing is damaged.
A new metal roof is a significant project. A professional crew will install thousands of screws to a consistent standard in a short time.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Replacement service or the towns we cover.
How to Drive Metal Roof Screws With an Impact or Screw Gun: Questions
Should I use a screw gun or an impact driver for metal roofing screws?
A screw gun with an adjustable depth-setting nose gives the most consistent results, because it disengages when the screw reaches the set depth. An impact driver is faster but easy to overdrive unless it has adjustable torque and you ease off the trigger. Many roofers use both, depending on the screw and the substrate.
How tight should metal roof screws be?
Tight enough to compress the rubber washer slightly so it seals, with a small even bulge at the edge of the cap, but not so tight that the rubber is squeezed out, cracked or the panel is dimpled. Test on scrap, compare against a correct sample and recheck as the temperature changes.
What happens if you overdrive a metal roof screw?
The washer is crushed and cracks, which breaks the seal and lets water in. It can also dimple the panel, strip the thread in wood or pull the screw out under load. The screw should be removed and replaced with a new one, and the depth setting should be corrected before you continue.
What size socket is used for metal roofing screws?
Most metal roofing screws have a hex head that takes a 1/4 inch or 5/16 inch magnetic hex socket, though it depends on the screw. Use the socket that fits the head snugly, and replace it when it wears, since a loose socket wobbles, rounds the head and can scratch the paint.
Why do metal roof screws leak?
The common causes are overdriven screws that crush the washer, underdriven screws that leave a gap, screws driven at an angle, stripped holes in the framing, and washers that have aged and cracked in the sun. Backed-out screws from thermal movement are another cause. Replacing or resetting the screw usually fixes it.
Related Guides
All Roofing Tools guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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