Metal Roof Screws: Types, Pattern, Washer Compression and Replacement
The right screw, the right number in the right place, driven until the washer is just compressed and no further.
- Intermediate
- Part of the install, or 2 to 4 hours for a screw replacement pass
- Updated
The short answer
Metal roof screws are coated self-drilling or self-tapping fasteners with an EPDM or neoprene washer, usually number 10 or 12, long enough to reach 1 inch into wood. They go in the flat or rib as the panel manual shows, with extra screws at eaves, ridge and laps. Drive until the washer is slightly compressed and bulges evenly, not crushed, and replace any stripped screw with an oversize one.
On an exposed-fastener roof, screws are the waterproofing. Each one passes through the panel and seals with a rubber washer, and a roof can have thousands. Screw problems are the biggest service issue on this style of roof: washers crack from UV, screws back out as the metal moves, and crushed washers leak from day one. Even on concealed-fastener panels, screws through flanges and trims have to be right.
Getting them right is about four things: the screw itself, where it goes, how many, and how hard it is driven. The numbers below are common values. Panel manufacturers publish their own pattern, which takes priority, particularly on engineered wind load roofs.
Screws pair with the installation of the panel: see exposed-fastener panels for the whole sequence and ProLock snap-lock panels for flange fastening. Setting up the screw gun and clutch is covered in how to drive metal roof screws with a screw gun.
Tools
- Variable-speed screw gun or impact driver with clutch
- Depth-setting nose piece or magnetic bit holder
- Correct hex or square drive bits, spares on hand
- Magnet or sweeper for loose screws
- Replacement screws in one size larger
- Sealant for old holes where needed
Materials
- Metal roofing screws with EPDM or neoprene washers, coated and colour-matched if desired
- Oversize replacement screws
- Butyl or polyurethane sealant for repairs
- Touch-up paint for heads
Safety first
- Dropped screws on a metal roof roll and become ball bearings. Keep them in a pouch and sweep stray ones up before walking.
- A screw gun kicking off a wet panel can throw you off balance. Tie off and keep both feet planted.
- Fall protection is required at 3 metres (about 10 feet) under WorkSafeBC rules, and anywhere a fall could cause serious injury.
Step by Step
Step 1: Pick the right screw for the substrate
For wood purlins or plywood, a number 10 or 12 screw with a sharp point and coarse threads is standard, typically 1 to 1-1/2 inch long for exposed panels on a solid deck, and longer where strapping sits on foam or over shingles. Reach at least 1 inch of penetration into the solid wood, or through the strapping into the rafter if the manual requires it. For steel framing, use self-drilling screws that clear the steel by three threads.
The washer should be EPDM or a similar UV-stable rubber, fitted to a steel cap. Screw heads come in hex or square drive. Match the coating to the panel: a zinc or ceramic-coated screw with a painted head stays good for decades, while an uncoated screw rusts and stains the panel.
Tip: Buy a box with a few spare bits in it. Worn bits slip and scratch the panel paint, and they are the main cause of cam-out on a wet roof.
Step 2: Follow the pattern in the panel manual
A typical exposed-fastener pattern puts a screw at every purlin or strap line on each side of the rib, or in every second pan. The layout usually has a tighter pattern at the eaves, the ridge and rakes, where wind uplift is strongest. Laps get a screw at the lap rib every 24 to 36 inches to pull the overlap tight on the tape.
Do not invent a pattern. Panel manufacturers test their roofs at a pattern and spacing, and cutting back on screws voids the wind rating. On a roof with heavy wind exposure, such as a ridge top, the engineer or the manual may call for closer spacing.
Step 3: Start the screw square and use the nose piece
A screw that goes in crooked seats one edge of the washer and leaves the other open, which is a leak. Hold the gun so the screw is at 90 degrees to the flat. A depth-setting nose piece or a clutch-controlled driver keeps the screw straight and stops at the right depth.
Start slowly to let the point bite through the paint and metal, then run at full speed. A high speed start skates across the paint and scratches it.
Step 4: Stop when the washer is just compressed
This is the most important step. Under-driven, the washer is loose and leaks. Over-driven, it splits, squeezes out and is crushed flat, and the steel cap dishes the panel. The right point is a slight bulge around the washer, with the rubber still visible and the cap sitting flat on it.
Check the first screws visually and change the clutch or depth setting if needed. Re-check after changes in temperature, because a cold panel and a warm panel drive differently.
Watch out: Do not leave a screw under-driven intending to tighten it later. Loose screws wobble as the panel expands and enlarge the hole.
Step 5: Replace stripped or crooked screws properly
If a screw spins without holding, the hole in the wood is stripped. Remove the screw and put a bigger diameter one, such as a 14, in a spot about an inch away, or fill the old hole with sealant first and fit a longer screw if the strapping is deep enough. Do not reuse a stripped hole with the same screw.
Fill the old hole in the panel with a dab of sealant or use a screw with a larger washer that covers it. A screw hole left open is a leak.
Step 6: Check the pattern and inspect as the roof ages
Before finishing, walk the roof looking for missed screws, dished panels and washers that are crushed or not bulging. Sweep loose screws and shavings, because steel pieces rust and stain the paint.
As the roof ages, screws back out because panels expand and contract and the holes wallow. Schedule an inspection every few years and replace any with cracked washers or rust. Exposed washers commonly last 15 to 25 years depending on sun exposure. See exposed-fastener installation for context.
Screws for Trims, Closures and Flashings
Trim screws are smaller, often number 10 or 9 with a washer, and the pattern is shown with the trim. Eave trims, rake trims, ridge caps and wall flashings each have their own spacing, commonly 12 to 24 inches. Fasteners for ridge and hip caps go through the closure line, and fasteners for sidewall flashing go high on the wall leg so the cladding covers them.
Thermal Movement and Screws
Screws hold a panel that wants to expand and contract. On long runs, screws in wide slots or oversize holes let the panel move, while a tight hole bends or wallows. See thermal movement in metal roofs. Keep screw spacing to the manual and avoid extra screws the manual does not show.
Common Mistakes
- Over-driving until the washer splits or squeezes out, which leaks from the start.
- Under-driving and leaving the washer loose, so water gets past.
- Screws going in crooked, seating only one side of the washer.
- Uncoated screws that rust and stain panels within a few years.
- Cutting back on the pattern at eaves, ridges and rakes, where wind loads are highest.
- Reusing stripped holes so the screw never holds.
- Leaving loose screws and shavings on the roof, where they rust and stain the finish.
When to Call a Professional
If a roof is leaking at screws, or many are backing out or have cracked washers, a roofer can tell whether the fix is replacing screws or the panels have moved too much for that to work. Some roofs benefit from a systematic replacement with larger screws.
Replacement on a steep or high roof is a fall risk, so use a contractor if you cannot tie off safely.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Metal Roofing service or the towns we cover.
Metal Roof Screws: Pattern, Torque and Washers: Questions
What size screws are used for metal roofing?
Most residential and farm roofs use number 10 or 12 screws, 1 to 1-1/2 inches long for wood, with an EPDM washer under a steel cap. The screw should reach at least 1 inch into solid wood or three threads past steel. The panel manufacturer's manual gives the exact type, length and pattern for the profile.
How tight should metal roof screws be?
Tight enough to compress the rubber washer slightly so it bulges a little at its edge, and no further. If the washer is squeezed flat, split or pushing out, the screw is over-driven. If it is loose or the cap still spins, it is under-driven. A clutch or depth nose piece helps get it consistent.
How many screws does a metal roof need?
It depends on the panel and the wind load. A common pattern is a screw at every purlin on each side of the rib, with extra screws at eaves, ridges and rakes and a lap screw every 24 to 36 inches. Always follow the manufacturer's installation manual or engineered layout for the roof.
Why do metal roof screws back out?
Panels expand and contract daily with temperature, and that cycling works on the screws. Wood strapping also shrinks and swells with moisture, which loosens the hold. Screws backing out is most common on exposed-fastener roofs after 10 to 20 years. Replace them with a larger diameter screw in a fresh spot rather than retightening.
How do you fix a leaking metal roof screw?
If the washer is cracked or the screw is loose, remove it and fit a larger diameter screw a short distance away, filling the old hole with sealant. Sealant alone over a failed washer is a short-term fix. If many screws are failing, plan a full replacement pass rather than patching one at a time.
Related Guides
All Metal Roofing 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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