On exposed-fastener metal roofing, drive the screws through the flat of the panel, not the raised rib. The flat sits tight against the wood decking or purlin below, so the screw pulls the panel down and lets the rubber washer compress into a watertight seal. A screw in the rib bridges a 5/8 to 3/4 inch air gap, flexes over time, and works its hole loose. The one big exception is standing seam: those panels are held by hidden clips and take no field screws at all.
This guide covers the rib-versus-flat call by panel type, where each fastener sits, how many screws a roof needs, spacing, and how tight to run them so the washers seal instead of blowing out.
Rib or flat: where do metal roofing screws go?
Screw exposed-fastener panels through the flat, immediately beside each rib, on both corrugated and R-panel (also called PBR) roofs. The flat has solid backing, so the panel clamps down and the neoprene washer seats evenly. The old “screw the high rib” habit comes from agricultural through-fastened work and trades a marginally better water shed for a screw that spans an air gap and loosens. Manufacturer installation manuals from Fabral, McElroy Metal, and Western States Metal Roofing now specify the flat.
The rib-versus-flat argument only applies to exposed-fastener panels, the kind where you see the screw heads. It does not apply to standing seam, where fastening works differently. Match your method to the panel profile below before you pick up a driver.
| Panel type | How it fastens | Screw location |
|---|---|---|
| Corrugated (wavy) | Exposed fastener | Flat, in the valley next to the rib |
| R-panel / PBR | Exposed fastener | Flat, beside the major rib |
| Ag-panel / 5V | Exposed fastener | Flat (older specs said high rib) |
| Standing seam | Hidden clip or nail flange | No field screws in the panel face |
Why screwing the flat beats screwing the rib
Flat fastening wins on seal, holding power, and appearance. Because the flat rests on the substrate, tightening the screw draws the panel snug and squeezes the washer flat. On the rib there is no wood directly under the screw, so the fastener has to span roughly 5/8 to 3/4 inch of open space, which is where the long-term problems start.
- Better seal: the washer compresses against a backed surface instead of an unsupported bridge.
- Less flex fatigue: a rib screw rocks with every wind and thermal cycle, wallowing out its hole until it weeps.
- No crushed ribs: over-tightening a rib collapses the rib profile and dents the panel line.
- Cleaner look: flat screws sit in even rows in the pan; rib screws telegraph every misalignment.
The trade normally raised for rib fastening is water: a flat screw sits in the channel where water runs. In practice the sealing washer, set at the right torque, handles that. A properly seated washer on a flat outlasts a loosening screw on a rib.
Where standing seam fasteners actually go
Standing seam panels carry no screws through the visible face. The panel legs snap or seam together, and concealed clips fastened to the deck hold the panels while letting them expand and contract. Some nail-flange standing seam products screw through a flange that the next panel hides. If you can see screw heads on the panel surface, you are looking at an exposed-fastener roof, not true standing seam.
That distinction matters for cost and lifespan. Exposed-fastener roofs need those washers re-checked over time, while concealed-clip systems remove the field fastener as a failure point. For a fuller cost and profile breakdown, see the guide to standing seam metal roof cost and how it compares to exposed panels.
How many screws per square of metal roofing?
Plan on roughly 80 screws per square (100 square feet) for exposed-fastener panels, with individual manufacturers ranging from about 75 to 80. That figure covers both the field screws holding the panel down and the smaller lap screws that stitch the side laps. Use it to buy fasteners, then confirm against your panel maker’s chart, because rib width and purlin spacing shift the exact count.
To estimate a whole roof, take the roof area in squares and multiply. The math below shows a typical mid-size roof.
| Roof size | Squares | Screws at 80 per square |
|---|---|---|
| 1,200 sq ft | 12 | about 960 |
| 2,000 sq ft | 20 | about 1,600 |
| 2,400 sq ft | 24 | about 1,920 |
Buy 5 to 10 percent extra for stripped heads, trim, and closures. If you are still sizing the job, the shed metal roofing walkthrough shows how panel and fastener counts scale on a small structure.
What screw pattern and spacing should you use?
On exposed-fastener panels, put field screws in the flats at every purlin or every solid-decking line, spaced about 18 to 24 inches apart down the panel, and run lap screws 12 to 18 inches on center along the side seams. At the eave, ridge, and rake ends, tighten the spacing because those edges take the most wind uplift.
- Snap chalk lines across the panels over each purlin or framing member so your screw rows stay straight.
- Place a field screw in the flat beside each major rib along that line, keeping every row consistent.
- Add lap screws through the overlapping rib every 12 to 18 inches to stitch panels together.
- Double the fastener density at eaves, ridges, hips, and rakes, where uplift loads concentrate.
- Keep the pattern uniform panel to panel so the finished roof reads as clean rows.
Wind zone changes the spec. High-wind and coastal jurisdictions often require tighter spacing and rated fasteners, so check local code and the panel’s tested assembly. The metal roof installation guide walks the full sequence from underlayment through ridge cap.
How tight should metal roofing screws be? (torque)
Run each screw until the rubber washer bulges out slightly to the edge of the metal cap, then stop. That is the seal. A washer that is barely touched will leak, and one crushed past the cap edge is over-driven, splits early under UV, and lets water in. Aim for even washer compression on every screw rather than a set torque number, because gauge and washer size vary by product.
| Drive state | What the washer looks like | Result |
|---|---|---|
| Under-driven | Washer flat, gap under cap | Leaks, screw backs out |
| Correct | Washer bulges just to cap edge | Watertight seal |
| Over-driven | Washer squeezed past the cap, dimpled panel | Splits early, panel dents, leaks |
Use a screw gun with an adjustable clutch or depth-sensitive nose so you stop at the same point every time. A standard drill run flat-out will strip heads and blow washers. Choose the right fastener too: hex-head, gasketed screws long enough to bite at least 1 inch into wood, or self-drilling metal-to-metal screws over steel purlins, in a coating matched to the panel to avoid galvanic corrosion.
Common screw mistakes that cause leaks
Most exposed-fastener leaks trace back to the screw, not the panel. Screwing the rib, over-driving the washer, and missing the framing member underneath are the three that show up most. Each is easy to avoid once you know the seal you are aiming for and where solid backing sits.
- Rib fastening: spans an air gap, flexes loose, and wallows the hole over time.
- Over-tightening: crushes the washer and the panel, so the seal fails within a few seasons.
- Missing the purlin: a screw that misses framing holds in bare metal and pulls out under load.
- Angled screws: a fastener driven off-square tips the washer and opens one side.
- Wrong washer material: cheap washers dry out and crack; use EPDM or bonded neoprene.
If leaks have already started, the fix is usually re-driving or replacing loose screws with a larger-diameter fastener and a fresh washer. The metal roof leak repair guide covers diagnosing whether the source is the fastener, the lap, or a penetration. For more install fundamentals across panel types, the roofing learn hub collects the related guides.
Frequently asked questions
Do you screw metal roofing on the rib or the flat?
Screw exposed-fastener metal roofing through the flat, right next to the rib, on corrugated and R-panel roofs. The flat sits on solid backing, so the panel pulls down tight and the washer seals evenly. Screwing the rib leaves the fastener spanning a 5/8 to 3/4 inch air gap where it flexes loose and leaks over time. Standing seam panels take no field screws at all.
How many screws do I need per square of metal roofing?
Budget about 80 screws per square (100 square feet) for exposed-fastener panels, including both field and lap screws. Manufacturers range from roughly 75 to 80 depending on rib width and purlin spacing. A 2,000 square foot roof works out to around 1,600 screws. Buy 5 to 10 percent extra for stripped heads and trim, and confirm against your panel maker’s chart.
How far apart should metal roof screws be?
Space field screws about 18 to 24 inches apart down the panel, landing on each purlin or decking line, and run lap screws 12 to 18 inches on center along side seams. Tighten the spacing at eaves, ridges, hips, and rakes, where wind uplift concentrates. High-wind and coastal codes often require closer spacing and rated fasteners, so check local requirements.
How tight should you drive metal roofing screws?
Drive each screw until the rubber washer bulges just to the edge of the metal cap, then stop. That even compression is the watertight seal. Under-driven screws leak and back out; over-driven screws crush the washer and dent the panel, failing within a few seasons. Use a screw gun with an adjustable clutch so every fastener stops at the same point.
Why should you not screw metal roofing in the rib?
A rib screw spans the open gap between the top of the rib and the wood below, usually 5/8 to 3/4 inch. With no backing, the screw flexes with wind and thermal movement, wallowing out its hole until it weeps. It also over-tightens easily, crushing the rib and distorting the panel. Flat fastening avoids all three problems.
Do standing seam metal roofs use exposed screws?
No. Standing seam panels are held by concealed clips or a hidden nail flange fastened to the deck, with no screws through the visible panel face. The seams lock the panels together while allowing thermal movement. If you see screw heads on the panel surface, it is an exposed-fastener roof, not true standing seam.
Reviewed by The Roofing Brief Team. Last reviewed July 2026.