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Flat or Rib? Where Should You Install Your Metal Roofing Screws?
- Mason Buchette
- Jun 9
- 3 min read
If you’re gearing up to install a metal roof, you’ve likely stumbled upon one of the most fiercely debated topics in the metal roofing world: Should the screws go into the flat part of the panel, or right through the top of the rib?
Ask three different contractors, and you’ll probably get three different, incredibly passionate answers.
So, what’s the actual verdict? Let’s break down the pros, the cons, and the industry standard so you can fasten your roof with total confidence.
The Contenders: Rib vs. Flat
Before we dive into the "where," let’s clarify what we’re talking about. We are discussing exposed-fastener metal roofing (like Tuff-Rib). These panels are held down by screws that feature a metal washer and a rubber gasket designed to seal out water.
Option 1: Driving into the Flat (The Industry Standard)
Most major metal roofing manufacturers and structural engineers recommend placing the screws in the flat section of the panel, right next to the rib.
Why the Flat wins:
Solid Wood Contact: The flat part of the metal sits flush against the wooden roof decking or purlins. When you drive the screw in, it immediately bites into the wood, creating a tight, incredibly secure grip.
Proper Gasket Compression: Because the metal is backed by solid wood, you can easily torque the screw to the perfect tightness, allowing the rubber washer to compress evenly and create a watertight seal.
Wind Resistance: Fastening to the flat provides maximum hold-down strength against high winds because the metal can't easily bend or pull over the screw head.
Option 2: Driving into the Rib (The Old-School Way)
Go back a few decades, and almost everyone was screwing through the top of the rib. The logic seemed bulletproof: Water runs down the flats, so if I put the screw on top of the rib, it will never see water.
Why the Rib sounds good:
Water Shedding: It’s true that the top of the rib gets less water volume than the flat. Many installers view this as all the proof you need to install through the rib.
Different Fastener Styles: In the old days, metal roofing was installed using lead nails with a rubber washer on it. These nails had to be installed through the rib because they would back out so quickly. For many installers, this old-school method has carried over with updated fastener technology purely out of tradition.
Why the Rib falls short:
The "Air Pocket" Problem: There is an empty space between the top of the rib and the wood decking below. When you drive a screw through the rib, it’s incredibly easy to over-tighten it, which crushes the rib, distorts the panel, and ruins the look of your roof. This can also create detrimental ice-damming effects.
Weak Grip and Rocking: Because the screw is bridging a gap of air before hitting the wood, it has a longer unbraced length. Over time, as the metal expands and contracts with the weather, the screw can "rock" back and forth, widening the hole in the metal and loosening its grip on the wood.
Poor Seal: It is notoriously difficult to get a perfectly square, properly compressed seal on a narrow, sloped rib.
The Verdict: Go with the Flat
The consensus among modern building codes and manufacturers is clear: Install your screws in the flat. The structural integrity and superior seal of a flat-fastened screw outweigh the water-channeling risks, provided you install the screws correctly.
Because flat installation relies entirely on the rubber washer doing its job, execution is everything.
How to Install Screws Like a Pro
To ensure your flat-fastened roof never leaks, avoid these three common mistakes:
Installation Type | What it Looks Like | The Result |
Under-Driven | The washer is loose and can spin. | Leak Hazard: Water will slide right under the gap. |
Over-Driven | The screw is too tight; the rubber washer is crushed and squeezing out the sides. | Leak Hazard: The rubber will crack and degrade rapidly under the sun. |
Crooked | The screw went in at an angle. | Leak Hazard: The washer can't seal evenly on all sides. |
Final Thoughts
While screwing into the rib might seem like a clever way to dodge water, modern fasteners are engineered to handle the flats perfectly. By fastening into the flat, you ensure your roof can withstand heavy winds and thermal movement without backing out or buckling the panels.
This is such a helpful breakdown of a detail most homeowners never think about. When I planned my own roofing project, I had no idea screw placement mattered so much. I worked with home renovation los angeles specialists and they guided me through every detail. It is amazing how small choices affect long-term performance.