Where Does the Screw Go in a Tuff Rib Ag Panel — in the Rib or the Flat Plane Beside It?

If you're installing Tuff Rib Ag panels, one question comes up again and again — from DIY homeowners, and honestly, from some contractors too: do I put the screw in the rib, or in the flat plane beside the rib?

If you've poked around online, you've probably seen debate about it. People arguing both sides. It's one of the most common questions we hear, and it's the kind of thing that can make you second-guess an otherwise straightforward job.

So let's settle it: the screw goes in the flat plane beside the rib. That's it. It's absolutely correct, and we'll explain exactly why.

The confusion didn't come out of nowhere. It goes back to continuous corrugated panel — the old, wavy metal panel that was a common roofing material roughly from the 1960s through the 70s and into the 80s.

If you picture continuous corrugated, it looks like waves: a gentle curve up, a gentle curve down, all the way across. Symmetrical. And here's the key detail — it had no flat part at all. Every surface was curved.

Back then, panels were fastened with nails that had rubber washers. And you could not put that nail in the valley at the bottom of the corrugation. It wasn't flat, so the rubber washer couldn't seal flat against it — it left a gap. In fact, it did something worse: it literally formed a dam. Water running down the roof would pile up behind the washer and go around it, looking for a way through.

So installers did the only thing that worked: they put the nail up in the rib. Even though the top of the rib was rounded, the soft, flexible rubber washer compressed enough to form a good seal. Installers tapped the nails in until snug.

There was another reason too. A smooth nail shaft could flex with expansion and contraction as the metal roof heated up in direct sunlight and cooled back down at night. The nail had to tolerate that movement over the life of the roof.

That's where the "put the fastener in the rib" advice comes from. It made sense — for a panel and a fastener that don't exist in modern roofs the same way anymore.

Here's the thing: metal roofing and fasteners have improved a lot since then. And Tuff Rib Ag is a completely different animal from old continuous corrugated.

Today, we don't drive nails. We drive screws with screw guns. And if you try to put a screw in the rib of a Tuff Rib Ag panel, it will easily crush and deform the rib. The rib is meant to hold the panel's shape and strength — not to take a fastener through its crown. And even setting the crushing aside, the rounded top of the rib wouldn't seal well with a screw anyway.

In short: the old rule was built for a rounded panel and a soft nail. Neither applies here.

Tuff Rib Ag has a much wider flat plane than old continuous corrugated ever had. The panel measures about 9 inches from rib to rib, which means the flat part is probably at least 7 inches wide. That's a lot of working room.

Put the screw in that flat plane, right beside the rib. That placement gives you three things:

1. It seals best. In the flat plane, the washer clamps tight against the substrate — say, a purlin — with only the thickness of the 28-gauge panel between them. A flat surface means a full, even seal.
2. It doesn't deform the panel. The rib keeps its shape, which is what gives the panel its strength and its clean, finished lines.
3. Screws don't flex. A screw is essentially a smooth metal shaft with an inclined plane — the thread — wrapped around it. That thread creates bracing support, so the screw stays rigid. It resists the expansion-and-contraction movement that made smooth nails work loose over time.

The washers changed right along with the fasteners — and it's a major upgrade.

They're no longer rubber. Modern metal roofing screws use EPDM washers — ethylene propylene diene monomer, if you want the full technical name. EPDM is built to last basically the length of the roof, roughly 40 years. Compare that to the 10 years or so a rubber washer from the 60s and 70s would give you.

That's the difference between a fastener you can forget about, and one that's starting to dry out and leak a decade down the road.

Simple version: in a Tuff Rib Ag panel, the screw goes in the flat, right beside the rib. It seals best. It doesn't deform the panel. And unlike an old smooth nail, the screw won't flex and work itself loose.

One honest note from our shop: if you're unsure about your roof's substructure, fastener schedule, or anything else about your specific installation, it's always worth consulting a roofing contractor before you start. We're glad to answer questions — that's part of what we're here for — but for on-the-roof decisions, a good local contractor is your best partner.