How to Measure Your Roof for Tuff Rib Ag Metal Roofing Panels

How do you figure out what length of metal roofing panel to order? It's a fair question. A custom order needs two specific numbers, and for Tuff Rib Ag it comes down to simple measuring and a little arithmetic.

At our Westfield, MA shop, every panel is rolled to order. There are no stock lengths, so the lengths you give us matter. Here's how to get them right.

This is the first of three measuring guides. This one covers Tuff Rib Ag (glossy, 28 gauge) and Textured Tuff Rib Ag (26 gauge). Both are Grade 80 steel, and the measuring is identical for the two.

For each roof plane (one flat surface of the roof), you need:

1. Panel length: from the ridge down to the drip edge at the eave.
2. Roof width: the length of the ridge, or the length of the eave. On a simple roof, they're the same.

Measure each plane separately and label it. A simple sketch with numbers beats a paragraph of description.

Tuff Rib Ag covers 36 inches (3 feet) per panel. Divide your roof width by 36 inches and round up.

• A 24-foot-wide roof: 288 inches ÷ 36 = 8 panels
• A 25-foot-wide roof: 300 inches ÷ 36 = 8.33, so you need 9 panels

At the top (ridge): Keep the panels back from the very peak by 1 to 2 inches at most, on both sides. Panels from opposite sides should never touch at the top. That gap allows for venting (if you want it) and, just as important, for thermal movement. Steel expands and contracts with temperature. If panels are installed touching on a cold day, summer heat can push them into each other, causing buckling or damage. So keep the 1 to 2 inch setback whether the ridge vents or not.

At the bottom (eave): A carpenter will typically install a fascia board along the rafter tails, and the drip edge goes onto that fascia. The panel should hang past the drip edge by no less than 1 inch and no more than 3 inches. Two inches is the sweet spot, and it's what we recommend.

Measure the top chord of the truss (or the rafter) from the very top down to the end of the top chord. With trusses, start where the top chords meet. With rafters, start from the middle of the ridge pole. Then add the thickness of the fascia board, whatever thickness is used on your building (1.5 inches thick in our example, though 3/4 inch is also possible). The exact number isn't the point. What matters is that you add it.

Now adjust the ends:

• Subtract up to 2 inches at the top (ridge setback)
• Add 2 inches at the bottom (overhang)

Those two adjustments roughly cancel each other out. So in practice:

Panel length ≈ top chord (or rafter) length + fascia board thickness

Shorten or lengthen by an inch to fine-tune within the 1 to 3 inch overhang range.

Say your truss top chord measures 8 feet (96 inches), and your carpenter installed a fascia board that is 1.5 inches thick.

• Top chord: 96 inches
• Plus fascia thickness: 1.5 inches
• Subtotal: 97.5 inches
• Minus 2 inches at the ridge: 95.5 inches
• Plus 2 inches of overhang at the eave: 97.5 inches

Your panel length is about 97.5 inches, or 8 feet 1.5 inches. Same answer as the shortcut. If your crew prefers a 1-inch overhang, take an inch off.

The ridge cap covers the gap at the top, and what goes under it depends on your building. You have two options:

• Ridge cap with foam closure strips: This seals the ridge tight. It does not breathe. Use it when you don't want venting, for example when the building has gable vents, or when spray foam insulation is against the underside of the roof deck.
• Ridge cap with mesh (vent) closure: This lets the ridge vent while keeping rain and snow out. Use it for most attic applications (any time there's an actual attic), barns, and cathedral-type ceilings with fiberglass insulation and proper baffles or vents against the underside of the plywood, since that cavity needs to breathe.

Either way, the 1 to 2 inch setback on both sides stays the same. Ask your installer which closure fits your building.

Trim keeps water out and finishes the edges. Common pieces include:

• Ridge cap: covers the peak where two planes meet
• Eave or drip edge: guides water off the bottom edge and into the gutter
• Rake trim: finishes the sloped gable edges
• Valley flashing: channels water where two roof planes meet at an inside angle
• Sidewall and headwall flashing: seals where a roof meets a vertical wall, like a dormer or an addition

We make trim to order, in standard profiles or custom profiles. Trim comes in standard lengths of 10 feet and 12 feet. Ten feet is the most common, and we use 12-footers only where 10s would create too much waste.

To figure out how many pieces you need, add up each trim run and divide by 10 feet, rounding up. For example, a drip edge run of 28 feet on one side and 28 feet on the other is 56 feet total. That's 6 pieces of 10-foot drip edge (60 feet). Do the same for each trim type: ridge cap, rake, valley, and wall flashing. Tuff Rib Ag and Textured Tuff Rib Ag use the same trim.

A deeper trim guide is coming soon.

Screws go in the flat plane right beside the rib, never in the rib itself. A screw driven through the rib can crush or deform it, and it doesn't seal well on the rounded top. The flat between ribs is 9 inches wide, with about 7 inches of truly flat surface, so the washer seats solidly. We have a separate post on exactly where the screws go: where to put your panel screws.

Modern panel screws use EPDM washers that last roughly 40 years, compared with about 10 years for the old rubber washers.

• Round up on panel counts for every plane
• Add a spare panel or an extra piece of trim for mistakes, damage, or surprises. Trim comes in 10-foot pieces, so rounding up usually gives you a little extra anyway
• Order extra screws. Dropped and stripped ones happen on every job

With a 24 to 48 hour turnaround on stock-color coil*, a small top-up order is easy if you come up short.

*24–48 hour turnaround applies to stock coil colors only. Custom colors may take longer.

You don't need perfect plans. A good request includes:

1. Glossy Tuff Rib Ag or Textured Tuff Rib Ag (or tell us about the building and we'll help you choose)
2. Color
3. Panel length and count for each plane, or your measurements and sketch
4. Trim list: the total length per trim type (or the piece count), and whether you need standard or custom profiles
5. Photos of the roof
6. Delivery address or pickup plan, and your timeline

Your measurements are a great starting point. But we recommend having your contractor or installer confirm them before you order, especially on steep pitches or roofs with several planes. Whoever is doing the install, whether that's a subcontractor, a contractor, or you as a DIY homeowner, should verify the panel lengths, because they know exactly how they'll install it.

We deliver across New England, New York, and New Jersey, or you can pick up at our shop in Westfield, MA. That's our only location.