Cover width vs overall width is the cheapest mistake in the building-materials trade to make and the most expensive one to fix. A roofing panel quoted at 1,000 mm wide covers 840 mm once it is installed. Order enough panels for a 12-metre roof using the width on the quotation and you will be three panels short — on site, after the crane has left.
The two numbers are both real, both correct, and only one of them belongs in a quantity calculation. Here is how to tell which, and how to write a quotation that does not create the shortfall in the first place.
The Two Widths, Defined
Overall width (also called total width, or 展开宽度 in Chinese mill specs) is the edge-to-edge dimension of the finished panel — the width of the steel coil it was rolled from, minus nothing.
Cover width (also called effective width, covering width, or net coverage) is the width one panel adds to the finished surface after the side lap with its neighbour is taken up.
The difference is the side lap, and it is consumed on every joint except the last one. That means:
Panels required = roof width ÷ cover width, rounded up — never roof width ÷ overall width.
The gap between the two is far larger than most buyers assume. Profiles with deep, wide ribs lose the most, because a corrugation has to overlap by a full rib to seal.
Run the Numbers on a 12-Metre Roof
Take a single roof face 12,000 mm wide, and a common Chinese-profile roofing sheet whose code is YX35-280-840: rib height 35 mm, rib pitch 280 mm, cover width 840 mm, rolled from 1,000 mm coil.
| Method | Calculation | Panels ordered | Result |
|---|---|---|---|
| Using overall width (wrong) | 12,000 ÷ 1,000 | 12 | Covers 10,080 mm — 1,920 mm of bare roof |
| Using cover width (right) | 12,000 ÷ 840 = 14.29 | 15 | Covers 12,600 mm — 600 mm trimmed off the last panel |
Twelve panels versus fifteen. That is 25% more material than the naive count, and the gap is not waste — it is the amount that was never going to be there. The 12-panel order is not "a bit tight," it leaves nearly two metres of open deck.
Now add what a real order needs on top:
- End laps for any roof longer than one panel: typically 150–300 mm depending on pitch, taken off the length, not the width.
- A cut allowance of 10–15% for hips, valleys, penetrations and damaged sheets.
- The last panel, which is almost never a full one. Round up, always.
An order built on cover width plus 12% for cuts lands close. An order built on overall width does not get there even with the allowance added.
Cover Width by Profile
Every profile family loses a different amount. These are the ones that show up most often in export quotations:
| Profile | Overall width | Cover width | Loss | Notes |
|---|---|---|---|---|
| YX35-280-840 (roof) | 1,000 mm | 840 mm | 16% | Code's third number is already the cover width |
| YX38-300-900 (wall) | 1,130 mm | 900 mm | 20% | Wider cover, but more coil per panel |
| YX35-125-750 (decking) | 1,000 mm | 750 mm | 25% | Deep-rib decking is the worst offender |
| Ag panel / multi-rib | 38" (965 mm) | 36" (914 mm) | 5% | 1" side lap per side |
| PBR / R-panel | 36" (914 mm) | 34–36" | 0–6% | Depends on whether the purlin bearing leg is lapped |
| ⅞" corrugated | 39" (991 mm) | 34.67" (881 mm) | 11% | 4.33" side lap; needs ~12% extra material |
| ½" corrugated | 26" (660 mm) | 21.33" (542 mm) | 18% | 4.67" side lap; needs ~23% extra material |
| Standing seam | 18" (457 mm) | 12–18" | 0–33% | Cover varies by seam type and panel width option |
Two readings worth taking from that table. The wider-looking panel is not always the more efficient one — YX38-300-900 covers more per sheet than YX35-280-840, but it eats 1,130 mm of coil to do it, so per square metre of finished roof it uses more steel. And ⅞" corrugated beats ½" corrugated by roughly 11% on material despite looking like the heavier profile, purely because the side lap is a smaller fraction of a bigger sheet.
Where the Number Hides in the Product Code
Chinese profiled-sheet designations under GB/T 12755 encode it directly: YX (压型, profiled) then rib height, rib pitch, cover width. In YX35-125-750, the 750 is the cover width and the coil is 1,000 mm — a 75% utilisation rate stated openly in the part number, if you know that the third number is coverage rather than sheet width.
European and UK box profiles usually publish both figures on the datasheet, labelled "overall width" and "cover width." North American profiles are typically quoted by coverage alone — a "36-inch panel" is 36 inches of coverage off a 38-inch sheet — which is why the same phrase means different things depending on which side of the quotation you are standing on.
That inconsistency is the root problem. The supplier is not lying and the buyer is not careless; they are reading two different conventions with the same words.
Four Places the Shortfall Actually Bites
1. The quotation. A price quoted per panel with only the overall width shown lets the buyer build a bill of quantities that is 15–25% short. A price quoted per square metre is worse, because it is ambiguous — per square metre of steel or per square metre of covered roof? Those differ by exactly the loss column above.
2. The packing list. Panels ship in bundles counted by piece. If the site count was done on overall width, the shortage is discovered when the last bundle is opened, which is the most expensive possible moment.
3. Cladding and trims. Wall cladding runs are usually shorter and more frequently interrupted, so the rounding-up loss per opening is proportionally larger. Every window reveal creates a new run with its own part-panel.
4. Invoicing disputes. When the buyer's calculation and the supplier's calculation disagree by three panels, the argument is never resolved by the sales email. It is resolved by whichever party has a drawing that states which width was used.
Next Steps
If you sell profiled sheet, three changes remove this whole category of dispute:
- State both widths on every quotation and datasheet, labelled explicitly — "overall width 1,000 mm / cover width 840 mm" — never one alone.
- Do the panel-count arithmetic in the quotation itself, showing roof width ÷ cover width, rounded up, with the cut allowance as a separate line. A buyer who can see the division will not redo it wrongly.
- Put the section drawing next to the price. A cross-section showing the rib profile with the side lap dimensioned, the overall width and the cover width called out as two separate dimensions, and the end-lap requirement noted, answers the question before it is asked.
That third one is what converts a quotation into an order. It also has to be dimensionally honest: the cover width dimension has to run between the actual lap points on the actual profile, because the buyer will scale their bill of quantities off it. Software that snaps dimensions to the real geometry and exports the drawing at the size a marketplace or a tender portal requires produces a figure a quantity surveyor can rely on; a redrawn or AI-generated section will produce a number that looks like a cover width and is not one. The same rule governs how you label building material dimensions generally, and it is the core of any building material spec diagram. For products sold by the box rather than the panel, the parallel trap is coverage per box.
FAQ
What is the difference between cover width and overall width?
Overall width is the panel's edge-to-edge dimension; cover width is how much finished surface one panel adds once its side lap is taken up by the next panel. Only cover width belongs in a quantity calculation. The difference ranges from about 5% on a shallow-rib agricultural panel to 25% on deep-rib decking.
How many roofing sheets do I need?
Divide the width of the roof face by the panel's cover width and round up, then repeat for each face. Add 10–15% for cuts, hips, valleys and penetrations, and account for end laps of 150–300 mm on runs longer than one panel. Using overall width in that division under-orders by the full side-lap percentage.
Does the number in a YX panel code mean the sheet width?
No — in the GB/T 12755 designation YX35-280-840, the three numbers are rib height (35 mm), rib pitch (280 mm) and cover width (840 mm). The coil the sheet is rolled from is wider, typically 1,000 mm for an 840 profile. Reading the third number as sheet width is the most common way this code is misused.
Should roofing be priced per panel or per square metre?
Per panel is unambiguous. Per square metre requires you to state whether the square metres are of material or of covered area, because those two figures differ by the side-lap loss. If a quotation says "per m²" with no qualifier, ask which one before building a bill of quantities on it.
Why does a wider panel sometimes use more steel?
Because coverage efficiency, not width, decides material use. A 900 mm cover profile rolled from 1,130 mm coil uses 1.256 m² of steel per square metre covered; an 840 mm cover profile from 1,000 mm coil uses 1.190 m². The narrower-covering panel is the more efficient one.
Sources & References
- ⅞" corrugated vs ½" corrugated — overall width, coverage width and material premium
- Metal roof panel coverage width — profile coverage table and panel-count formula
- Box profile sheet sizes and cover width — UK convention for quoting both figures
- Metal Construction Association — technical resources for metal panel systems