A buyer from Tanzania sent me three different profile drawings from three different suppliers, all labeled "840 type roofing sheet." Two of the drawings had different rib heights. One had a different pitch. He asked: "How can they all be 840 type if the dimensions are different?" The answer is that "840" refers to the effective coverage width, not the complete profile specification. Two manufacturers can produce 840 mm coverage sheets with slightly different rib geometry — and they won't interlock on the same roof.
I'm Iris Xu, sales engineer at ZTRFM. I've been matching roofing sheet profiles to buyer requirements for 8 years. This article is the reference guide I wish every buyer had before placing an order. It covers the main profile families, their dimensions, and the naming conventions that cause confusion.
Profile Naming: Why It's Confusing
In the Chinese roll forming industry, roofing profiles are named by their effective coverage width in millimeters. "840 type" means the sheet covers 840 mm of roof width when installed. "900 type" covers 900 mm. The feed width (coil width before forming) is always wider — typically 1000 mm for 840/900 types.
The confusion arises because the coverage width doesn't specify the rib geometry. Two 840 sheets can have different:
- Rib height (25 mm vs 28 mm vs 35 mm)
- Number of ribs (2, 3, 5, or 7)
- Rib pitch (distance between rib centers)
- Bend angles at the rib shoulders
These differences mean sheets from different manufacturers may not interlock. Always verify the complete profile drawing, not just the coverage width.
The Main Profile Families
1. Corrugated (Wave Profile)
The oldest and simplest profile. A continuous sinusoidal wave with no flat sections between ribs. Think of the classic "tin roof" shape.
Common types:
- Type 828: 828 mm coverage, 76 mm pitch, 18 mm wave height. Feed width 1000 mm.
- Type 840 (corrugated variant): 840 mm coverage, 80 mm pitch, 20 mm wave height.
- Type 750: 750 mm coverage, smaller wave, used for wall cladding as well as roofing.
Material thickness: 0.3–0.8 mm. Corrugated is the easiest profile to form (fewest stands needed) and the cheapest to produce. It's common in rural roofing, agricultural buildings, and budget residential construction.
2. Trapezoidal (Ribbed Profile)
The most widely used roofing profile globally. Flat pan between raised trapezoidal ribs. The ribs provide stiffness and water-shedding capacity.
Common types:
- Type 840 (trapezoidal): 840 mm coverage, 5 ribs, 25–28 mm rib height. Feed width 1000 mm. The most popular roofing profile in China and Southeast Asia.
- Type 900: 900 mm coverage, fewer ribs, wider flat pan. Feed width 1000 mm. Popular for wall cladding (wider flat area looks better from a distance).
- Type 750 (V-125): 750 mm coverage, smaller ribs, used for both roofing and wall panels. Feed width 1000 mm.
- Type 820: 820 mm coverage, taller ribs (35 mm), higher load-bearing capacity. Used for longer spans.
- IBR (Inverted Box Rib): 686 mm coverage (762 mm feed), 32 mm rib height. Popular in Africa and Australia. Fewer, deeper ribs than 840 — better for longer spans.
Material thickness: 0.3–0.8 mm for roofing, 0.4–0.6 mm for wall cladding.
3. Standing Seam
Covered in detail in our standing seam article. Key dimensions: 600 mm coverage, 50 mm rib height, 360-degree Pittsburgh double-lock seam. Material thickness: 0.5–0.7 mm. Premium product — highest weathertightness, highest cost.
4. Tile-Profile (Decorative)
Profiles that mimic the appearance of clay or concrete roof tiles, formed from steel sheet. The sheet has raised sections that look like individual tiles.
- Coverage: typically 765–1,050 mm depending on the tile pattern
- Rib height: 20–30 mm (at the tile steps)
- Material thickness: 0.4–0.6 mm (prepainted coil required — the color is the finish)
- Applications: Residential roofing where aesthetics matter — villas, resort buildings, decorative facades
Coverage Width vs Sheet Width
This is the source of most buyer confusion. The coverage width is always less than the total sheet width because of the side overlap.
Example: Type 840 trapezoidal roofing. The total sheet width (after forming) is approximately 900 mm. When two sheets are installed side by side, they overlap by one rib (about 60 mm). The effective coverage is 900 – 60 = 840 mm.
When calculating how many sheets you need for a roof:
Number of sheets = roof width / coverage width (rounded up)
Not roof width / sheet width. Using sheet width will underestimate the number of sheets by 5–10%.
Choosing the Right Profile
| Application | Recommended profile | Why |
|---|---|---|
| Budget residential, rural roofing | Corrugated 828/840 | Lowest cost, easy to install, adequate for short spans |
| Standard industrial/commercial roofing | Trapezoidal 840 | Best balance of cost, strength, and water shedding |
| Long-span roofing (purlin spacing > 1.5 m) | Trapezoidal 820 or IBR | Taller ribs provide higher load-bearing capacity |
| Wall cladding | Trapezoidal 900 or 750 | Wider flat pan looks better from a distance; fewer ribs |
| Premium / hurricane zones | Standing seam 600 | No exposed fasteners; highest wind uplift resistance |
| Decorative residential | Tile-profile | Aesthetics of tile with weight and cost of steel |
Thickness and Material Recommendations
Profile selection is half the decision. Material specification is the other half:
- 0.3–0.4 mm: Budget markets, temporary structures, wall cladding. Adequate for mild climates. Will deform under foot traffic during installation.
- 0.5 mm: The sweet spot for most roofing. Stiff enough to walk on during installation, strong enough for standard purlin spacing (1.0–1.2 m).
- 0.6–0.8 mm: Heavy-duty roofing, industrial buildings, long spans. Can handle wider purlin spacing and heavier loads.
- Material: PPGI (prepainted galvanized) for most applications. PPGL (prepainted galvalume) for coastal or industrial environments — the aluminum-zinc coating provides better corrosion resistance. AZ150 coating weight for aggressive environments, AZ100 or Z275 for standard.
The Tanzania Buyer's Resolution
Back to the buyer with three different "840 type" drawings. I had him request the complete profile cross-section drawing from each supplier, including all dimensions: rib height, rib pitch, rib width, bend radius, and side overlap detail. Two of the three matched. The third had a different rib pitch — it was technically an 840 mm coverage sheet but from a different tooling design.
He ordered from one of the two matching suppliers and had his installer verify the profile against the drawing before installation. No interlock problems.
The lesson: "840" is a coverage width, not a complete specification. Always get the full profile drawing, and if you're buying sheets from multiple suppliers for the same project, verify that the profiles are dimensionally identical. A 2 mm difference in rib pitch will cause visible misalignment at the side laps.
