"Can one machine make both 2x3 and 3x4 downpipes?" A distributor in Argentina asked me this question last month. He wanted to serve the residential market with 2x3 inch downpipes and the light commercial market with 3x4 inch, but didn't want to buy two machines. The answer is yes — but with an important caveat about changeover time that most suppliers won't mention.
I'm Iris Xu, sales engineer at ZTRFM. Downpipe and gutter machines are a niche but growing product line — every steel building needs rainwater management, and metal downpipes outlast PVC by decades. This article covers the profiles, sizes, machine configuration, and what to ask before buying.
Downpipe Profiles and Sizes
Metal downpipes (also called downspouts or rainwater pipes) come in two cross-section shapes:
Rectangular Downpipes
The most common type. Sizes are specified in inches:
- 2 x 3 inch (51 x 76 mm): Residential — single-story houses, small roof areas
- 3 x 4 inch (76 x 102 mm): Residential and light commercial — two-story houses, medium roof areas
- 4 x 5 inch (102 x 127 mm): Commercial — warehouses, industrial buildings, large roof areas
Material thickness: 0.4–0.6 mm. Galvanized steel, prepainted steel (PPGI), or aluminum. The coil feed width varies by pipe size — a 2x3 downpipe might need 280 mm wide coil, while a 3x4 needs 350 mm.
Round Downpipes
Less common but used in European and some Asian markets:
- 75 mm diameter — residential
- 90 mm diameter — residential/commercial
- 110 mm diameter — commercial
Gutter Profiles
Gutters (the horizontal channels that collect rainwater from the roof edge) are produced on separate roll forming machines:
- K-style (ogee): The most popular in North America. The front face has a decorative ogee curve. Sizes: 5 inch and 6 inch (referring to the opening width).
- Half-round: Semicircular cross-section. Common in Europe and high-end residential. Sizes: 125 mm, 150 mm, 200 mm diameter.
- Box gutter: Rectangular, used on commercial and industrial buildings. Custom sizes.
Machine Configuration
A complete downpipe production line includes these stations:
1. Manual decoiler (3-ton capacity). Downpipe coil is narrow (280–400 mm) and light. A manual decoiler is sufficient for most operations.
2. Feeding guide. Aligns the strip and feeds it into the roll former.
3. Roll forming section. 21–26 forming stands. Shaft diameter typically 70 mm. Roll material: 45# steel with hard chrome plating (adequate for thin, light material) or Cr12MoV for longer life. Forming speed: 8–15 m/min.
4. Hydraulic cutting device. Cuts the formed downpipe to length. Cut accuracy: ±1.5–2 mm. The blade is profile-matched — each pipe size needs its own blade.
5. Bending/elbow machine. This is the station that makes downpipe production different from other roll forming. After the straight pipe is cut to length, it passes to the bending machine, which creates elbows (bends) at preset angles. The bending die forms a smooth curve — typically 45 degrees or 75 degrees — that allows the downpipe to navigate from the roof edge to the ground. Minimum distance from cut to bend: typically 250 mm (can be customized to 100 mm).
6. Necking/shrinking device. One end of each downpipe section is necked (reduced in size) so it slides into the next section. This creates a male-female joint that allows sections to connect without separate couplers.
7. Output table. Typically 2 meters long. Supports the finished pipe as it exits.
Typical machine footprint: 15 m long x 1 m wide x 1.5 m high. Machine weight: approximately 8,000 kg. Power: 5.5 kW main motor + 7.5 kW hydraulic station = 13 kW total.
Producing Multiple Sizes on One Machine
Back to the Argentina distributor's question. Yes, one machine can produce multiple downpipe sizes — but the roll set and cutting die are size-specific. To switch from 2x3 to 3x4:
- Change the roll set (swap all forming rolls) — 2–4 hours
- Change the cutting die — 30 minutes
- Change the bending die — 30 minutes
- Change the necking die — 30 minutes
- Change the decoiler spacer to match the new coil width — 15 minutes
- Test run and adjust — 30–60 minutes
Total changeover: 4–6 hours. If you switch sizes daily, that's 20–30 hours per week of non-production time. For the Argentina distributor, I recommended buying a second roll set and keeping it pre-mounted on a removable raft. This cuts changeover to 1–2 hours.
Alternatively, some suppliers offer a combo machine that can produce two sizes (e.g., 2x3 and 3x4) with a quick-change mechanism. The changeover time drops to 30–60 minutes, but the machine costs 30–40% more and has more complex tooling.
What to Check Before Ordering
Verify the local standard. Downpipe sizes and shapes vary by market. North America uses inch-based rectangular pipes (2x3, 3x4). Europe uses metric round pipes. Some markets use both. Make sure the machine's tooling matches your market's standard.
Check the elbow angle options. The bending machine should handle at least two angles (commonly 45 and 75 degrees). If your market uses other angles, confirm the die is available.
Confirm the material range. If you plan to form aluminum as well as steel, the rolls need to handle both materials. Aluminum is softer and requires different forming pressure. Some machines come with a material selection switch on the HMI that adjusts motor torque.
Ask about the PLC and HMI. A downpipe line is simpler than a roofing line, but the control system still matters. Delta PLC with a 7-inch HMI is standard. The HMI should support recipe storage for different pipe sizes, lengths, and elbow angles.
Plan for coil supply. Downpipe coil is narrow (280–400 mm). You can buy pre-slit narrow coil, or buy full-width coil (1000–1250 mm) and slit it yourself. Pre-slit is easier but costs 10–15% more per kilogram.
Downpipe machines are straightforward — simple profiles, thin material, moderate speed. But the bending and necking stations add complexity that a standard roofing line doesn't have. Make sure your supplier has experience with the complete line, not just the roll forming section.
