Understanding Common Sizes and Thread Types of Bimetal Screws for Metal Roofing Applications
Why Bimetal Screws Matter in Metal Roofing
Metal roofs look simple on the outside, but the fastening system underneath is what actually keeps everything stable. Constant rain, wind load, and daily temperature swings put serious pressure on every screw. When a fastener fails, the result isn’t just a small leak — it can mean panel movement or even roof deformation over time.
That’s why bimetal screws have become the go-to choice for modern roofing systems. They combine two materials in one body — stainless steel for corrosion protection and carbon steel for drilling strength. This mix gives you both endurance and efficiency in one screw.
At Fasto, we’ve seen contractors use bimetal screws successfully in coastal factories, solar structures, and long-span warehouses. The idea is simple: install once, and don’t worry for years.
👉 Explore Fasto’s range of customized bimetal screws for roofing and structural applications.
Common Sizes and Dimensional Range
Bimetal screws are available in a range of diameters and lengths to fit different roof setups. The most common sizes fall between #10 and #14 in diameter, with lengths from 25 mm up to 150 mm.
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#10 x 25–50 mm – Light-duty use, such as thin sheet or cladding connections.
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#12 x 50–100 mm – Standard roofing panels fixed to steel purlins.
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#14 x 75–150 mm – Heavy-duty fixing for insulated or multi-layer systems.
Most roofing screws come with 5/16" or 3/8" hex washer heads, paired with bonded EPDM washers for a tight, watertight seal. While imperial sizing is still the industry norm, many Asian and European suppliers now also use metric equivalents like M5.5 and M6.3 for better compatibility.
A good rule of thumb: the thicker the material or insulation layer, the longer the screw you’ll need to achieve full thread engagement without over-penetration.
Thread Types and What They Do
Threads are often overlooked, but they determine how securely a screw holds to the metal sheet.
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Coarse Thread – Better for thin sheets; grabs quickly and avoids stripping.
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Fine Thread – Works best on thicker steel sections, offering tighter grip and vibration resistance.
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Self-Tapping – Cuts threads into softer materials like aluminum.
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Self-Drilling – Made for steel-to-steel connections; skips the pre-drilling step.
A quick installation tip: don’t use fine-thread screws on thin sheet metal — they can spin or under-tighten, especially if the drill speed is too high. Always check your sheet gauge and match the thread type before ordering in bulk.
Head Styles and Drill Point Selection
Head design affects both function and appearance. The hex washer head is still the favorite for roofing jobs since it allows solid torque transfer and easy sealing with EPDM washers.
Other options include:
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Pan Head / Truss Head – Low-profile choice for visible facade panels.
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Drill Points #2–#5 – Define how thick a material the screw can penetrate:
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#2 – For sheets up to 1.2 mm
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#3 – For mid-thickness materials up to 3 mm
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#5 – For structural steel up to 12 mm
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Choosing the right drill point matters more than most realize. Too small, and the tip burns out. Too large, and you lose holding power. Always consider the material stack and total penetration depth.
Material Structure and Corrosion Resistance
Each bimetal screw is made from two metals fused together by a solid-state welding process.
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Head & Upper Shank: Stainless steel (A2 or A4) — corrosion-resistant and ideal for outdoor or coastal projects.
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Drill Point & Lower Thread: Hardened carbon steel — provides cutting power and holding strength.
This fusion is what gives bimetal screws their edge. They resist rust where it matters most, around the exposed head, while keeping the lower section hard enough to drill through steel.
At Fasto, production follows strict thermal fusion control to make sure the stainless and carbon sections bond cleanly — avoiding weak weld lines or joint corrosion. In low-quality imports, that bond is often the first point of failure.
Choosing the Right Drill Tail for Different Sheet Thicknesses
Selecting the correct drill tail (point type) depends entirely on the base material thickness.
| Base Material Thickness | Recommended Drill Point | Typical Application |
|---|---|---|
| ≤ 1.2 mm | #2 | Thin aluminum or steel sheets |
| 1.2–3 mm | #3 | Steel purlins, wall panels |
| 3–12 mm | #5 | Structural beams, heavy frames |
Picking the wrong drill point can waste time on-site — screws dull faster, and holes misalign. Always verify sheet thickness before mass installation.
Engineer’s Insight:
“In coastal roofing, an A4 stainless + carbon steel bimetal screw can last 15 years or more with only minimal inspection. The drill point selection is just as important as the alloy itself — that’s where most early failures start.”
Conclusion — A Dependable Choice for Long-Term Roofing
Bimetal screws have changed how modern metal roofing systems are built. When you choose the right size, thread, and drill type, you don’t just get faster installation — you get a longer-lasting roof.
Fasto continues to support global contractors with high-quality, customizable bimetal screws engineered for consistency and field reliability. From industrial sheds to coastal plants, precision fastening starts here.
We supply high-quality, customizable fasteners to meet a wide range of project needs.
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