Key takeaways
- Carbon steel screws drill fastest and cost least, but rely entirely on their coating for corrosion resistance.
- Solid 304/316 stainless screws resist corrosion but cannot be hardened to drill steel; use them in aluminium or pre-drilled holes.
- Bi-metal screws combine a stainless body with a hardened tip and are the default for coastal, solar and long-life roofing.
Roofing screws spend decades in rain, sun and salt. The choice between carbon steel, stainless steel and bi-metal is really a choice about where corrosion resistance comes from and how much drilling the screw has to do.
Carbon steel with a coating
Case-hardened C1022 steel drills fast and is the most economical choice. Its corrosion resistance depends on the coating: electro-zinc reaches about 72 hours to white rust in neutral salt spray, while ceramic and zinc-flake coatings reach 500 to 1,000 hours or more. Once the coating is scratched by the driver or worn away, the steel underneath rusts.
Solid stainless steel
Austenitic 304 (A2) and 316 (A4) stainless steel cannot be hardened by heat treatment, so a solid stainless drill point is soft compared with a steel purlin. Solid stainless self-drilling screws work in aluminium and thin sheet only. They are excellent near the sea, but they are not a substitute for a structural screw into steel.
Bi-metal
A bi-metal screw welds a hardened carbon or martensitic tip (SS410, SS420 or SCM435) to a 304 or 316 body. The tip drills, then sits inside the joint while the stainless body and head face the weather. You pay more per piece than for carbon steel, but replacement labour on a failed roof costs far more.
| Carbon steel | Solid stainless | Bi-metal | |
|---|---|---|---|
| Drills steel | Up to 12.5 mm | Aluminium, thin sheet | Up to about 6 mm |
| Corrosion source | Coating | Base metal | Base metal (body) |
| Coastal use | Not recommended | Yes (316) | Yes (316 body) |
| Relative cost | Low | Medium | Medium to high |
Which one to specify
- Inland roofing with a 15–20 year design life: carbon steel with ceramic or AS 3566 Class 3 coating.
- Within roughly 5 km of the coast, or in industrial atmospheres: bi-metal with a 316 body.
- Solar arrays: bi-metal, because the screws must outlast the panel warranty.
- Aluminium-to-aluminium: solid 316 stainless.
Whatever you choose, ask your supplier for salt spray and hardness test reports for the actual lot, not a generic brochure figure.
Frequently asked questions
Will bi-metal screws stain the roof?
The exposed head and washer are stainless, so they do not produce rust streaks. The hardened tip is inside the joint and out of the weather.
Is 410 stainless the same as bi-metal?
No. 410 is a hardenable martensitic stainless with moderate corrosion resistance. Bi-metal uses 304 or 316 for the body and only uses a hardened alloy at the tip.
