Sheet Grade and Body Construction in Steel Doors
The short answer: Sheet grade matters, but it does not determine a steel door's strength on its own. What really decides it is how the body is built: whether the leaf is one piece, whether the lock and hinge zones are reinforced, and what the leaf thickness and infill are. A "galvanized" door made from thin sheet is weaker than a thick, reinforced body.
What is DKP sheet?
DKP is cold-rolled steel sheet. Its surface is smooth, it holds paint and cladding well, and it forms easily, which is why it is widely used for frames and leaves. Its quality depends on the rolling quality; when a specific grade such as Ereğli A1 is stated, it means the sheet has been inspected for thickness tolerance and surface flatness.
What is galvanized sheet?
Galvanized sheet is steel coated with zinc. The zinc oxidizes before the steel does, protecting the metal beneath. It improves corrosion resistance on surfaces in direct, constant contact with moisture.
So which is better?
The question is framed wrong, so there is no clean answer. In a steel door, the sheet sits beneath paint, primer and the cladding layer applied over it. The door's outer surface is not bare sheet — it is finished with compact laminate, composite, wood or powder coating. In practice, corrosion resistance is decided by this cladding layer and the gasket sealing.
So before asking "which sheet", ask these:
- Is the leaf one piece (monoblock)? Multi-piece bodies give way at the joints.
- Are the lock and hinge zones reinforced? Forcing always starts at these two points.
- How thick is the leaf, in mm? Thickness directly affects resistance to bending.
- What is the infill, and what is its density? Sound and heat insulation come from here.
- Are there fixed pins on the hinge side? Without them, the hinges can be removed and the leaf lifted out.
- How many sides does the gasket run around? A missing gasket is where moisture and dust get in.
Bayrakdoor's body construction
Our doors are built to this specification:
| Frame | Ereğli A1 grade 2 mm sheet, omega-bend profile |
|---|---|
| Frame reinforcement | Lock and hinge zones backed with 3 mm |
| Leaf | 1.5 + 1.5 mm omega-bend, monoblock reinforced pan system |
| Leaf reinforcement | 3 mm angle reinforcement at the lock and hinge zones |
| Leaf thickness | 70 mm bare metal — 90 mm total |
| Infill | 80 kg/m³ rock wool |
| Fixed pins | 5 on the hinge side, 20 mm diameter |
| Gaskets | Rubber gaskets on 3 sides of the frame, 4 sides of the leaf |
Building the leaf as an omega-bend, monoblock pan system spreads load across the body instead of concentrating it at a single point. The 3 mm reinforcement at the lock and hinge zones targets exactly the two points where forcing begins.
How does cladding affect corrosion?
On an exterior door, the real protection is the cladding. Compact laminate and composite absorb no water and hold their color under UV for a long time; marine plywood suits sheltered entrances; solid wood and iroko give the most natural look and need annual maintenance. See the cladding options page for details.
What to ask for when buying
The quote should list each of these as separate lines: frame and leaf sheet thickness, reinforcement thickness, total leaf thickness, infill type and density, number of fixed pins, gasket layout and lock brand. The phrases "steel door" or "security door" carry no information on their own.
Related pages
So what should you look at?
The sheet-grade debate gets so much attention because it looks like the one measurable number. In reality, what determines a door's resistance to forcing is how the body is built.
Was the leaf bent from one piece?
A monoblock pan system means the leaf is bent and closed from a single sheet. The fewer the joints, the more rigid the door; every joint is a potential weakness.
Is there reinforcement, and how many mm?
The lock and hinge zones are the most heavily stressed parts of a door. 3 mm sheet and angle reinforcement in these zones makes a marked difference over an unreinforced door of the same sheet thickness.
How thick is the leaf?
Bare-metal thickness and total thickness are different things, and they are often confused. Ours is 70 mm bare metal and 90 mm total including the cladding.
What is the infill, and how many kg/m³?
Insulation comes from the infill, and density is critical. Rock wool at 30 kg/m³ and rock wool at 80 kg/m³ are not the same product; polyurethane foam, meanwhile, settles over time and leaves a void at the top.
Cladding matters more than the sheet
On a door facing an exposed facade, what determines its lifespan is not the sheet grade but the cladding over it. The right cladding fully isolates the body from the elements; the wrong one deforms within three years and exposes the metal beneath.
That is why, if the budget needs trimming, the right place is the cladding choice — not the sheet thickness or the reinforcement.
Frequently Asked Questions
Does a steel door have to use galvanised sheet steel?
No. In a steel door the sheet steel sits beneath the paint and the outer finish, so in practice it is the finish and the seals that determine resistance to corrosion. What really matters is how the door is built: a single-piece leaf, reinforcement around the lock and hinge areas, the thickness of the leaf and the density of the core fill.
Why does leaf thickness matter?
Thickness has a direct effect on how well the leaf resists bending and forcing. On Bayrakdoor doors the bare metal thickness of the leaf is 70 mm, and 90 mm in total with the finishes applied.
What are fixed pins for?
The fixed pins on the hinge side slot into the frame when the door is closed, so the leaf cannot be lifted out even if the hinges are removed. Our doors have 5 fixed pins of 20 mm diameter on the hinge side.
Is thicker sheet steel always better?
Only up to a point. As thickness increases the leaf gets heavier and puts more load on the hinges; a thick leaf with no reinforcement may perform worse than a thinner, reinforced one. What counts is the construction as a whole.
Doesn't the grade of sheet steel make a difference to rusting?
It does, but on an exterior door the body is already protected by the finish and the steel never comes into direct contact with the air. The risk lies in deep scratches, where oxidation can start and spread.
What does Ereğli A1 grade mean?
It means the sheet has been inspected and classified for thickness tolerance and surface flatness. Regardless of the type of steel, stating the quality grade gives you a measurable piece of information.