Concrete Cover Thickness Lookup
Minimum cover from exposure class and member type, with every code correction applied.
Quick answer
With the default inputs: Exposure class Class 1 · Member type Slab / wall / shell · Concrete grade C30 etc.Minimum cover c 20 mm
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Result
Minimum cover c
20mm
| Tabulated value | 15mm |
| Bar diameter d | 20mm |
| Governed by bar diameter (c ≥ d) | 20mm |
| Exposure class | Class 1 (dry indoor) |
Based on GB 50010-2010 (2015 edition) Table 8.2.1 and notes 2-5. Cover is measured from the outer edge of the outermost bar to the concrete surface. Where durability demands are higher, the environment is aggressive, or the design documents say otherwise, the full code and design documents govern.
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Formula
| Lookup | c = Table 8.2.1 (exposure class × member type) |
| Grade correction | add 5 mm when concrete is C25 or below |
| Life correction | ×1.4 for a 100-year design life, rounded up to 5 mm |
| Precast | 5 mm reduction allowed, floors 15 / beams 20 |
| Bar check | c not less than the bar diameter d |
| Foundation | ≥40 mm with blinding, ≥70 mm without |
Class 2a, beam, C30, 50-year life, cast in situ, 20 mm bars: table gives 25 mm; no grade or life correction; the 20 mm bar does not govern, so cover is 25 mm.
How to work it out
- Establish the exposure class first. Class 1 is dry internal (living rooms, bedrooms, offices); class 2a is damp internal or non-severe external (kitchens, bathrooms, canopies); 2b is alternating wet and dry or severe external; 3a is de-icing salt and sea air; 3b is saline soil and coastal. Get this wrong and everything downstream is wrong.
- Select by element type: slabs, walls and shells differ from beams, columns and ties, because beams and columns must cover the links, adding 5 mm.
- Read the base value: class 1 gives 15 mm for slabs and 20 for beams; 2a gives 20/25; 2b 25/35; 3a 30/40; 3b 40/50.
- Adjust for strength: where concrete is C25 or lower, add 5 mm, since low-strength concrete carbonates faster.
- Adjust for design life: at 100 years, multiply by 1.4 and round up to the next 5 mm.
- Apply the two final checks: cover not less than the bar diameter, and at foundations not less than 40 mm with blinding or 70 mm without.
Where it goes wrong
- Using slab cover for beams and columns. Beams and columns must cover the links, adding 5 mm; using slab values leaves links exposed, and spalling follows within two or three years.
- Confusing whether cover is measured to the link or the main bar. The code measures to the outermost steel, which in beams and columns is the link, not the main bar.
- Classifying bathrooms and kitchens as class 1. These rooms are permanently damp and belong in 2a, taking cover from 15 to 20 mm. Five millimetres short means earlier corrosion.
- Treating basement walls and foundations as internal exposure. Below ground calls for 2b or 3a, especially with groundwater or aggressive soil.
- Using too few spacers. Spacers are normally at 500 mm centres in a staggered pattern; use too few and the steel sags during placing, making cover uncontrollable.
Material and cost
Every additional 5 mm of cover costs 1-2% more reinforcement because the effective depth reduces and more steel is needed. But rectifying corrosion from inadequate cover costs many times the design difference — repairing a basement soffit can run to several hundred CNY per square metre.
On site
- More cover is not automatically better. Excessive cover widens surface cracks, since crack width scales with cover; in class 2a it is unwise to exceed 30 mm.
- Foundation cover is 40 mm with blinding and 70 mm without — a large difference. Steel laid straight on soil with no blinding never achieves adequate cover.
- Precast elements may reduce cover by 5 mm, but never below 15 mm for slabs and walls or 20 mm for beams and columns.
- For coastal projects in classes 3a and 3b, add corrosion inhibitor or epoxy-coated bar; thicker concrete alone is usually insufficient.
Basis
- GB 50010-2010 (2015 edition) Code for design of concrete structures, minimum cover to reinforcement
- GB 50010-2010 (2015 edition) Code for design of concrete structures, durability provisions
- GB 50204-2015 Code for acceptance of construction quality of concrete structures, reinforcement
FAQ
Is cover measured to the stirrup or to the main bar?
To the outermost bar. Where stirrups exist the outermost bar is the stirrup, so cover runs from the stirrup surface to the concrete face and the main bar sits one stirrup diameter deeper. Spacers must be set to the stirrup, which is the most commonly mis-set item on site.
Why add 5 mm for C25 and below?
Lower-grade concrete is less dense and more permeable, so carbonation reaches the bar sooner and the passive layer breaks down. The extra 5 mm compensates. It is also why C20 is rarely used outdoors today - not for lack of strength but for lack of durability.
Is this table enough for coastal or chemical plant structures?
No. This table stops at class 3b. Class 4 (seawater) and class 5 (aggressive substances) are covered by separate codes and usually need coatings, corrosion inhibitors or sulfate-resisting cement as well. Chemical plants, treatment works and marine structures need the specific codes.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Concrete Cover Thickness Lookup. 2026-10-25. https://jianzhujisuan.com/en/concrete-cover
Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.