The cover table, then the three numbers it feeds: chair height, tier gap and bars per layer
← Back to the calculatorCover is not a finish tolerance. It is the dimension that decides how long the bar takes to corrode, and it is also the first term in three other numbers: the chair height that holds it, the clear gap between two mats, and the effective depth the section is designed on.
That is what the rest of this page does. Table first, then the chain from cover to chair height to tier gap to how many bars fit in a layer, because a cover that is right on paper and wrong in the chair is the same as no cover at all.
ACI 318-19 Table 20.5.1.3.1, specified cover for cast-in-place nonprestressed concrete. Read it by exposure first, then by member, then by bar.
| Exposure | Member | Bar | Cover (in) |
|---|---|---|---|
| Cast against and permanently in contact with ground | All | All | 3 |
| Exposed to earth or weather | All | #6 to #18 | 2 |
| #5 and smaller | 1½ | ||
| Not exposed to weather, not in contact with ground | Slabs, joists, walls | #14, #18 | 1½ |
| #11 and smaller | ¾ | ||
| Beams, columns | Primary, ties, stirrups, spirals | 1½ | |
| Shells, folded plates | #6 and larger | ¾ | |
| #5 and smaller | ½ |
The blue row is the one that gets missed. A footing poured directly against soil needs 3 in of cover on the sides and bottom, and most residential footing details are drawn with 2 or 3 in of bottom cover and nothing said about the sides. That is a code requirement, not a preference, and it is the reason a footing has to be 2 in wider than the cage plus the bar if the form is the soil itself.
Plant-controlled precast gets its own table, and the numbers are lower because the cover is not fighting a field-formed surface. ACI 318-19 20.5.1.3, second table.
| Exposure | Member | Bar | Cover (in) |
|---|---|---|---|
| Exposed to earth or weather | Wall panels | #14, #18 | 1½ |
| #11 and smaller | ¾ | ||
| Other members | #14, #18 | 2 | |
| #6 to #11 | 1½ | ||
| #5 and smaller | 1¼ | ||
| Not exposed, not in contact with ground | Slabs, walls, joists | #14, #18 | 1¼ |
| #11 and smaller | ¾ | ||
| Beams, columns | Primary reinforcement | db, min ½ max 1½ | |
| Ties, stirrups, spirals | ⅜ |
Bundled bars. Cover is the equivalent diameter of the bundle, need not exceed 2 in, and stays at 3 in where the concrete is cast against and permanently in contact with the ground. ACI 318-19 20.5.1.3.5. Three #5 bars bundled have an equivalent diameter of about 1.25 in, so the cover is 1.25 in and not the 1.5 in the table would give for a single #5.
Six steps. Every one of them is a number somebody on site has to set, and the last one is the only one that changes the design.
| Symbol | Meaning | Unit | Where it comes from |
|---|---|---|---|
| c | Specified cover | in | Table 20.5.1.3.1 |
| db | Bar diameter | in | ASTM A615 |
| h | Chair height | in | Step 2 |
| t | Member depth | in | Drawing |
| g | Clear gap between mats | in | Step 3 |
| s | Minimum clear spacing | in | ACI 318-19 25.2.1 |
| b | Member width | in | Drawing |
| N | Bars per layer | count | Step 5 |
1. c = table value for exposure, member and bar 2. h = c + db ÷ 2 (height to the bar centreline) 3. g = t − c_b − c_t − db_b − db_t 4. s = max(db, 1 in, 4/3 × dagg) 5. N = (b − 2c + s) ÷ (db + s), rounded down 6. As = N × Ab
Step 2 is where the chain usually breaks. Step 3 is where a slab that is 1 in too thin quietly loses a mat. Step 5 is where the cover you chose costs you a bar: an extra half inch of cover on a 12 in wide member is half an inch less clear width, and at 1.33 in spacing that can be a whole bar.
Chairs are sold by height, and two conventions are in circulation: height to the underside of the bar, and height to the centreline. They differ by exactly db ÷ 2, which is 0.25 in on a #4 and 0.5 in on a #8. Order the wrong one and you are a quarter inch short of cover across the whole slab.
| Bar | Cover (in) | Height to underside | Height to centreline |
|---|---|---|---|
| #4 | 1.5 | 1.50 | 1.75 |
| #4 | 2.0 | 2.00 | 2.25 |
| #5 | 1.5 | 1.50 | 1.81 |
| #5 | 2.0 | 2.00 | 2.31 |
| #6 | 1.5 | 1.50 | 1.88 |
| #6 | 2.0 | 2.00 | 2.38 |
| #8 | 1.5 | 1.50 | 2.00 |
| #8 | 2.0 | 2.00 | 2.50 |
The convention that sets cover is the first one: the underside of the bar sits at the chair height, so the chair height is the cover. The centreline convention is what you use when you are checking effective depth, because d = t − c − db ÷ 2 for the bottom mat. If a supplier's catalog does not say which, measure one chair.
The clear gap is what is left of the depth after both covers and both bar diameters, and ACI 318-19 25.2.2 puts a floor of 1 in on it.
| Slab depth | Clear gap between mats | Two #4 mats? |
|---|---|---|
| 4 in | 0.0 in | No — below the 1 in minimum |
| 5 in | 1.0 in | Exactly the 1 in minimum |
| 6 in | 2.0 in | Yes |
| 8 in | 4.0 in | Yes |
| 10 in | 6.0 in | Yes |
| 12 in | 8.0 in | Yes |
The 4 in row is the useful one. A 4 in slab with 1.5 in cover and #4 both ways has no clear gap: 4 − 1.5 − 1.5 − 0.5 − 0.5 = 0. Nothing about that detail is ambiguous, and no amount of chair ordering fixes it. The minimum depth for two #4 mats at 1.5 in cover is 2 × 1.5 + 0.5 + 0.5 + 1 = 5 in.
It depends on exposure and bar size, and there is no single number. Direct cast against earth is 3 in. Exposed to earth or weather is 2 in for #6 and larger and 1½ in for #5 and smaller. Not exposed, in a slab or wall, is ¾ in for #11 and smaller. All of these are ACI 318-19 Table 20.5.1.3.1, and the 3 in row is the one that surprises people: a footing poured against soil has to hold 3 in, not 2.
Enough for a #6 or larger bar exposed to weather, and enough for any bar cast against earth only if you are not in contact with the ground. Two inches fails the 3 in requirement for anything cast directly against soil, which is the single most common cover error on a footing. Two inches also fails the ¾ in interior slab requirement only in the other direction: it is more than needed, and that costs you effective depth.
To the outermost steel, which in a beam or column is the stirrup. A beam with a #3 stirrup and 1½ in of cover has the main bar centre at 1½ + ⅜ + half the main bar, so 2.19 in for a #5 main bar. Measuring cover to the main bar and getting 1½ in means the stirrup is outside the concrete, which is the failure mode that shows up as rust spots along a beam line.
It depends on which convention your supplier quotes. Height to the underside of the bar is 1.5 in, and that is what sets cover. Height to the bar centreline is 1.5 + db/2, so 1.75 in for #4 and 1.81 in for #5. Chairs are stocked in 1 in steps, so both land on a 2 in chair, but a 1 in chair would leave 0.75 in of cover on a #4 and fail.
At least 1 in of clear space, ACI 318-19 25.2.2. The gap you actually get is t − c_bottom − c_top − db_bottom − db_top. On a 6 in slab with 1.5 in cover and #4 both ways that is 6 − 1.5 − 1.5 − 0.5 − 0.5 = 2.0 in. On a 4 in slab the same arithmetic gives zero, which is the check telling you the slab is too thin for two mats, not that the gap is small.
Yes, but only across one boundary. Below #6 the exposed-to-weather cover is 1½ in and at #6 and above it is 2 in, so going from #5 to #6 adds half an inch of cover and takes it straight out of the effective depth. Interior slabs split at #14, which almost never appears in building work. The step is per bar, not per member.
The equivalent diameter of the bundle, but never more than 2 in, and never less than 3 in where the concrete is cast against and permanently in contact with the ground. A bundle of three #5 bars has an equivalent diameter of about 1.25 in, so 1.25 in of cover, not 1.5. ACI 318-19 20.5.1.3.5.