How to Build Cinder Block Steps

Why block courses and the riser rule do not line up

Block is sold by nominal size and made smaller. CMHA's CMU-TEC-001 says specified dimensions are "typically 3/8 in. (9.5 mm) less than nominal dimensions," so an 8x8x16 block is actually 7-5/8 inches tall. Add the standard 3/8 inch mortar bed, which the Mason Contractors Association gives from TMS 602 along with a tolerance of plus or minus 1/8 inch, and each course rises exactly 8 inches.

The code limit for stairs serving a house is 7-3/4 inches per riser, with no more than 3/8 inch between the tallest and shortest riser in a flight (IRC R311.7.5.1, reproduced in Placer County's stair handout). If every step gets the same cap, the cap cancels out and each riser is still one course tall. Squeeze the joints to the thinnest the tolerance allows and you get 7-7/8 inches, which still fails. This is our arithmetic, not a published figure, but it is the reason so many block step builds get flagged.

So there are three honest ways to do it:

• Use the block as the core, not the face. Build the block a little low and set the actual tread heights with a poured concrete cap or shell. The block saves concrete and the poured part makes the risers legal. This is the most forgiving method, because the concrete absorbs whatever the block does not divide into.
• Use solid units on their side for a 6 inch course. CMHA lists nominal thicknesses of 4, 6, 8 inches and up, and a solid 6 inch unit laid on its broad face rises 5-5/8 inches plus the joint, so 6 inches a course. Six inch risers with 12 to 15 inch treads are comfortable outdoors.
• Check your local amendment. Some jurisdictions allow taller risers than the model code. Massachusetts and New York allow 8-1/4 inches, and Oak Park, Michigan's porch guidelines do the same, so an 8 inch course passes there. Ask the building department before you rely on it.

Garden steps that do not serve a door are a different case. IRC R311.7 exempts stairways "not within or serving a building, porch or deck," and the US Forest Service's trail guidance treats a 6 to 8 inch rise as reasonable, so 8 inch block courses work fine on a landscape path. Steps on a slope covers the proportions for that.

Lay out the steps

Measure from the finished landing or door sill down to the finished walk, divide by the number of risers, and build every step to that figure. The bottom riser is measured to the finished walk, not to the footing, and it has to match the others within 3/8 inch like every other step.

The rest of the geometry is the same as poured steps. Treads at least 10 inches, or 11 inches and up so you need no nosing. A clear width of at least 36 inches. A landing at the door at least 36 inches deep in the direction of travel and as wide as the door, sloped no more than 2 percent, per IRC R311.3 as reproduced by Indiana's building code office. A handrail at four or more risers. The stairs calculator has every dimension with its code section.

Because block comes in 16 inch lengths, a stoop that is 48 or 64 inches wide lays out with fewer cuts than one that is 54. Treads are easier: they are set by where the next course steps back, so a 12 inch tread means each course sits 12 inches behind the one below.

The footing comes first

Block steps attached to a house in a frost climate need a footing below the frost line, the same as poured steps. Minneapolis's exterior entry landing bulletin is blunt about it: "Frost depth footings are required for landings that are attached to a dwelling or garage that has frost depth or greater footings." The frost depth there is 42 inches. Minnesota's state porch handout says footings must extend to frost depth at the extremities of the porch.

A block stoop is heavier and more rigid than a wood one, and it is mortared together, so a footing that heaves under one corner cracks the whole thing rather than just tipping it. Get your frost depth from the building department and size the footing before anything else.

Freestanding garden steps on a slope are usually built on a compacted base instead. County Materials' step construction guidelines call for 6 inches of 3/4 inch crushed stone or road base compacted to 95 percent of Standard Proctor density, with fill compacted in 8 inch lifts.

Mortar, and what goes in the cores

Mortar. ASTM C270's selection table, reproduced by the Mason Contractors Association, recommends Type S for exterior masonry at or below grade, including walks and patios, with M or N as alternatives. Steps sit in the same conditions, so use Type S. The mortar calculator works out how much to mix.

Cores. You have two choices, and both are legitimate:

• Grout. For a solid, mortared stoop, fill the cores with masonry grout rather than concrete. CMHA's TEK 09-04A sets the slump at 8 to 11 inches, far wetter than concrete, with fine grout using only sand under 3/8 inch and coarse grout adding pea gravel under 1/2 inch. The International Masonry Institute explains why it is so wet: the high water content "is rapidly reduced because the masonry absorbs much of the water." The 3/4 inch stone in ordinary concrete is also bigger than coarse grout allows.
• Drainage stone. For dry-laid landscape steps, County Materials fills "all cores and 6" behind blocks with drainage material." Stone drains, grout does not, and on a hillside that matters more than strength.

The space inside a stoop's block perimeter can be filled with compacted gravel or clean rubble before the cap goes on. Quikrete's steps and ramps project guide says to fill with "gravel, broken brick, or chunks of concrete block," kept back from the form sides so it does not break through the surface. The block calculator counts the units, the mortar and the grout for any wall, which is a useful way to price the perimeter.

Capping the treads

Bare block makes a poor tread. The footnote to the ASTM C270 table warns that "Masonry exposed to weather in a nominally horizontal surface is extremely vulnerable to weathering," and a step tread is exactly that: flat, wet, walked on and salted. The open cores of a hollow unit also hold water. Cap every tread.

• Poured concrete cap. Form over the block and pour the treads, which also lets you set the riser heights exactly. Pitch each tread forward about 1/4 inch per foot so water runs off, which is Quikrete's figure and also the most the code allows: IRC R311.7.7 limits tread slope to 1 in 48, or 2 percent. Put a 1/2 inch expansion joint against the house.
• Cap units. For landscape steps, County Materials sets caps with a 1 inch overhang on the front and sides, glued down with masonry adhesive, and says to allow 48 hours before use.

For the concrete in a poured cap, steps exposed to weather need 3,000 to 3,500 psi and air entrainment in moderate and severe climates under IRC Table R402.2. What concrete to use for steps covers which bagged mixes qualify. Keep de-icing salt off new concrete for the first winter.

Precast landscape step units

Garden centers also sell solid concrete step units, typically a single block about the size of one riser and tread, meant to be stacked on a compacted base. Belgard's Landings step, for example, is 18 by 48 inches and 6 inches tall, with riser faces designed to nest together. These are a landscape product. They make good garden steps on a slope and are not intended to replace an entry stoop with a frost footing.

For full one-piece entry steps delivered by truck, see precast concrete steps. For the poured version, see how to build concrete steps.