Skip to content

Masonry Guide: Block, Brick, Mortar & Retaining Walls

Masonry estimating is unusually predictable. Units come in standard sizes, mortar consumption per hundred units is well established, and the arithmetic barely changes from job to job. What varies — and what decides whether the wall is still standing in twenty years — is what happens at the base and behind it.

Nominal versus actual dimensions

Every masonry unit has two sizes. A standard concrete block is nominally 8 × 8 × 16 inches and actually 7-5/8 × 7-5/8 × 15-5/8. A modular brick is nominally 8 × 2-2/3 × 4 and actually 7-5/8 × 2-1/4 × 3-5/8.

The missing 3/8 inch in each direction is the mortar joint. Nominal dimensions are designed so that unit plus joint divides evenly into standard wall dimensions, which is why you estimate with nominal figures and lay to actual ones. It is also why a consistent 3/8-inch joint matters: run your joints at half an inch and by the tenth course you are an inch off your intended height.

Concrete block

A standard block covers 0.889 square feet of wall face including its joint — 1.125 blocks per square foot. That figure holds for 4, 6, 8, and 12-inch blocks, because all of them present the same 8 × 16 inch face. Only the wall thickness changes.

An 8-foot wall is exactly twelve courses, since each course is 8 inches nominal. This is not a coincidence; it is why 8 feet is such a common wall height.

Mortar runs about three 70-lb bags per 100 blocks at a 3/8-inch joint. If you are mixing from separate cement and sand rather than a pre-blended bag, allow roughly 1.1 cubic feet of masonry sand per bag equivalent.

Structural and retaining walls generally need grouted cells with vertical rebar at intervals your code or engineer specifies. A non-loadbearing garden wall usually does not, though grouting the top course improves durability at little cost.

Use the concrete block calculator for block and mortar counts.

Brick

Brick takes far more mortar than block for the same wall area — about seven bags per 1,000 modular bricks — simply because there are many more joints.

Bricks per square foot by format:

| Format | Face size | Per ft² | |---|---|---| | Modular | 7-5/8 × 2-1/4 | 6.86 | | Queen | 7-5/8 × 2-3/4 | 5.76 | | King | 9-5/8 × 2-5/8 | 4.61 | | Utility | 11-5/8 × 3-5/8 | 3.00 |

Almost all modern residential brick is veneer, not structural: a single wythe tied back to a framed wall with a drainage cavity behind it. That cavity is not optional detail, it is how the wall works. Brick is porous, water gets through it, and the cavity plus weep holes at the base is what lets that water drain back out. Weep holes mortared shut — which happens more often than it should — turn a working wall into a water trap.

Order from one batch and blend from several pallets as you lay. Brick colour varies between firings, and working a wall pallet-by-pallet leaves a visible band that cleaning will not remove.

Use the brick calculator and the mortar calculator.

Mortar types

| Type | Compressive strength | Where it belongs | |---|---|---| | M | 2,500 psi | Foundations, retaining walls, below grade | | S | 1,800 psi | Below grade, high wind, structural | | N | 750 psi | General above-grade work — the usual default | | O | 350 psi | Repointing soft historic masonry |

The instinct to use the strongest available mortar is wrong. Mortar should be weaker than the units it joins, so that movement cracks the joint — which is repointable — rather than the brick, which is not. Using Type M on soft historic brick destroys the brick.

Mortar stays workable for 90 minutes to two and a half hours. You may re-temper with a little water once inside that window; after it has begun to set, throw it out. Re-tempering stiff mortar is one of the most common causes of weak joints.

Retaining walls

Retaining walls fail from water, not from soil weight. The forces involved are counter-intuitive: saturated soil behind a wall exerts roughly twice the lateral pressure of the same soil drained. Nearly every leaning or bulging block wall you have seen failed because water had nowhere to go.

The essential build-up:

  1. Compacted base, 6 inches of crushed stone under the first course, level and thoroughly compacted.
  2. Buried first course, at least one-tenth of the wall height and never less than 6 inches. This is what resists the wall sliding forward.
  3. Drainage stone, 12 inches of clean angular rock the full height behind the wall.
  4. Drain pipe, perforated, at the base of the drainage stone, daylighted to somewhere water can escape.
  5. Batter, each course stepped back about an inch so the wall leans into the slope. Most segmental blocks set this automatically with a lip or pin. Do not build it plumb.

Height triggers engineering. Above 3 to 4 feet — the exact figure is local — you need an engineered design, a permit, and usually geogrid extending back into the retained soil. A wall supporting a driveway or structure often needs engineering at any height. This is the single most expensive corner to cut in landscaping, because the remedy for a failed wall is demolition and rebuild.

Use the retaining wall calculator for blocks, courses, drainage stone, and base.

Cold weather

Mortar that freezes before it sets loses most of its strength permanently, and the loss is not recoverable by curing later. Below 40°F you need cold-weather procedures: heated mixing water, protected materials, and enclosure or blankets to keep the work above freezing while it cures. Most masons simply stop.

Estimating checklist

  1. Measure gross wall area, deduct openings over about 10 ft².
  2. Multiply by units per square foot for your material.
  3. Add waste — 5% for block, 10% for brick, which needs more culling for colour.
  4. Calculate mortar from the unit count, not the wall area.
  5. For retaining walls, size the base and drainage stone separately — they are usually a larger volume than people expect.
  6. Check the footing requirement. Typically twice the wall thickness in width, 8 inches deep, below the frost line. Frost depth varies enormously by region.

Related calculators

Ad space