Question 1

How many blocks do I need for a retaining wall?

Accepted Answer

Divide the wall height by the block height for courses, and the wall length by the block length for blocks per course, then multiply. A 20 ft long, 3 ft high wall of 12 × 6 in blocks needs 6 courses of 20 — about 120 blocks, plus caps.

Question 2

How deep should the base of a retaining wall be?

Accepted Answer

Bury the first course at least one-tenth of the wall height, and never less than 6 inches, on top of 6 inches of compacted base. That buried course is what stops the wall sliding forward under load.

Question 3

Do I need drainage behind a retaining wall?

Accepted Answer

Yes, always. Twelve inches of clean angular stone behind the wall with a perforated drain pipe at the base. Water pressure behind a wall is what pushes it over — far more walls fail from hydrostatic pressure than from the weight of the soil.

Question 4

How high can I build a retaining wall without an engineer?

Accepted Answer

Typically 3 to 4 feet, but this is set by local code and varies. Above that threshold you generally need an engineered design, a permit, and often geogrid reinforcement extending back into the retained soil.

Question 5

What is the setback on a retaining wall?

Accepted Answer

Each course steps back slightly, usually about an inch, so the wall leans into the slope. Most segmental blocks have a lip or pin system that sets this automatically. The batter is what makes the wall stable, so do not build it plumb.

Question 6

What is geogrid and when do I need it?

Accepted Answer

A synthetic mesh laid between courses and extending back into the retained soil, tying the wall into the mass behind it. It is typically required above 3 to 4 feet, and the required length and spacing come from an engineered design.

Question 7

How much does a retaining wall cost?

Accepted Answer

Segmental block runs $15–35 per square foot of wall face installed, including base and drainage. Engineered walls with geogrid cost considerably more. Material alone is usually a third to a half of the installed figure.

Question 8

Do I need a permit for a retaining wall?

Accepted Answer

Usually above 3 or 4 feet, and often at any height if the wall supports a surcharge such as a driveway or structure. Check locally before building — a wall that has to come down is far more expensive than the permit.

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