Question 1

How many retaining wall blocks do I need?

Accepted Answer

Multiply the wall length (in inches) by the wall height (in inches) and divide by the face area of one block. For an 18 by 8 inch standard SRW block, the face is 144 sq in. A 20 ft long, 3 ft tall wall has a face area of 8,640 sq in, requiring about 60 blocks - plus an extra row buried below grade and capstones on top. Always order 5 to 10 percent extra for cuts and breakage.

Question 2

How deep should the base of a retaining wall be?

Accepted Answer

Industry rule of thumb is to bury one full course (block row) for every 8 inches of exposed wall height, with a minimum of 6 inches of buried wall and 6 inches of compacted base gravel underneath. A 4 ft tall wall therefore needs roughly a 12 inch deep trench - 6 inches of gravel plus one buried block course.

Question 3

Do I need geogrid for a retaining wall?

Accepted Answer

Most segmental retaining walls over 4 ft tall require geogrid soil reinforcement every 2 courses. Walls under 4 ft can usually rely on block weight alone, but check the manufacturer's design tables and your local code. Walls over 6 ft almost always require an engineered design and a permit. The geogrid extends back into the retained soil to lock the soil mass into the wall.

Question 4

How much drainage gravel goes behind a retaining wall?

Accepted Answer

Plan for at least 12 inches of clean 3/4 inch crushed stone behind the wall, running the full height. For a 20 ft long, 3 ft tall wall (with one buried course), that is roughly 80 cubic feet or 3 cubic yards of drainage stone. Always include a perforated drain pipe at the base to carry water away - hydrostatic pressure is the leading cause of retaining wall failure.

Question 5

How tall can I build a retaining wall without a permit?

Accepted Answer

Most jurisdictions allow unpermitted SRW walls up to 4 ft tall measured from the bottom of the footing to the top of the wall. Anything taller usually requires a permit and an engineered design. Some areas have stricter limits (3 ft) or require permits for any wall near a property line, slope, or structure. Always check with your local building department.

Question 6

What is the best gravel for a retaining wall base?

Accepted Answer

Use 3/4 inch clean (washed) crushed stone for the leveling base under the first course - it compacts well and allows water to drain. The drainage zone behind the wall should also be clean stone (no fines, also called chips or angular gravel). Avoid pea gravel, river rock, and crusher run with fines for these areas - they hold water and undermine wall stability.

Block Type	Face Size	Per 100 sq ft
Small (garden)	4 x 12 in	300
Standard (6 in)	6 x 16 in	150
Versa-Lok / Allan	6 x 16 to 18 in	134 to 150
Jumbo (8 in)	8 x 18 in	100

Wall Height	Geogrid	Embedment
Under 3 ft	Not required	n/a
3 to 4 ft	1 layer mid-wall	4 to 6 ft back
4 to 6 ft	Every 2 ft of height	6 to 8 ft back
Over 6 ft	Engineer required	Per design

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