How to build a retaining wall

Overview

Retaining walls hold back soil, and soil holds water. Get the base and the drainage right and the wall will stand for decades; skip them and it will bulge or tip within a season. For DIY, keep walls low — under about 3–4 ft — where proper block and drainage practices are enough. Anything taller is an engineer's job.

Step 1: Plan and check height limits

This is the most important step. Walls above roughly 3–4 ft, and any wall near a slope, driveway, or building, typically need an engineered design and a permit. Call 811 to have underground utilities located before digging. On sloping ground, confirm which side is the high side and how much drop the wall will hold.

Step 2: Dig the base trench

Excavate a trench about a third wider than the block and deep enough for the gravel base plus a buried first course — the first course should sit below the finished grade so the wall can't slide. Keep the trench bottom level across its width and along its length.

Step 3: Compact a gravel base

Fill the trench with crushed gravel in 2–3 in layers, compacting each layer with a plate compactor. A firm, level base is what stops the wall from settling and leaning. Estimate the volume with the gravel calculator — it includes waste.

Step 4: Lay the first course

Set the first course of blocks on the compacted base, fully buried and level in both directions. Use the concrete block calculator for the block count and the mortar calculator for mix. Spend the extra time getting this course perfect — every course above it follows.

Step 5: Build the courses with setback

Stack each course with a slight setback into the slope — the wall leans back against the soil it holds. Stagger the vertical joints between blocks like brickwork, and check plumb and level as you go. Many block systems use a lip or pin that sets the setback for you.

Step 6: Install drainage

This is the step that keeps the wall standing. Place a perforated pipe along the base behind the wall, sloping slightly toward daylight, and bury it in pea gravel so water filters down and away instead of building pressure. Add weep holes through the wall near the base. Use the pea gravel calculator to size the drainage material.

Step 7: Backfill, cap, and finish

Backfill behind the wall with drainage gravel at the base and clean fill above it, compacting in thin lifts. Cap the top course with cap stones set in adhesive or mortar. Finish the front with whatever suits the space, and keep drainage outlets clear of debris.

Cost and time

• Materials: roughly $8–$20 per square foot of wall face for blocks, gravel, and pipe.
• Time: 2–4 days for a low backyard wall, more for larger or excavated sites.
• Tools: shovel or mini-excavator, plate compactor, level, rubber mallet, tape measure.

Safety and permitting notes

• Walls over ~3–4 ft need engineered plans and usually a permit.
• Never build a retaining wall without drainage — water pressure is what topples them.
• Call 811 to locate buried utilities before digging.
• Set the first course below grade so the wall can't slide forward.
• Keep the base trench level and fully compacted.