Homeowner wants a 5-foot tall CMU retaining wall holding back the neighbor's yard, and a fence on top.

Critical Elements for a 5-ft CMU Retaining Wall (IRC 2021 R404.4 / ACI 530):

1. Engineered Design Required: At 5 ft retained height (exposed), this exceeds IRC prescriptive limits (typically max 4 ft unbalanced backfill). You need a stamped structural drawing. Do not build without it.

1. Foundation (Key to Survival): Width: Min. 24–30" wide (usually 2x wall thickness). Depth: Bottom of footing ≥ 12" below undisturbed grade (frost/depth per local code). Key/Shear Key: Mandatory. 12" deep x 12" wide key cast into footing bottom, centered under wall stem. Prevents sliding. Reinforcement: Continuous #4 or #5 bars top & bottom (2" clear), plus vertical dowels (matching wall verticals) extending 40d_b into footing.

1. CMU Wall Construction: Grout: Fully grouted all cells (not partial). Vertical Rebar: #4 @ 16" or #5 @ 24" o.c. (per design), centered in cell. Lap splices 48d_b min. Horizontal Rebar: Joint reinforcement (ladder/truss) every 2nd course (16") PLUS bond beams with 2-#4 @ 48" o.c. max. Mortar: Type M or S (ASTM C270).

1. Drainage (Non-Negotiable): Backfill: 12" min. clean ¾" crushed stone (No. 57) behind wall, full height. Pipe: 4" perforated PVC (SDR 35) at footing bottom, daylighted or to sump. Slope 1% min. Filter Fabric: Non-woven geotextile (Mirafi 140N equiv.) wrapping stone, separating from native soil. Weep Holes: 3" PVC every 4 ft (backup only; pipe is primary).

1. Waterproofing: Bituminous coating or peel-and-stick membrane on back face before backfill.

1. Fence Connection: Do not surface-mount. Core drill footing/stem for fence posts set in epoxy/grout, or weld post bases to embedded plates in bond beam. Fence wind load adds overturning moment.

1. Backfill Compaction: 95% mod. Proctor in 8" lifts. No heavy equipment within 3 ft of wall.

Red Flags: No permit/engineer, no keyway, no drain pipe, partial grout, native clay backfill, fence lag-bolted to block tops.

Bottom line: Hire a structural engineer for the design. Build exactly to those drawings. Inspect rebar placement before grout.

To ensure a 5-foot-tall CMU (concrete masonry unit) retaining wall remains stable for 10+ years — especially with a fence on top — it must be properly engineered and constructed to resist lateral earth pressure, hydrostatic forces, and surcharge loads. Here’s what’s required, based on IRC (International Residential Code) and NCMA (National Concrete Masonry Association) standards:

Critical Requirements:

1. Footing (Foundation): - Minimum 12" thick, extending at least 6" below frost line (check local code; often 12–24" deep). - Width: At least 2x the wall thickness (e.g., 16" for 8" CMU). - Reinforced with #4 rebar at 16" o.c. both ways (top and bottom).

1. Wall Reinforcement: - Vertical #4 rebar every 24" o.c., grouted solid in cells. - Horizontal bond beams (two #4 rebars) at top and every 4' vertically. - Critical: For walls >4 ft, vertical reinforcement is typically required by IRC R404.1.2 and NCMA TEK 14-7B.

1. Drainage (Non-Negotiable): - 12" wide gravel drain behind wall, wrapped in geotextile fabric. - Perforated drain pipe (4" PVC, sloped 1/8" per ft) at base, discharging to daylight or storm system. - Weep holes every 24" o.c. in first course above grade.

1. Backfill: - Use free-draining granular material (e.g., crushed stone or gravel) within 12" of wall. - Compact in 6" lifts to 95% Proctor density. - Never use native clay or expansive soil directly against wall.

1. Surcharge from Fence: - A fence adds ~20–30 psf lateral load. Treat as surcharge: - Increase wall design height by 1–2 ft equivalent (per NCMA). - Or, increase footing width and reinforcement accordingly. - Fence posts must not be set directly into the wall — use a separate post bracket or set posts in concrete behind the wall, clear of the CMU.

1. Waterproofing: - Apply bituminous coating or membrane to the back of the wall below grade to prevent moisture intrusion and efflorescence.

Red Flags (Requires Licensed Engineer/Pro):

• Wall >4 ft tall in seismic zones (check IBC/IRC seismic maps).
• Poor soil (clay, organic, or high water table).
• Slope behind wall >3:1.
• Fence adds significant wind load (e.g., solid privacy fence).

Bottom Line: A 5-ft CMU wall with fence on top must be reinforced, drained, and footed per code — not just stacked blocks. If soil conditions are poor or you