Retaining Wall Planning Guide

Most jurisdictions in Tennessee and Georgia set a threshold — commonly 4 feet of exposed face — above which a retaining wall needs engineered drawings and a permit, regardless of the material. Below that threshold, many walls can be built without formal engineering, though local code varies and it's worth confirming with your county before assuming. Height also determines footing depth and width: a wall that's twice as tall isn't twice the structure, it's several times the structure, because the soil pressure behind it increases non-linearly with height.

If you're not sure whether your planned wall crosses the engineering threshold, measure the actual exposed face height at its tallest point, not an average — that's what code typically references.

Walls that need a permit typically need stamped engineering drawings showing footing dimensions, reinforcement, drainage details, and sometimes a geotechnical soil report if the site has known problem soils. This isn't paperwork for its own sake — it's what protects you if the wall does fail and there's a liability question later, and lenders and insurers sometimes ask for it during a sale. Budget both the time (engineering review adds weeks) and cost (a stamped design isn't free) into your planning if your wall is tall enough to need it.

Soil holds an enormous amount of water weight, and a retaining wall built without a way for that water to escape is holding back hydrostatic pressure it was never designed for. This is the single most common cause of retaining wall failure we see — not undersized walls, but walls with no drainage behind them.

A properly built wall needs:

• Free-draining backfill directly behind the wall — typically crushed stone, not the native clay that was excavated out
• A perforated drain pipe at the base of that backfill, sloped to daylight or to a discharge point away from the wall
• Weep holes through the wall face at intervals, as a backup outlet if the drain system gets overwhelmed
• Proper grading at the top of the wall so surface water doesn't pool and soak straight down into the backfill

Skipping any of these to save cost is the most expensive mistake we see homeowners make on wall projects, because the failure shows up months or years later — bulging, leaning, or a section that's shifted — after everything's backfilled and landscaped.

The footing has to sit on undisturbed, adequately bearing soil — not on loose fill or topsoil. On sites with clay or poor-draining soil, footing design sometimes needs to go deeper or wider than a straightforward calculation would suggest, specifically because clay's bearing capacity changes with moisture content. This is part of what an engineer is accounting for on taller walls, and part of what an experienced excavation crew is watching for even on smaller ones.