Retaining Wall Design: How Builders Plan a Wall That Lasts
The decisions behind a block retaining wall in the order builders make them, with code thresholds, drainage and geogrid specs, and a worked example.
Retaining wall design is mostly settled before the first block goes down. When a wall bulges, leans or cracks, the cause is usually a call made on paper, or never made: a wall too tall for its block, water with no way out, or a driveway above that nobody counted as load.
Here are the decisions in the order we make them on a real plan, from layout and height through base, drainage, geogrid, corners, fences and permits, then who draws the wall, what a section drawing shows and a worked example from one of our plans. Still comparing wall types, costs and which option gives the best value? Start with our retaining wall ideas.
Quick answer
Good retaining wall design is a handful of decisions made in order: where the wall runs and how tall it gets, which block, how it sits (a 6 in. compacted base about 24 in. wide, first course buried), how water leaves (12 in. of clean stone and a 4 in. perforated pipe to daylight), and whether it needs geogrid and an engineer. Under the IRC, walls holding back more than 4 ft of soil, or more than 2 ft with a driveway or fence above, need an engineered design.
Step 1: Lay out the wall and set its height
Start on a site plan, not in the dirt. Mark where the wall runs, then the top-of-wall and bottom-of-wall elevations along its length. Together they give you the exposed height at every point and show where the base has to step. The wall’s total height adds a buried course and the base below it.
Decide early whether the wall cuts into the hill or holds back new fill. A cut wall retains soil that has settled for decades; fill has to be compacted in lifts, or the ground behind the wall sinks. Plan each end too: turned back into the slope as a return, or stepped down with the grade.
Builder’s note: the dig behind a wall is bigger than the wall. Geogrid at least 4 ft long runs back from the block, so plan on excavating 5 ft or more behind the face, and make sure trees, fences and the property line aren’t in that zone.
Planning a retaining wall?
Central PA: we build it. Anywhere else: get it designed.
In Clinton, Lycoming, Centre and Union counties we build it for a fixed price in writing, from $6,900 installed, with 33% down, never more. Anywhere else, our design studio lays out a retaining wall for your yard: a Groundwork Single Space plan ($349) gives you a scaled site plan (layout, sizes, materials, plants) plus a planting list.
Retaining wall design by height
Height drives nearly every other decision. A few numbers apply to almost every block wall; the table shows what changes as a segmental block wall gets taller, assuming level ground above and below and decent soil. The block maker’s charts and your local code have the final say.
| Exposed height | Base and burial | Drainage | Reinforcement | Engineer and permit |
|---|---|---|---|---|
| Under 2 ft | 6 in. compacted stone, first course buried | 12 in. clean stone; pipe if the soil stays wet | None | Rarely needed |
| 2 to 3 ft | 6 in. base about 24 in. wide, at least 6 in. of block buried | 12 in. clean stone plus a 4 in. perforated pipe to daylight | Usually none with full-size block | Engineer if a driveway, fence or other load sits above |
| 3 to 4 ft | Same base; bury deeper per the block maker | Stone and pipe, always | Often geogrid, depending on block, soil and slope | Engineer if loaded; permit likely under the model code, which counts the 4 ft from the bottom of the footing (Pennsylvania measures from the lowest grade) |
| Over 4 ft, or any surcharge | Per the engineer’s design | Per the design; outlets about every 50 ft on long walls | Geogrid at least 0.6H and 4 ft long | Engineered design, usually a permit |
A wall under about 3 ft with nothing heavy above it is a reasonable DIY project with the block maker’s guide in hand. Past that, or with any load above, it’s a job for a pro and often an engineer.
Step 2: Choose the block
Block retaining wall design starts with the unit. Heavier, deeper units hold more soil on their own, and each course steps back a set amount, called setback, so the wall leans into the hill. Most systems publish a gravity-wall chart showing how tall the block can go without reinforcement; the usual answer is around 3 to 4 ft.
Small garden-wall blocks are fine for beds, not banks. Big units go taller but make tight curves harder. Our comparisons of Allan Block vs. Versa-Lok and the best retaining wall blocks by brand cover the tradeoffs between brands.
Concrete and wood retaining wall design
Poured concrete is rigid, so it needs a reinforced footing below the frost line, steel sized for the height, and weep holes or a drain behind the wall; past 4 ft it’s an engineer’s drawing. Wood walls need ground-contact lumber tied into the slope with deadmen, and they typically last 10 to 40 years; This Old House puts ground-contact pressure-treated pine at up to 40. Expect a wood wall to be the first one you replace.
Step 3: Base, burial and frost
Dig a trench about 24 in. wide and lay at least 6 in. of crushed stone, compacted, so the pad extends about 6 in. past the front and back of the block. Bury the first course at least 6 in., deeper on taller walls or where the ground slopes away below the wall.
A block wall on firm soil doesn’t need to reach the frost line: CMHA treats segmental walls as flexible structures, and Allan Block’s embedment guidelines say the granular leveling pad can accommodate movement from freeze-thaw cycles, unless local code requires a deeper base. Poured concrete footings are different; look up the depth where you live in our frost line depth by state guide. On sloping ground, step the base up in full block heights and keep the block at every step fully buried. Allan Block’s installation guide shows the trench, base and compaction sequence.
Step 4: Drainage behind and above the wall
Water is what pushes walls over. Behind the block, place a column of clean, angular stone at least 12 in. wide for the full height. At the bottom, toward the back, lay a 4 in. perforated pipe that runs to daylight, with outlets about every 50 ft on a long wall.
Allan Block requires the pipe on reinforced walls, gravity walls over 4 ft and poorly drained sites. We put one behind nearly every wall, because it costs little and clay holds water. On top, cap the stone with a few inches of compacted soil, shape a shallow swale so runoff goes around the wall, and carry downspouts past it in solid pipe.
Rule of thumb: if you can’t find a place for the pipe to daylight, the wall isn’t designed yet.
Step 5: Geogrid and when an engineer is required
Geogrid is a stiff polymer mesh laid between courses and rolled back into compacted fill, tying the wall to a block of soil behind it. CMHA’s design tech note for segmental walls suggests grid at least 0.6 times the wall height and never shorter than 4 ft, with layers no more than 24 in. apart. Pull it taut, compact fill over it in lifts of 8 in. or less, and never drive on it.
The engineering line comes from the code. Since the 2015 edition, IRC section R404.4 has required an engineered design for walls retaining more than 48 in. of unbalanced fill, or more than 24 in. while resisting other lateral loads; ICC names vehicles parked above and fences on top as examples. A slope above the wall, a nearby building or pool, poor soil, standing water and terraced walls set too close together are the other usual triggers.
Our Groundwork plans are drawn by builders, not engineers or landscape architects. We lay out the wall, but our plans don’t include engineering or an engineering review: a wall over 4 ft or under a surcharge needs a licensed engineer’s stamped design before it’s built.
Retaining wall design software
Engineers size grid and check stability in programs such as CMHA’s SRWall 5 and Allan Block’s AB Walls 3D + Terraces, which Allan Block offers free to engineers and architects. For homeowners, the useful free tools are the block makers’ installation guides and gravity-wall charts.
Step 6: Corners, curves, steps and caps
Corners use corner units, and grid needs care there: each side of an outside corner is reinforced on its own, and one layer of grid should never sit directly on another. On an outside curve, the setback tightens the radius with every course, so check the curve at the top of the wall, not just at the base.
Finish every end with a return into the slope or step it down with the grade; never leave a raw end. Steps built into a wall need equal risers of 5 to 7 in. and 12 to 16 in. treads on the same compacted base. Caps go on last, glued with a concrete adhesive made for block.
Step 7: Plan fences and railings before you build
A fence on a retaining wall is part of the design, not an add-on. Posts never get anchored to caps or bolted to block: Allan Block recommends fence footings at least 3 ft behind the back of the wall, or post sleeves set in the reinforced zone while the wall goes up. Augering through a finished wall’s geogrid weakens it.
A fence also catches wind, and the IRC counts that as an extra load, so a wall over 24 in. with a fence on top needs an engineered design. Some cities require a guardrail on walls over 30 in. that sit within 3 ft of a walking surface. A vehicle guide rail gets its own footing.
Step 8: Permits, HOA and utilities
Under the model residential code, a wall up to 4 ft tall, measured from the bottom of the footing to the top of the wall, needs no permit unless it supports a surcharge. Many towns change that limit or add setbacks from property lines, so call the building department with your plan in hand.
An HOA may need to approve the look separately, and a call to 811 a few business days before you dig gets buried utilities marked for free; notice periods vary, so check yours in our 811 dig laws by state. You or your contractor handle all three; a design plan doesn’t replace any of them.
Who designs a residential retaining wall?
Who should draw your wall depends on how tall it is and what sits above it. On residential work there are three usual answers, and a lot of walls pass through more than one of them:
- The installer, working from the block maker’s guide: a short gravity wall on level ground with nothing above it. Block makers publish installation guides and gravity-wall charts for exactly this wall, and a good contractor builds from them.
- A designer or design-build contractor: lays the wall out on a scaled plan, sets top and bottom elevations, picks the block and drainage, draws the section and writes a quantity takeoff so every bid prices the same wall. Groundwork, our online retaining wall designer, does this with a Build-Ready plan ($1,795), which adds construction details and a quantity takeoff to the scaled plan. It is not an engineered design.
- A licensed professional engineer: any wall past the IRC’s 48 in. line, any wall over 24 in. with a driveway or fence above, terraces closer than twice the lower wall’s height, and walls on wet or poor soil. The engineer checks stability, sizes the geogrid and seals the drawings the building department reviews.
Pennsylvania adds a wrinkle. The state’s Uniform Construction Code measures its 4 ft permit exemption from the lowest level of grade to the top of the wall, not from the bottom of the footing, and the exemption doesn’t apply once the wall supports a surcharge such as a driveway. Check your own state in our retaining wall permit guide and building codes guide before the design is final.
What a retaining wall section drawing shows
A plan view shows where the wall runs. A section is a slice through it, drawn as if you cut the wall and looked at the cut end. It’s the drawing that carries the construction details a plan can’t: how deep, how wide, what goes behind the block and where the water leaves. A good residential section shows:
- Grades and heights: finished grade in front of and above the wall, the exposed height and the total height including the buried course.
- Base: the trench and compacted leveling pad of crushed stone (Allan Block calls for at least 6 in. of wall rock) and how much block is buried.
- Block and cap: the unit, its setback and the glued cap.
- Drainage: 12 in. of clean stone behind the block and a perforated pipe at the lowest point toward the back of the trench, vented to daylight about every 50 ft.
- Geogrid: each layer, the course it sits on and how far it runs back from the face.
- Fill: the reinforced zone of compacted fill, placed in lifts of no more than 8 in., and the retained soil beyond it.
- Loads above: a slope, fence posts, a driveway or parking, with their distance from the face.
On an engineered wall, the engineer’s section replaces the generic one and adds the numbers only a stability check can give: grid type, grid lengths and spacing, and the soil values the design assumes. The example below shows how those details read on a real plan.
Retaining wall design example: a 4 ft geogrid wall under parking
Sample plan 02 is a driveway-edge wall: about 38 ft of segmental block, 4 ft exposed, holding up a parking area, with a 3 ft return and a low end that steps down with the grade. Vehicles park within 3 ft of the top, which makes it a surcharged wall, so the plan calls for stamped engineering and a permit at any height and labels its geogrid lengths as preliminary.
Here’s how the plan answers each step above:
- Base: a 6 in. compacted leveling pad, 24 in. wide, with the first course fully buried.
- Drainage: no filter fabric; a 12 in. column of clean stone and a 4 in. perforated pipe to daylight at the low end.
- Geogrid: 6 ft layers on courses 1 and 3 and 8 ft layers on courses 5 and 6 under the wheel path, pulled taut, with no splices.
- Backfill: compacted granular fill in 8 in. lifts, with only a light compactor within 3 ft of the face.
- Ends and rail: the wall steps down with grade, with no raw ends, and a guide rail, if required, gets its own footing.
- Takeoff: about 160 face sq ft of block, 7 corner units, 25 caps, 880 sq ft of geogrid, 9 tons of drain stone and 40 CY of fill.
That takeoff is what lets two contractors price the same wall. Our online retaining wall design service draws this kind of plan from photos and measurements with a Build-Ready plan ($1,795), which adds construction details and a quantity takeoff, and on a wall like this it becomes the starting point for the engineer.
Common design mistakes
Most of what we cover in why retaining walls fail starts on paper. Watch for these:
- Measuring only the exposed height, then learning the wall needed a permit or an engineer.
- A drain pipe with nowhere to go, or downspouts emptying behind the wall.
- Geogrid that’s too short, spliced or driven on during backfill.
- Fence posts augered through the grid or anchored to the caps.
- Garden-wall blocks stacked into a 3 ft retaining wall.
- Backfill dumped in one pass instead of compacted in 8 in. lifts.
Go deeper: our guides to retaining wall drainage, how segmental retaining walls work, when a wall needs geogrid, retaining wall base depth and walls over 4 feet, plus our retaining wall calculator.
Building this in central PA? We work through these same steps on every wall we build, from layout and base to drainage and grid. See how we build a retaining wall in Lock Haven or a retaining wall in Williamsport (the same crew covers South Williamsport, Loyalsock and Montoursville). In Centre County, see how we build a retaining wall in State College.
Related: All our retaining wall guides in one place
FAQ
How tall can a retaining wall be before it needs an engineer?
Under IRC section R404.4, walls retaining more than 48 in. of soil, or more than 24 in. while also resisting loads like parked cars or a fence, must be designed to accepted engineering practice. CMHA also recommends an engineered design for segmental walls over 4 ft of total height, and your town may be stricter.
How deep should a retaining wall base be?
For a segmental block wall, plan on at least 6 in. of compacted crushed stone about 24 in. wide, with the first course buried at least 6 in. Block walls don’t need to reach the frost line; poured concrete walls do.
How long should geogrid be in a retaining wall?
CMHA suggests geogrid at least 0.6 times the wall height and never less than 4 ft, with layers no more than 24 in. apart. Walls with a driveway or slope above need longer grid, and final lengths should come from an engineer.
Can you put a fence on top of a retaining wall?
Yes, if it’s planned before the wall is built. Set posts at least 3 ft behind the wall or in sleeves placed during construction, never on the caps. Under the IRC, a fence on a wall over 24 in. adds wind load that calls for an engineered design.
What software do engineers use to design retaining walls?
Engineers use programs such as CMHA’s SRWall 5 and Allan Block’s AB Walls 3D + Terraces to size geogrid and check stability. For a small wall, a homeowner needs the block maker’s free installation guide and gravity-wall chart, or a scaled plan.
Do I need a permit for a retaining wall?
Under the model residential code, walls up to 4 ft tall, measured from the bottom of the footing to the top of the wall, are exempt unless they support a surcharge. Many towns change that limit or add setback rules, so call your building department first.
Who designs a residential retaining wall?
A short gravity wall with nothing above it is usually built straight from the block maker’s installation guide and chart. A designer or design-build contractor draws the layout, section and takeoff. Walls over 4 ft of retained soil, or over 24 in. with a driveway or fence above, need a licensed engineer’s design.
What does a retaining wall section drawing show?
A slice through the wall: the grades in front and behind, the exposed and total height, the base pad and buried course, the block and cap, the drain stone and pipe, any geogrid layers with their lengths, the compacted fill zone and any load above the wall.
Does Pennsylvania measure retaining wall height differently?
Yes, for permits. Pennsylvania’s Uniform Construction Code measures the 4 ft permit exemption from the lowest level of grade to the top of the wall, not from the bottom of the footing, and the exemption doesn’t apply when the wall supports a surcharge such as a driveway.
Sources: ICC, 2015 IRC significant changes, R404.4 retaining walls; CMHA, SRW-TEC-004 segmental retaining wall design; Allan Block, basic installation; Allan Block, proper embedment guidelines (frost and the leveling pad); 34 Pa. Code § 403.62, permit exemptions.
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Published by Muthler Landscaping & Construction, owned by Thayne Muthler (PA HIC #PA212584). How we research and check facts: editorial policy.