Awaiting expert review. This page covers safety or regulatory rules that change over time. It is kept out of search results until a qualified reviewer has checked it; the sources below are dated so you can verify each point.
Quick answer
A segmental retaining wall (SRW) is a retaining wall made of manufactured concrete units stacked dry, without mortar, that holds back soil by its own weight and, in taller or loaded walls, by layers of geogrid buried in the backfill.
NCMA calls SRWs gravity walls and describes two types, conventional and soil-reinforced. Drainage, a prepared base and height limits decide whether one stands. Engineering and permit rules are local, so this page names no design numbers except where a named source prints them.
Quick facts
The key facts about Segmental Retaining Wall, with sources:
- Wall type
- Gravity wall of dry-stacked concrete units; conventional or soil-reinforced 1
- Unit standard
- Units should meet ASTM C1372 (dry-cast SRW units) 1
- Reinforcement
- Horizontal geogrid or geotextile layers extending into the backfill 1
- Unreinforced height
- Typically up to 3 to 4 ft, or less in poor soil, per CMHA 2
- Permit trigger (model code)
- Retaining walls over 4 ft are not exempt from a permit; measured from footing bottom in the model text read 23
- Not the main drain
- Gravel fill and drain pipe alone are not the wall's primary drainage, per NCMA 1
- Who decides
- Local code first; NCMA says check it before any size project 1
Key takeaways
- A segmental retaining wall is a gravity wall: stacked concrete units, no mortar, held in place by weight and, when needed, by geogrid buried in the soil behind.
- Water is the enemy. NCMA says gravel fill and a drain pipe are not meant to be the primary drainage path, and FHWA says most failures of reinforced soil walls involve prolonged rainfall.
- Height is not just what you see. CMHA counts the buried part below finished grade, and the model permit text measures from the bottom of the footing.
- Past low heights, or with a slope, driveway or building above it, an engineer and a permit usually come into the job. Local code decides where that line sits.
- Do not swap geogrid, fill or drainage parts on an engineered wall without the designer's approval.
What a segmental retaining wall is, in plain terms
The National Concrete Masonry Association (NCMA) says segmental retaining walls are gravity retaining walls that rely primarily on their mass to resist the force of the soil behind them.
The units are manufactured concrete blocks placed dry, without mortar, and usually linked by concrete shear keys or mechanical connectors.
They can also be paired with horizontal layers of soil reinforcement that reach back into the backfill and widen the working mass 1.
That differs from a poured concrete or mortared wall. Because the units are dry-stacked, the wall can flex slightly and move water through its face.
NCMA says SRWs are typically built on flexible aggregate leveling pads, so the footing does not need to go below the frost line as long as the foundation soil can bear the load 1. Whether your local code agrees is a separate question.
Retaining walls as a whole are a hub topic on this site. The retaining walls service page covers the work, and the permit thresholds research covers the height and permit lines. This page covers only the segmental type: what it is made of and why each part is there.
How the parts hold back soil
NCMA lists the parts of a wall as the foundation soil, a leveling pad, the units, the retained soil, gravel fill and, for soil-reinforced walls, the reinforcement 1.
Each has a job. The foundation soil carries the load. The leveling pad spreads the weight of the dry-stacked column over a wider area.
The units make the face and, in gravity walls, most of the mass.
The gravel fill extends behind the units, helps compaction and lets incidental water drain away from the face 1.
In a soil-reinforced wall, layers of geogrid or geotextile run between courses and back into compacted fill.
NCMA says the units and the reinforced soil are then treated as one heavier gravity mass, and that geogrids are the most common reinforcement 1.
This is why a wall can be taller than blocks alone would allow. It is also why the geogrid, the fill and the units are chosen as a system.
NCMA says materials on an engineered wall cannot be swapped without the designer's approval, and that not every geogrid works with every block system 1.
Batter, the backward lean of the face, adds stability. NCMA says increasing the batter generally increases the stability of the wall, and that it comes from the setback between courses, often fixed by the unit's lips, keys or pins 1.
- Finished grade and cap (optional)Slope for positive drainage at the top
- Segmental unitsDry-stacked, meeting ASTM C1372, with shear keys, lips or connectors
- Geogrid or geotextile layersBetween courses, extending back into the reinforced soil
- Gravel fill and drain pipeBehind the face; incidental water only, not the primary drain
- Reinforced (infill) soilCompacted structural fill to the tail of the reinforcement
- Leveling padCompacted gravel or low-strength concrete under the first course
- Foundation soilMust bear the load; organic and expansive soils are unsuitable
Summary of NCMA's component list and best-practice notes. A site designer sets every dimension. Source: [1]
Conventional and soil-reinforced walls compared
| Feature | Conventional (gravity) SRW | Soil-reinforced SRW |
|---|---|---|
| What holds it | Mass of the units, single or multiple depths (crib) 1 | Units plus layers of geogrid or geotextile in compacted fill 1 |
| Typical height | Unreinforced walls typically up to 3 to 4 ft, less in poor soil or with surcharge 2 | No theoretical maximum when properly designed; walls over 50 ft have become more common 2 |
| When used | Short walls with sufficient mass against sliding and overturning 1 | When gravity height is exceeded or there are surcharges, slopes or poor foundation soil 1 |
| What decides it | Manufacturer height charts, or an engineer if local code requires 1 | A designer's approved documents; do not change without the designer 1 |
| Typical risk | Sliding or overturning if too tall for the system 1 | Water in the fill and geogrid layout errors 15 |
Why "how tall can I go" has no one answer
CMHA gives 3 to 4 ft as a typical limit for unreinforced walls, and says less if soil is poor or a load sits above.
That is an industry range for a standard block system, not a rule for your yard. The block maker's chart for the exact unit, and local code, set the limit 21.
Example: how a "3.5 ft wall" can pass 4 ft
The dimensions are invented to show the arithmetic. The two measuring rules are CMHA's and the model code text read through UpCodes 23. Your local code decides which one binds.
| Step | Example input | What the source says |
|---|---|---|
| Visible face above grade | 42 in. (3.5 ft) | Example only |
| First course buried below finished grade | 8 in. | CMHA: wall height includes the bottom portion that extends below finished grade 2 |
| Height by the CMHA method | 42 + 8 = 50 in., about 4.2 ft | Measured from the top of the leveling pad to the top unit, excluding cap 2 |
| Model permit line | 4 ft | Exemption covers walls not over 4 ft, measured from the bottom of the footing to the top of the wall, unless supporting a surcharge 3 |
| Result | The wall is over 4 ft by the measure printed | It is not exempt under that model text; ask the building department |
A wall that shows 3.5 ft can still count as more than 4 ft. Many jurisdictions also count tiers together, and any slope, driveway or building above the wall can make it a surcharge case. Use the retaining wall calculator for block counts, not for permit decisions.
Where you meet the term: bids, block charts and permits
On a bid. A retaining wall bid should name the block system, the wall height by its own measuring method, the base and drainage layers, the geogrid (if any) and who stamps the design. If the bid just says "block wall," ask. The landscaping estimates guide shows how to compare lines.
On a block maker's chart. NCMA says that if engineering is not required, you should follow the block and geogrid manufacturers' design charts without exceeding the heights 1.
On a permit form. The form may ask for wall height, setback from the property line, loads above the wall and drainage. The research page named above shows how thresholds differ.
On site plans for steps and terraces. A wall often runs with stairs or terraces. See landscape steps and garden stairs and landscape grading.
What the system offers, and what it asks of you
What NCMA and CMHA credit it with
- Design flexibility in look, placement and tight spaces, and ability to handle site drainage, per NCMA 1.
- Dry-stacked, mortarless units can be installed quickly, even by one person for short walls, says NCMA 1.
- Can be corrected or adjusted for alignment during construction, unlike cast-in-place concrete, per NCMA 1.
- Reinforced versions can be built very tall when designed for it, per CMHA 2.
Limits and burdens
- Needs the right foundation soil; NCMA calls organic and expansive soils unsuitable 1.
- Water is a main hazard, and it needs a drainage design, not just gravel 15.
- Reinforced walls need a designer and often inspection and testing 1.
- Heights, permits and design rules are local, and a wall over the line needs more paper 23.
Limits of the evidence, and who wrote it
Most of the technical detail comes from NCMA and CMHA, which represent the industry that sells the product.
The NCMA installation guide we read is an undated distributor copy that cites the Design Manual for Segmental Retaining Walls, 3rd Edition (2009) and 2008 to 2010 ASTM and TEK documents.
A newer Design Manual edition may exist, and we did not read it 1.
The CMHA page is marked "Revised 2014" 2. The California DSA document applies to school buildings and state-owned essential services buildings, not homes.
We use it only as an example of how a formal design process looks 4.
FHWA's statement covers geosynthetic reinforced soil walls in general, and is not a failure rate for residential block walls 5.
We did not find a university Extension guide with sourced residential height rules. Oregon State has a guide to building a landscape retaining wall, but it did not load (HTTP 403), so we do not cite it. We give no design numbers for your wall. Heights, setbacks and reinforcement lengths come from a designer.
What drives the price, and how long a wall lasts
We do not publish prices. The main drivers are wall height and length, how much soil must be excavated and hauled, the leveling pad and drainage, the number of geogrid layers, access for equipment, site engineering, inspection and permits.
A taller wall costs more than proportionally, because reinforcement and engineering come in at a certain height. See the cost hub for what shapes a bid.
On lifespan, a source-based answer is "it depends." NCMA says the performance of mechanically stabilized earth walls in everyday applications and extreme events has proven excellent, and that after several recent earthquakes they held up better than conventional retaining walls when properly designed 1.
That is an industry statement, and it depends on design and drainage. We give no number of years.
Ongoing costs are small if drainage works: clearing outlets and watching for movement. If water backs up, costs rise quickly.
How a wall job runs, and who does what
The is below a contractor and engineer job, not a weekend project for tall walls. The sequence follows NCMA's guide; it is not a build instruction.
- Check local code first. NCMA says to learn local retaining wall requirements to decide whether engineering is needed 1. Ask the building department about permits and heights.
- Decide if engineering is needed. NCMA's rule: if yes, follow the approved documents and change nothing unless the designer signs; if not, follow the manufacturer's charts without exceeding the heights 1.
- Call 811. NCMA says every state has a Call Before You Dig service. Allow time for utilities to be marked, and tell the designer if a utility sits in the reinforced zone 1.
- Survey and approve the layout against property lines, easements and grades before work starts 1.
- Prepare and inspect the foundation. NCMA says foundation soils are examined by an inspector, and a geotechnical engineer evaluates soils that do not meet the design 1.
- Build to the approved system with the specified units, fill, pipe and geogrid 1.
- Inspect and record changes. NCMA lists monitoring of earthwork, fill and compaction, and wall and geogrid installation. Any field change goes to the designer 1.
Engineering, permits and digging
Call 811 before any digging. Retaining walls over the local height line, walls with a slope, driveway, pool or building above them, and walls near a property line need an engineer and often a permit.
Do not exceed the block maker's height chart. Do not send wall drainage onto a neighbor or into a sanitary sewer. NCMA says heavy equipment should stay at least five feet behind the wall face 1.
Keeping a wall in service
We found no source that gives a maintenance schedule for residential SRWs, so these are tasks tied to what the sources say about drainage and loads.
- Keep the outlets clear. NCMA says water should be directed away from SRWs whenever possible, and drain outlets must keep gravity flow 1.
- Keep surface water off the wall. Grade the top so runoff goes away from the face. See hardscape drainage.
- Keep heavy loads back. NCMA says avoid heavy equipment on top of a finished wall unless the design allowed for it 1.
- Do not add loads later such as a new driveway, pool or shed above the wall without asking the designer, since surcharges change the design 1.
- Look after the wall after heavy rain, checking for bulges, leaning, cracks or wet spots at the face.
Warning signs, and when to call a pro
- Bulging or a belly in the face. NCMA says uneven geogrid tension or compaction can cause localized movement or bulging 1.
- Leaning or rotation from the original batter. NCMA's construction tolerance is 2 degrees from the plan batter right after building 1. Later movement beyond that is a signal to ask an engineer.
- Settlement or dips along the top. NCMA's tolerance for differential settlement is 1% of the distance between two points on the wall 1.
- Water seeping or soil washing through the face. NCMA says a gravel and filtration design is needed to keep retained soil from washing through 1.
- Cracks in the ground behind the wall, or a sinking patio above it. This points to settlement in the backfill.
Any of these on a wall that supports a slope, driveway or building calls for an engineer. For drainage, see the drainage diagnostic.
Standards, codes and what each one is
Product standard (voluntary). NCMA says SRW units are required to meet or exceed ASTM C1372, the standard specification for dry-cast segmental retaining wall units 1. ASTM standards are voluntary until a contract or code adopts them. We did not read the paid text.
Design standard (voluntary). NCMA's Design Manual for Segmental Retaining Walls (3rd Edition, 2009, as cited in the guide we read) is the design reference NCMA cites.
California's DSA, for school projects only, requires design to that manual's 3rd edition or AASHTO methods, with a geotechnical report and stamped analysis 14. NCMA also points to its TEK 18-11 for recommendations where local code gives no guidance 1. We did not read TEK 18-11.
Building code (law, as adopted locally). CMHA says SRWs fall under the International Building Code Section 105.2, which requires a permit for earth retaining structures over 4 ft in total height, and that local codes may require a design by a design professional 2.
The model text we read (Texas Windstorm Insurance Association Building Code, 2024, section 105.2) exempts walls not over 4 ft measured from the bottom of the footing, unless supporting a surcharge, and says exemptions do not authorize work that breaks other rules 3. Adoption, edition and local amendments vary by state and city.
Product labels and manufacturer charts. Block and geogrid makers publish height limits for their systems. NCMA says to follow them 1.
Other rules. Zoning, easements, HOA covenants and property-line setbacks are separate from the building code. The permit thresholds research above compares a few.
Segmental walls compared with the terms near them
| Term | What it is | Key difference | Where to read more |
|---|---|---|---|
| Segmental retaining wall | Dry-stacked concrete units, with or without geogrid 1 | Mortarless; flexible; gravity or reinforced | This page |
| Retaining wall (general) | Any wall that holds back soil | Includes poured, timber and stone | See the service page linked above |
| Raised garden bed | A framed growing bed above grade | Not meant to hold a slope; often low | See raised garden bed |
| Swale or berm | A shaped channel or ridge of soil | Moves water; does not hold a grade change | See swale vs. berm |
| Interlocking pavers | Concrete units for flat surfaces | Carries foot or vehicle loads, not soil pressure | See interlocking concrete pavers |
Common retaining wall myths
- Myth The blocks hold the soil back.
- Reality NCMA says SRWs rely primarily on their mass, and soil-reinforced walls use buried reinforcement to extend that mass 1.
- Myth A gravel strip and a drain pipe make drainage safe.
- Reality NCMA says the gravel fill and pipe should not be relied on as the primary drainage medium for the wall 1.
- Myth Under 4 ft never needs a permit.
- Reality The model text says exemption is not permission to break other rules, and a wall with a surcharge is outside it. Local code can set lower lines 3.
- Myth Any geogrid fits any block.
- Reality NCMA says geogrids cannot be exchanged without the engineer's evaluation, and not all types are compatible with all block systems 1.
- Myth Pea gravel is a fine base.
- Reality NCMA's best-practice note says to avoid pea gravel for the leveling pad 1.
- Myth Any soil works as a foundation.
- Reality NCMA says organic and expansive soils are unsuitable foundation soils 1.
The local variable: soil, water, frost and permit lines
Four local facts change a wall. Foundation soil: NCMA calls organic and expansive soils unsuitable 1, so expansive-clay regions need special handling.
Rain: FHWA says a great majority of failures of reinforced soil walls involve prolonged rainfall 5.
Frost: NCMA says SRWs on flexible leveling pads need not go below the frost line if the foundation soil can bear the load 1, but your local code and engineer decide.
And the permit line: CMHA and the model code both use 4 ft, but local amendments differ 23. Our research pages cover expansive soils and permit lines.
When a segmental wall fits, and your next step
A segmental wall suits a grade change where the soil is stable, water can be drained away and the height is within the block maker's chart or an engineer has designed it.
It is a poor fit where there is a slope, driveway, pool or building above the wall, where soil is organic or expansive, or where drainage has no outlet, unless an engineer designs for it.
Your next step is to call the building department, ask whether the wall needs a permit or a stamped design, and ask each bid to say how it measures height. Ask who is responsible for the design and for inspection.
To find help, read choosing a landscaping company. A contractor that cannot name the block system, the drainage design and the geogrid is not ready to bid a tall wall.
Questions about Segmental Retaining Wall
What is a segmental retaining wall?
A segmental retaining wall is a wall of manufactured concrete units stacked dry, without mortar, that holds back soil by its own weight. NCMA describes two types: conventional gravity walls, and soil-reinforced walls that add layers of geogrid in the backfill to increase the effective width and weight of the mass.
How tall can a segmental retaining wall be?
It depends on the system and design. CMHA says unreinforced SRWs typically reach 3 to 4 feet, or less with poor soil or surcharge loads, and that properly designed reinforced walls have no theoretical maximum height. Follow the block maker's height chart, or an engineer's approved design.
Do segmental retaining walls need geogrid?
Tall or loaded walls do. NCMA says soil-reinforced walls should be used when the gravity wall height is exceeded or when there are surcharges, slopes or poor foundation soils. Geogrid layers are chosen as part of the system, so they cannot be swapped without the designer's evaluation.
Do segmental retaining walls need drainage?
Yes. NCMA says water can increase loads and scour soil, and should be directed away from the wall. Its gravel fill and drain pipe handle incidental water and are not meant to be the primary drainage path. FHWA notes most reinforced soil wall failures involve prolonged rainfall.
Do I need a permit for a segmental retaining wall?
Often, over a certain height. CMHA says the International Building Code Section 105.2 requires a permit for earth retaining structures over 4 feet in total height. The model text we read measures from the bottom of the footing and excludes walls supporting a surcharge. Local code decides, so ask your building department.
Does a retaining wall need an engineer?
NCMA says to check local code first. If local code or the site requires engineering, follow the approved documents. If not, follow the manufacturer's charts without exceeding the heights. Walls with slopes, driveways, pools or buildings above them usually call for an engineer.
How is retaining wall height measured?
It depends on the rule. CMHA measures from the top of the leveling pad to the top unit, excluding the cap, and includes any part below finished grade. The model permit text measures from the bottom of the footing to the top of the wall. Both can exceed the height you see.
Does a segmental retaining wall need a footing below the frost line?
NCMA says SRWs are typically built on flexible aggregate leveling pads, so the footing does not need to go below the frost line if the foundation soil has enough bearing capacity. Local code and your engineer decide for your site. Organic and expansive soils are unsuitable foundations.
Why do segmental retaining walls fail?
Water is a common factor. FHWA says a great majority of failures of reinforced soil walls involve prolonged rainfall. NCMA lists other causes to guard against: poor foundation soil, unplanned loads, uneven compaction or geogrid tension, and changed materials. A designer and inspections reduce these risks.
Sources
- National Concrete Masonry Association, Segmental Retaining Wall (SRW) Installation Guide (undated distributor-hosted PDF citing Design Manual 3rd Ed., 2009; text extracted with pdftotext), retrieved .
- Concrete Masonry & Hardscapes Association, SRW-FAQ-001: Segmental Retaining Wall height (revised 2014), retrieved .
- Texas Windstorm Insurance Association Building Code, 2024, Section 105.2 Work exempt from permit (model administrative text, via UpCodes), retrieved .
- California Division of the State Architect, IR 16-3: Earth Retaining Systems with Precast Concrete or Concrete Masonry Units (rev. 11/07/17; school and essential-services buildings; PDF text extracted with pdftotext), retrieved .
- Federal Highway Administration, Synthesis of Geosynthetic Reinforced Soil (GRS) Design Topics, FHWA-HRT-14-094 (February 2015; PDF text extracted with pdftotext), retrieved .
Expert review
Written by the HyreYard Research Desk. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Held G-REV. Component, drainage and construction facts are from NCMA's installation guide and CMHA, both industry bodies; permit text is a model code read through UpCodes; FHWA and California DSA are government documents for other uses, labeled as such.
No wall design numbers are given beyond what NCMA prints. The Oregon State, Iowa DOT and Prince George's County pages returned 403 and are not cited. A licensed civil or geotechnical engineer must review before release.
Suggest a correction. We fix errors and say what changed.