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Retaining Wall Guide: Design, Drainage and Block Systems
A retaining wall stands or falls on what you can’t see: the base, the drainage stone behind it and the geogrid in the backfill. These are the articles we’ve written from building block, boulder and timber walls in central PA, grouped from planning through repair.
Quick answer
A retaining wall is decided by its base, drainage and reinforcement, not the face block. Plan at least 6 in. of compacted base stone, bury the first course at least 6 in., and keep at least 12 in. of clean stone behind the block with a drain pipe vented at least every 50 ft. Past the block’s rated gravity height, the wall needs geogrid at least 0.6 times the wall height. In Pennsylvania, a wall over 4 ft, measured from the lowest level of grade to the top, or any wall holding back a surcharge, isn’t exempt from a permit.
Start with your question
| Your situation | What decides it | Read |
|---|---|---|
| Planning a new wall | Height, soil, what sits above it and where the water goes | Retaining wall design |
| Over 4 ft, or holding up a driveway, slope or fence | Pennsylvania’s permit line and the code’s engineering trigger | Retaining wall permits, walls over 4 feet |
| How much block to order | Face area, the buried course, caps and a little extra | How many blocks you need, block calculator |
| Picking a block | Gravity height, setback and how the units connect | Block systems compared, block specs by brand |
| “What will it cost?” | Material, height, length and access | Wall cost calculator, wall cost in central PA |
| A steep yard you want in levels | How far apart the walls sit compared with the lower wall’s height | Terraced backyard ideas, sloped backyard ideas |
| Wet soil behind the wall, or water weeping through the face | Drain stone, pipe and outlets behind the block | Retaining wall drainage |
| A wall leaning, bulging or cracking | Whether the base, the drainage or the reinforcement gave out | Fixing a leaning wall, why walls fail |
| An old timber or railroad tie wall rotting out | Tear-out, creosote disposal and what to build instead | Replacing a railroad tie wall |
| Steps through the wall, or a fence on top | Riser height, setback and guard rules | Retaining wall steps, fence on a retaining wall |
The numbers that come up most
| What | Number | Where it comes from |
|---|---|---|
| PA permit exemption | Walls not over 4 ft, measured from the lowest level of grade to the top of the wall, unless the wall supports a surcharge | 34 Pa. Code 403.62(c)(2) (permit guide) |
| Engineered design | Over 48 in. of unbalanced fill, or over 24 in. tall when the wall resists loads beyond the soil | IRC R404.4, per the ICC summary of the 2015 changes (permit guide) |
| Base | At least 6 in. of compacted wall rock in a trench about 24 in. wide | Allan Block installation guide (design guide) |
| Buried first course | At least 6 in. (CMHA); Allan Block says at least 6 in. or 1 in. per foot of wall height | CMHA SRW-TEC-004 (design guide); Allan Block estimation worksheet (block count guide) |
| Drain stone | At least 12 in. of free-draining stone, measured from the back of the block | CMHA (drainage guide) |
| Drain pipe outlets | Toe drain vented at least every 50 ft | Allan Block (drainage guide) |
| Geogrid | At least 0.6 times the wall height and never shorter than 4 ft; layers no more than 24 in. apart | CMHA SRW-TEC-004 (design guide) |
| Gravity height | Set by each block’s chart. One example: Belgard rates Diamond Pro for gravity walls up to 4 ft including the buried course | Belgard (block count guide) |
| Terraces | Walls at least twice the lower wall’s height apart to act on their own; closer than that, design them as one taller wall | CMHA SRW FAQ-001 (terraced backyard guide) |
| Extra block | Order 5% extra | Allan Block estimation worksheet (block count guide) |
| Steps in a block wall | Allan Block units are about 8 in. tall, so a 48 in. rise takes 6 steps | Allan Block Tech Sheet 1022 (steps guide) |
Start here
- Retaining Wall Design: How Builders Plan a Wall That Lasts: height, base, drainage and reinforcement decided before you dig.
- Why Retaining Walls Fail (and How to Build One That Lasts 30+ Years): the common failures and how to avoid each one.
- Retaining Wall Drainage: the stone, pipe and outlets that keep a wall standing.
- When Does a Retaining Wall Need Geogrid?: a plain-English guide to reinforcement.
- Retaining Wall Block Systems Compared: Allan Block, Versa-Lok, Keystone, Belgard and Fortus side by side.
Planning and design
- Retaining Wall Design: How Builders Plan a Wall That Lasts: the builder’s planning process, step by step.
- Retaining Wall Ideas: 20 Options With Heights, Costs and Limits: wall styles and where each one makes sense.
- Terraced Backyard Ideas: tier math, wall spacing and 11 designs.
- Sloped Backyard Ideas: 15 Ways to Use a Hill: options sorted by how steep your slope is.
- How to Landscape a Sloped Backyard: assess, drain, grade, retain, plant, in that order.
- How to Level a Sloped Backyard: grading a hill flat enough to use before you build on it.
- How Many Retaining Wall Blocks Do I Need?: the block count, row by row.
Block systems, brands and wall types
- Retaining Wall Block Systems Compared: five major block systems on specs, setback and fit.
- Retaining Wall Block Sizes, Weights and Specs by Brand: 18 wall blocks charted from manufacturer spec sheets.
- Allan Block vs Versa-Lok: specs, setback, corners and colors compared.
- Versa-Lok Standard: block size, weight, pins, colors and wall heights.
- LibertyStone Retaining Walls and Pavers: Fortus specs, lineup, where to buy and warranty.
- Segmental Retaining Wall: what it is, how it works and when it needs geogrid.
- Timber Retaining Wall vs. Block: which one to build for your site and budget.
- Boulder Retaining Wall: rock sizes, height limits and how it’s built.
Base, drainage and reinforcement
- How Deep Should a Retaining Wall Base Be?: the part nobody sees and every wall depends on.
- Retaining Wall Drainage: Stone, Pipe and Outlets: getting water out from behind the wall.
- When Does a Retaining Wall Need Geogrid?: height, load and soil triggers for reinforcement.
- Retaining Wall Batter: why a good wall leans back on purpose.
Caps, fences and seat walls
- Retaining Wall Caps: overhang, adhesive and how to install them.
- Fence on Top of a Retaining Wall: setback, sleeves and guard rules.
- Seat Wall Dimensions: height, depth, length and what goes under it.
- Retaining Wall Steps: layouts, riser counts and how to build steps into a block wall.
- How to Build a Curved Retaining Wall: radius, setback, curves and corners.
Permits, height and engineers
- Retaining Wall Permit: when you need one, and when you need an engineer.
- Retaining Walls Over 4 Feet: engineers, permits and what they add to the cost.
Lifespan, failure and repair
- How Long Do Retaining Walls Last?: lifespan by wall type.
- Why Retaining Walls Fail: drainage, base and reinforcement mistakes that bring walls down.
- How to Fix a Leaning Retaining Wall: when a repair works and when it’s a rebuild.
- Retaining Wall Maintenance: a seasonal checklist for block walls.
- Replacing a Railroad Tie Retaining Wall: tear-out, creosote disposal and what to build instead.
Free tools
Count block, caps, stone and geogrid with our retaining wall block calculator, price the wall by material with the retaining wall cost calculator (or see what walls cost in central PA), measure your hill with the slope calculator, figure the dirt you’ll move with the cut and fill calculator, and check retaining wall permit rules in your state and 811 dig laws before you excavate.
Want the wall laid out before you buy block? Groundwork online landscape design draws it to scale with a full materials list, and online retaining wall design covers walls on their own. In central PA, our crews build it: see retaining walls in Lock Haven, hardscaping services or get a quote. More topics on the guides page.