How to Build a Retaining Wall

Updated September 2026 · Home Project Resources

A retaining wall is only as good as its base and its drainage. Almost every leaning, bulging, or collapsed wall you have ever walked past failed for one of those two reasons, not because the blocks were wrong. Get a level, compacted gravel base and give the water behind the wall somewhere to go, and a segmental block wall will stand for decades.

So the real work happens before the first block goes down and behind the wall where nobody sees it. Digging the trench deeper than feels necessary, compacting it in layers, and shoveling in clean drainage stone are the boring steps that decide the outcome.

Working out how many blocks and how much stone? Our retaining wall calculator turns your wall length and height into block counts, base volume, and backfill stone.

Updated September 2026

A note on numbers. This page gives you dimensions, quantities and scope, not dollar figures. Material and contractor pricing varies by region, by season and by the specifics of your job, and we do not set any of it. Work out what you need here, then get quotes that can be compared to each other.

How to build a retaining wall, step by step

1. Check the height rules with your local building department and call 811 before digging.
2. Lay out the wall with stakes and string, marking the front face.
3. Dig the trench wide enough for the block plus 12 inches of drainage stone behind it.
4. Compact 6 inches of crusher run in two 3 inch lifts, then screed it flat.
5. Set the first course dead level, checking every block front to back and side to side.
6. Place the perforated drain pipe at the back of the base, holes facing down.
7. Stack the next course, sweeping the top of each course clean before setting blocks.
8. Fill the block cores and the 12 inches behind them with clean angular stone.
9. Backfill native soil behind the stone and compact in 6 to 8 inch lifts as you climb.
10. Wrap filter fabric between the drainage stone and the native soil.
11. Cap the drainage column with low permeability soil and grade the ground away.
12. Glue on the cap blocks with masonry adhesive and let them cure.

A 20 foot wall at 3 feet exposed is a solid weekend for two people, plus a delivery day. The digging and compacting is most of it.

Before you dig: height, permits, and when you need an engineer

This is the part to settle first, because it can change the entire project.

Walls above roughly 3 to 4 feet of exposed height commonly require an engineered design and a building permit. So does almost any wall that holds back a slope rather than a flat pad, and any wall with a surcharge on top of it, meaning a load sitting within the failure zone behind the wall. A driveway, a parking area, a pool, a shed, or another wall above yours are all surcharges.

The exact trigger varies by jurisdiction. Some towns start at 30 inches, some at 4 feet, some measure from the bottom of the footing rather than from grade. Your local building department is the authority, and one phone call before you dig is cheaper than tearing out a finished wall. Ask them two things: at what height a permit is required, and whether they require a stamped design from a licensed engineer at that height.

Call 811 (or your regional dig line) before any excavation. It is free, it is usually required, and hitting a gas line or a buried service is a genuinely dangerous outcome.

A note on scale: a 4 foot wall retains several tons of soil per running foot once it is saturated. That is not a DIY judgment call. If your wall crosses the local threshold, hire the design out and build to it, or hire the whole job.

Why walls fail, and what actually holds one up

A block wall stays put through three things working together: the weight of the blocks, friction between the courses, and the soil in front of the buried bottom course pushing back.

Working against it is lateral earth pressure, the sideways shove of the soil behind the wall. That pressure is manageable when the soil is drained. It becomes enormous when the soil is saturated, because water behind the wall adds hydrostatic pressure on top of the soil pressure.

Water is heavier than most people expect, at about 62.4 pounds per cubic foot. Trapped behind a 4 foot wall, that pressure builds toward the bottom and pushes the wall out at exactly the point it is least able to resist. This is the single most common reason a wall built with good blocks and reasonable care starts leaning three winters later.

Freeze and thaw makes it worse. Water in the backfill freezes, expands, and ratchets the wall forward a fraction of an inch each cycle. It never ratchets back.

So the entire drainage system exists to make sure hydrostatic pressure never develops. That is the whole point of it.

The base trench: dig deeper than you think

The base is a compacted gravel pad that spreads the wall's weight and gives you a dead flat starting surface.

Trench depth. Plan on 6 inches of compacted base gravel plus enough depth to bury the bottom course. A typical trench is 10 to 14 inches deep from finished grade for a wall in the 2 to 4 foot range. Deeper is fine. Shallower is where trouble starts.

Trench width. Cut it at least twice the depth of your block, so roughly 24 inches wide for a 12 inch deep block. That extra width behind the block is where your drainage stone lives.

Dig into undisturbed soil. If you hit soft fill, topsoil, or old debris, keep going until you reach firm native soil, then bring the level back up with compacted gravel. Building on loose fill guarantees settlement.

Base material. Use a well graded crushed stone with fines, commonly sold as 3/4 inch minus, crusher run, or road base. The fines are what let it lock together under compaction. Clean drainage stone with no fines will not compact into a stable base, so save that for behind the wall.

Compact in lifts. Spread 3 inches, compact it, spread another 3 inches, compact again. A plate compactor is a cheap one-day rental and is worth every dollar here. Hand tamping a 20 foot trench is possible and miserable, and it is easy to convince yourself it is done when it is not.

Then get it flat. Screed the base and check it with a 4 foot level in both directions.

The first course is the whole wall

Every error in the first course multiplies as you go up. A block that sits 1/8 inch high throws off the course above it, and by course six you are shimming and cursing.

Set the first block, check it level side to side and front to back, and set the next one against it. Check both. Then check the run with a string line pulled tight from end to end. Tap blocks down with a rubber mallet to lower them and add a thin sprinkle of base material to raise them.

Expect the first course to take as long as the next four combined. That is normal and it is time well spent.

Bury the bottom course. A common rule of thumb is to bury 1 inch of wall for every 8 inches of total wall height, with a minimum of one full course. So a 4 foot wall gets about 6 inches buried, and a 3 foot wall gets a full block buried in most cases.

That buried course matters because it resists sliding. The soil in front of the wall provides passive resistance, pushing back against the whole wall trying to slide forward on its base. A wall set on top of the ground has almost none of that, and it is also exposed to erosion undermining the front edge. Burying the bottom course is cheap insurance for a few extra inches of digging.

Drainage: gravel, pipe, and an exit

Drainage is not optional and it is not the same thing as backfill. Here is the full system.

Drainage stone. Behind the blocks, place 12 inches of clean crushed stone, typically 3/4 inch angular with no fines, sometimes called clean stone or #57 stone. Angular stone locks together and stays open. Round pea gravel drains but shifts, and stone with fines clogs.

Perforated pipe. Lay a 4 inch perforated drain pipe at the base of the stone column, sitting on top of the base or just behind it, with the holes facing down. Holes down lets water enter the pipe from the bottom of the stone column, which is where it collects.

Filter fabric. Wrap non woven geotextile fabric between the drainage stone and the native soil behind it. Without it, fine soil particles migrate into the stone over a few seasons and turn your drain into a solid mass. Fabric is one of the smallest lines on the materials list for a residential wall, and it is the difference between a drain that works in year 15 and one that does not.

The exit is the part people forget. The pipe has to daylight somewhere lower than the pipe itself. Run it to an open end at the low end of the wall, into a swale, or to a drywell. Slope it at least 1/8 inch per foot toward that outlet. A perfect stone column feeding a pipe that dead ends in the ground does nothing at all.

Backfill in lifts. Once the stone is in behind a course, add native soil behind the stone and compact both in 6 to 8 inch lifts as you climb. Compacting after the fact is not possible.

Cap the top. Before the last few inches, place a layer of low permeability soil or a fabric cap over the drainage stone so surface water sheds off rather than pouring straight into your drain column. Then grade the ground behind the wall to slope away.

Setback and batter

Segmental blocks have a lip or a pin system that automatically sets each course back from the one below it, usually about 3/4 inch to 1 1/4 inch per course depending on the block.

That lean into the hill is the batter. It matters because it puts the wall's center of gravity back toward the retained soil rather than out over the toe. A wall that leans slightly into the slope has to rotate through vertical before it can fall forward, which buys a large margin.

Do not fight the block's built in setback. It was engineered with the block. If you are building with a block that has no lip, set your string line so the face leans back about 1 inch per foot of height and check it as you go with a level or a batter gauge.

Geogrid: when the blocks alone are not enough

Geogrid is a stiff plastic mesh laid horizontally between courses and extending back into the retained soil. It ties the soil mass and the wall together so they behave as one heavy block rather than a thin facing holding back loose dirt.

As a general expectation, walls taller than about 3 to 4 feet, walls holding a slope, and walls with a surcharge above them typically need geogrid. Below that, a well drained gravity wall on a good base is often adequate.

Here is the important part: grid spacing, grid length, and grid strength come from an engineered design, not from a rule of thumb on a website. Those numbers depend on soil type, wall height, slope above, groundwater, and the surcharge. A guess that is close is still a guess, and grid installed at the wrong spacing or cut too short gives you the confidence of reinforcement without the reinforcement.

If your wall is in geogrid territory, that is the same territory where you likely need a permit and a stamped design anyway. Get the design, then install exactly to it.

Caps and the finish

Cap blocks give the wall a finished top edge and keep water from sitting in the block cores.

Clean the top course, run a bead of masonry construction adhesive on each block, and set the caps. Two beads in a zigzag hold better than one straight line. Let the adhesive cure per the label before anyone sits on the wall, which is usually 24 hours.

Caps often overhang the face by about 1 inch. Cut the ends with a masonry blade for a clean corner.

Cutting blocks safely. Cutting concrete block dry with a saw produces respirable crystalline silica dust, which causes permanent lung damage and is a known carcinogen. Use wet cutting with a water fed saw, or a dust collection system, and wear a properly fitted N95 or better respirator plus eye and hearing protection. Work outdoors and stay upwind. A dust mask from the paint aisle is not a respirator.

Worked example: a 20 foot wall, 3 feet exposed

Say you are building a straight wall 20 feet long with 3 feet of exposed face, using a standard segmental block that is 12 inches wide, 8 inches high, and 6 inches deep at the face.

Total wall height. 3 feet exposed plus one buried course at 8 inches gives 44 inches, so 5.5 courses. Round to 6 courses at 8 inches each, which is 48 inches total, with the bottom 12 inches buried. Exposed face lands at 36 inches.

Block count. 20 feet is 240 inches. At 12 inches per block that is 20 blocks per course. Six courses is 120 blocks. Add 5 to 10 percent for cuts and breakage, so order 128 to 132 blocks.

Cap blocks. 20 more, plus a couple of spares, so 22.

Base gravel. Trench 20 feet long, 2 feet wide, 6 inches of compacted base. That is 20 x 2 x 0.5, which is 20 cubic feet. Divide by 27 to get 0.74 cubic yards. Order 1 cubic yard, because compaction and spillage eat the difference. Crusher run is the cheapest aggregate on the yard's list, but a single yard will attract a small load fee, so ask what the minimum load is before you order.

Drainage stone. A 12 inch wide column of clean stone, 20 feet long, running the full 4 foot height. That is 20 x 1 x 4, which is 80 cubic feet, or 2.96 cubic yards. Order 3 cubic yards. Clean 3/4 inch stone costs more per yard than crusher run because it is washed and screened, and this is the larger of your two aggregate orders.

Pipe and fabric. 24 feet of 4 inch perforated pipe to give yourself an outlet run, plus about 100 square feet of non woven geotextile.

Where the money goes. Take that takeoff to a supplier and the shape of the bill is always the same: blocks are well over half of it, the two aggregate deliveries together are the next largest chunk, and caps, adhesive, pipe and fabric together are a small tail. Because blocks dominate, the block you choose moves the total more than every other decision combined, and a plain gray standard block costs a fraction of a tumbled or split-face color-blended one for the identical wall. Installed by a contractor, a wall like this is quoted per square foot of face, and the face here is 60 square feet. A contractor quote lands well above the materials list, because it carries excavation, compaction, haul-away and labor. Get two or three local quotes on the written takeoff above so they are comparing the same wall.

Our gravel calculator will size the base and drainage stone from your own dimensions, and the cubic yard calculator converts any of those volumes into yards for ordering. If you are pouring a concrete footing instead of using a gravel base, the concrete calculator handles that volume.

Cost of materials at a glance

Materials and labor are listed separately on purpose. Contractor pricing bundles them, and comparing a materials list against an installed quote is how people end up thinking they are being overcharged when they are not.

Common mistakes that show up in year three

• Base too thin or not compacted. Settlement appears as a dip in the middle of the wall and opening joints at the top.
• No filter fabric. The drain silts up, water backs up, and the wall starts leaning even though there is a pipe back there.
• Pipe with nowhere to go. Very common. Walk the outlet before you bury it.
• Backfilled with the clay you dug out. Clay holds water against the wall. Clean stone against the block, native soil behind that.
• First course not level. Every problem above it traces back here.
• Building tall without a design. The wall that needed geogrid and did not get it usually looks fine for two to five years, then goes fast.
• Ignoring the surcharge. A driveway, hot tub, or slope above the wall changes the loads dramatically.

Frequently asked questions

How tall can I build a retaining wall without a permit?

It depends entirely on your jurisdiction, and there is no universal number. Many places set the threshold somewhere between 30 inches and 4 feet of exposed height, and many also require a permit at any height if the wall holds a slope or supports a surcharge like a driveway. Call your local building department before you dig and ask for their specific trigger height and whether an engineer's stamp is required.

Do I need a concrete footing for a retaining wall?

Not for a segmental block wall. A compacted crushed stone base is the standard and it is actually better, because it drains and it flexes slightly with frost instead of cracking. Poured concrete footings are used for mortared stone, brick, and cast concrete walls, and those need to sit below the local frost depth. Our concrete calculator covers the volume if you are going that route.

How much gravel do I need behind a retaining wall?

Plan on a 12 inch wide column of clean angular stone running the full height of the wall, from the base to a few inches below the top. For a 20 foot wall that is 4 feet tall, that comes to about 3 cubic yards. Fill the block cores with the same clean stone as you go, which adds drainage capacity and weight.

What happens if a retaining wall has no drainage?

Water saturates the soil behind it and adds hydrostatic pressure to the existing earth pressure, which can easily double the load the wall has to resist. The wall bulges near the bottom, leans forward, and eventually the courses shear apart. Freeze and thaw accelerates it, moving the wall forward a little each cycle. It is the leading cause of failure in walls that were otherwise built reasonably.

Should a retaining wall lean back?

Yes. That backward lean is called batter, and most segmental blocks build it in automatically through a lip or pin system that sets each course back about 3/4 inch to 1 1/4 inch. It shifts the wall's weight toward the hill so the wall has to rotate through vertical before it can tip out. Do not shim the courses plumb to make the face vertical.

When do I need geogrid in a retaining wall?

Typically above about 3 to 4 feet, on any wall holding a slope, and on any wall with a load above it. The specific spacing, length, and strength of the grid come from an engineered design based on your soil and loading, not from a general rule. If your wall is tall enough to need grid, it is almost certainly tall enough to need a permit and a stamped design as well.

How deep should the trench be for a retaining wall?

Typically 10 to 14 inches below finished grade for a wall in the 2 to 4 foot range. That covers 6 inches of compacted base gravel plus enough depth to bury the bottom course, which is roughly 1 inch of burial per 8 inches of wall height with a minimum of one full block. Always dig down to firm undisturbed soil, then build back up with compacted gravel if you had to go deeper.

How much does a retaining wall cost per foot?

Contractors quote segmental block walls per square foot of wall face, so the first thing to do is convert your run into face area: length times exposed height. Doing it yourself costs materials only, and materials are a fraction of an installed price, because labor, excavation, compaction and haul-away are most of a contractor's number. Four things push a quote up sharply: an engineered wall, which any wall over about 4 feet usually needs; poor access, meaning every ton comes through a gate by wheelbarrow instead of by machine; wet or clay soil that needs extra excavation; and a decorative block instead of a plain one. Get three quotes, hand each one the same block model, wall length and exposed height, and ask each whether the price includes the drainage stone column, the perforated pipe with its outlet, geotextile fabric, and haul-away of the spoil. A bid noticeably below the others has almost always dropped one of those four.

The short version

Dig a trench wide enough for drainage stone and deep enough to bury the bottom course. Compact 6 inches of crusher run in 3 inch lifts and get it dead flat, because every error in the first course grows as you climb.

Behind the blocks, run 12 inches of clean angular stone, a 4 inch perforated pipe with the holes down, filter fabric between the stone and the native soil, and an outlet that actually daylights. Compact the backfill in lifts as you go, and cap the top so surface water sheds away rather than into the drain.

Let the block's built in setback give you the batter, glue the caps down, and wet cut with a respirator if you are cutting block. Above roughly 3 to 4 feet, or with any slope or surcharge above, get an engineered design and a permit, because your local building department sets the real threshold.

Our retaining wall calculator gives you block counts and stone volumes from your dimensions. If you are planning hardscape around it, the paver patio cost guide covers what the surface work adds.