The short answer, and why the question is slightly wrong
Compacted fill dirt goes behind a retaining wall. Screened topsoil only belongs in the thin planting layer at the very top, and only if something is actually going to grow there. If you are choosing one product to dump into the whole void behind your wall, topsoil is the wrong pick every time.
But the question itself hides a bad assumption. Most people search this expecting to buy one material and be done. A retaining wall backfill is not one material. It is a layered section, and each layer does a different job. Buy one product for the whole thing and you have either overpaid for soil that will rot down and settle, or you have built a wall with nowhere for water to go.
A typical order for a small wall backfill includes three things. Clean angular drainage stone directly against the back of the wall face. Fill dirt for the bulk of the void behind that stone. And a shallow cap of screened topsoil at the surface, only in the footprint where grass or plants are going.
The rest of this post walks that cross section from the wall face outward, so by the end you can put a number on each of the three materials and order them together instead of guessing at the tailgate.
What each material actually is
Fill dirt
Fill dirt is subsoil. It comes from below the topsoil layer, and it is largely free of organic material. That sounds like a downside until you remember what you are asking it to do. Organic matter decomposes. As it breaks down, it loses volume, and anything sitting on top of it drops. Fill dirt has almost nothing left to decompose, so once it is compacted it holds its shape and stops moving.
That is why fill dirt is the standard material for changing grade, building embankments, and backfilling around structures like pools, utility trenches, and retaining walls. It is mass that stays put.
Screened fill dirt has been run through a screen to pull out rocks, roots, chunks of old concrete, and general debris. That matters more than it sounds. Unscreened fill with fist sized rock in it will not compact evenly. You get voids around the big pieces, and voids are where settling starts. Screened material goes down in consistent lifts and tamps flat.
Screened topsoil
Screened topsoil is native topsoil run through a finer screen than standard topsoil, which is what lets it spread thin and even without lumps. It is built for lawn top dressing, seed beds, and leveling before sod, and typical install depth is a couple of inches.
Every one of those strengths is useless three feet underground. Behind a wall, topsoil's organic content is a liability, not a feature. It decomposes and loses volume. It holds water where you specifically want water leaving. And it costs more per yard than fill dirt, so you would be paying extra for material that makes the job worse. There is no version of this trade where buried topsoil wins.
Drainage stone
Drainage stone means clean angular crushed stone, commonly 3/4 inch clean or a #57 size. The word clean is doing real work there. It means the fines, the dust and small particles, have been washed out. Water moves straight through clean stone instead of sitting in it. Stone that still has its fines in it, like crusher run or bank gravel, packs tight and drains about as well as the dirt you were trying to get water out of.
Rounded pea gravel is also not the same thing. Angular pieces have flat faces that lock against each other and stay where you put them. Rounded stone rolls and shifts under load. For the zone directly behind a wall, you want angular.
Why water, not weight, is what wrecks retaining walls
A block retaining wall is built to hold back soil. Soil pressure is the load it was designed for. Saturated soil pressure is the load it was not.
When rain soaks into backfill that has no drainage path, water builds up in the soil behind the wall and pushes forward against the back face. That water pressure is relentless and it acts across the whole height of the wall. Walls lean out at the top. Courses separate and show daylight between blocks. In bad cases the bottom courses slide off the base and the whole thing comes apart in a season.
Clay backfill makes all of this worse. Clay holds water for a long time and swells as it takes it on, so you get both the pressure of trapped water and the pressure of expanding soil. If the only dirt available on your site is heavy clay, that is not a reason to skip the stone zone behind the wall. It is a reason to make that stone zone wider.
This is also the second strike against topsoil behind a wall, separate from the settling problem. Topsoil is designed to hold moisture for roots. Put it behind a wall and it acts like a sponge in exactly the spot you want water draining away from.
The layered section exists to give water a fast, obvious path down to a pipe and out of the area before it ever has a chance to load the wall.
The backfill cross section, wall face outward
Drainage stone against the wall
Common practice is at least a foot of clean angular crushed stone directly behind the wall face, running the full height of the wall. If you are building with hollow block, fill the block cores with the same stone as you go up. The stone zone is the wall's escape route for water, and a foot is a working minimum, not a generous allowance. Wider is better, especially in clay.
Perforated pipe at the base
A perforated drain pipe sits at the bottom of the stone zone, in the stone rather than in the dirt, sloped so water runs to an outlet. That outlet can be daylight at one end of the wall where grade drops away, or another legal discharge point. What it cannot be is nowhere. A perforated pipe with no outlet is not a drain, it is a reservoir sitting at the base of your wall, which is the single worst place to store water.
Filter fabric between stone and dirt
Filter fabric separates the stone zone from the fill dirt behind it, either wrapped around the stone or run as a vertical layer between the two. Water passes through the fabric. Soil particles do not. Without it, fines from the fill dirt wash into the voids in the stone over a few seasons and the drainage zone silts up. It looks identical from the surface, and it has quietly stopped working.
Compacted fill dirt for the bulk
Fill dirt fills whatever void remains behind the stone zone. Compact it in lifts, roughly 6 to 8 inch layers, tamping each layer before you add the next. This is the step people skip, and it is the one that shows. Dumping the entire load in and running a tamper over the top surface only compacts the top few inches. Everything below stays loose, and within a year you have a settled trench running the length of the wall.
When you get near finished grade, shape the surface so water runs away from the wall rather than pooling behind it. A slight slope away from the wall does more for the wall's lifespan than most people expect, because every gallon that sheds off the surface is a gallon the drainage system never has to handle.
The topsoil cap
If you are seeding, sodding, or planting behind the wall, leave room for roughly 6 inches of soil over the compacted fill. That is the layer where screened topsoil earns its keep. Roots live there, and the fine screen means it levels out smooth for seed or sod. Run the cap over the fill dirt zone only. Where the stone zone meets the surface, keep the top of the stone open or covered with a thin layer of coarse material so water can still get in from above. Sealing the top of the stone under 6 inches of soil closes the one path you built for surface water.
How much of each to order
Work the three zones as three separate volumes. It takes five minutes and it is the difference between one delivery and two.
Stone zone. Wall length times wall height times the depth of the stone zone, all in feet, divided by 27 for cubic yards. Then add a bit for the block cores and for the pipe trench at the bottom.
Fill dirt. Take the total void volume, subtract the stone zone, subtract the topsoil cap. Order some extra on top of that number, because loose material loses noticeable volume once you tamp it in lifts. A load that looks like plenty sitting in a pile shrinks when it is compacted properly.
Topsoil cap. Length times width of the planting strip times cap depth in feet, divided by 27. A 6 inch cap is 0.5 feet, so the surface area times 0.5, divided by 27.
Worked example
Say the wall is 30 feet long and 3 feet tall, with a backfill void 2 feet deep behind it. The stone zone takes the first foot out from the wall face, and the remaining foot is fill dirt.
Total void is 30 times 3 times 2, which is 180 cubic feet, or 180 divided by 27, about 6.7 cubic yards.
Stone zone at 1 foot deep is 30 times 3 times 1, which is 90 cubic feet, or 90 divided by 27, about 3.3 cubic yards. Call it 4 yards with the block cores and pipe trench accounted for.
Topsoil cap sits over the fill dirt zone only, which is the outer 1 foot of the strip. So 30 times 1 is 30 square feet of surface. At 6 inches deep that is 30 times 0.5, which is 15 cubic feet, or 15 divided by 27, about 0.6 cubic yards. Order a full yard so you have enough to grade with.
Fill dirt is what is left. Total void 6.7 yards minus 3.3 yards of stone minus 0.6 yards of topsoil leaves about 2.8 cubic yards. Round up to 3.5 yards to cover compaction loss.
So the order is roughly 4 yards of clean stone, 3.5 yards of fill dirt, and 1 yard of screened topsoil. Getting all three on one delivery run is usually cheaper than three separate trips, and bulk fill dirt is dramatically less expensive per yard than anything you would buy in bags.
Mistakes that show up a year later
- Backfilling with topsoil or leftover garden soil because it was already sitting on site. The wall looks perfect in June and has a visible trench along the back by the next spring.
- Skipping compaction and letting rain settle the fill. Water settles soil unevenly, and it tends to pull fill away from the wall rather than snug against it.
- Over compacting right against the wall face with heavy equipment, which shoves blocks out of alignment. Hand tamp the first foot or two out from the wall and save the plate compactor for the outer zone.
- Using crusher run or bank gravel instead of clean stone. The fines plug the drainage path and you have built a wall with no drainage at all.
- No pipe outlet, or an outlet that gets buried by a later landscaping project. Mark it and keep it clear.
- Burying the capstone course under soil so the top of the wall disappears into the grade. Finish the backfill below the capstone, not over it.
- Sealing the top of the stone zone under heavy clay or a thick topsoil cap. Keep that strip open or covered with coarse material so surface water can still reach the stone.
How We Started
We started Mulch Mound because we got tired of the hassle that came with buying landscaping materials. The options were either loading bags into your car at a garden center or calling around to local suppliers, trying to figure out pricing, minimums, and delivery schedules. Neither option felt convenient or transparent.
Three of us – Alec, Mo, and Tyler – decided there had to be a better way. Alec and Tyler got their start back in 2013 running a landscaping business during college, moving mulch and mowing lawns to pay tuition. That experience taught them how frustrating it was to source materials, and years later, that frustration turned into Mulch Mound.
We focus on making it simple to get mulch, stone, and soil delivered directly to your home. Order online, pick your delivery date, and we handle the rest. No loading bags. No calling multiple suppliers. No wondering if you bought enough or paid a fair price.
We work with quality local suppliers in the areas we serve and aim to be straightforward about what we offer and what it costs. Landscaping is hard work. Buying the materials for it shouldn't be.
Frequently asked questions
Can you use topsoil to backfill behind a retaining wall?
No, and if it has already been done, the useful skill is telling settling apart from wall movement. Settling shows up behind the wall while the blocks stay in line. Sight down the face of the wall from one end. If the courses are still straight and tight and the only problem is a sunken strip behind it, the wall is fine and you are dealing with soil losing volume. If the top course is leaning out or you can see daylight between blocks, that is a drainage and pressure problem and topping up soil will not touch it. For pure settling, the low cost fix is to keep filling the depression with compacted fill dirt and re establish the slope away from the wall, saving topsoil for the last few inches. Expect to repeat it for a couple of seasons until the buried organic material finishes breaking down.
Is clay fill dirt safe to use as retaining wall backfill?
Clay can work in the outer zone if it is compacted properly in lifts and there is an adequate stone zone between it and the wall. Clay directly against the wall face is the classic setup for a leaning wall. It drains slowly, so water stays in it long after the rain stops, and it swells as it absorbs moisture. That combination puts sustained pressure on the back of the block. If clay is what you have, widen the stone zone and be strict about the drain pipe outlet.
Do I need filter fabric between the drainage stone and the fill dirt?
For any wall you want to last, yes, and the detail that matters most is the overlap. Fabric laid in strips needs a generous overlap at every seam, with the uphill piece lapped over the downhill piece so soil cannot ride down through the joint. Run it up the full height of the stone zone and fold the top edge over before you place the topsoil cap, otherwise soil simply pours in over the top and you have done the work for nothing. Use a woven or non woven landscape fabric rated for separation rather than the thin weed barrier sold in garden center rolls, which tears as soon as stone lands on it. Very clean, sandy fill is the one case where people skip fabric, but sandy fill is rare on most residential sites and fabric is cheap insurance either way.
How thick should the topsoil layer be over compacted fill?
About 6 inches for lawn or general planting behind the wall. That is enough root depth for turf and small perennials without burying the grade you worked out. Separately, once grass is established back there and you want to top dress it, screened topsoil spreads thin and even because of the finer screen, so apply a quarter to a half inch per pass and let the grass grow up through it rather than burying the blades.
Does the drain pipe have to slope?
Yes, and a small consistent fall beats a steep one that flattens out halfway. Set a string line along the pipe trench before you place stone and check it as you go, because it is very hard to correct once the stone is in. The usual outlet is daylight at whichever end of the wall sits lowest. If neither end daylights, the alternatives are a solid pipe run out to a lower point in the yard or a connection to an existing storm drain where local rules allow it. Put a rodent screen or grate on the open end, keep the discharge away from a neighbor's property, and photograph the outlet location before you backfill so you can find it in five years.