How Deep Should Topsoil Be for a New Lawn?
Four to six inches of topsoil is right for a new lawn. Four is the working minimum. Six is better if the ground underneath is poor. More than eight is usually wasted money, and over compacted clay it can actively hurt.
That last part is the bit nobody mentions, so it is worth explaining before you order.
Why deeper is not better
Grass roots do not politely stop at the bottom of your new topsoil. They grow until something stops them, and a sharp boundary between loose new soil and hard old ground is exactly that. Water hits the same boundary and stops too, sitting in the bottom of your topsoil layer instead of draining away.
Pile eight or ten inches of topsoil onto compacted clay and you have built a bathtub. The grass roots in the top few inches, sits in a wet layer after every storm, and browns off in the first dry spell because a shallow-rooted lawn has nothing to fall back on.
The fix is not more soil. It is breaking the boundary — see whether to till topsoil in or spread it on top.
Depth by situation
| Situation | Depth | Why |
|---|---|---|
| New lawn, decent soil underneath | 4 in | Enough root zone; the ground below is already usable |
| New lawn, poor or builder's subsoil | 6 in | The topsoil is the whole root zone, so it has to carry the lawn |
| Over compacted clay | 6 in, loosened first | Depth without loosening builds a perched water table |
| Topdressing an existing lawn | ¼–½ in per pass | Any more smothers the grass — see topsoil over grass |
| Vegetable beds | 8–12 in | Root crops need the room; no grass to smother |
| Trees and shrubs | Backfill, don't layer | Mix with native soil so roots leave the planting hole |
Order for the settled depth, not the spread depth
Delivered topsoil is fluffed up. It settles roughly 15–20% once it is raked, watered and walked on. Spread four inches and you have about three and a quarter inches of lawn soil a month later.
So if you want four inches settled, spread five. In volume terms, for 1,000 square feet:
- 4 in settled — spread 5 in, order about 15.5 cubic yards
- 6 in settled — spread 7 in, order about 21.5 cubic yards
The topsoil calculator handles the conversion for your own area. Add the settling allowance to the depth before you type it in.
How to check what is underneath
Before you decide on depth, dig one hole a spade deep somewhere in the middle of the area. Two minutes, and it changes the answer:
- Dark, crumbly, spade goes in easily. You have soil. Four inches is plenty.
- Pale, dense, spade needs boot pressure. Builder's subsoil. Six inches, and loosen the top of it first.
- Grey or orange, sticky, holds a thumbprint. Clay. Six inches, loosened, and take drainage seriously before you spend on soil.
- Rubble, brick, plastic. Dig it out. Topsoil over construction spoil will always be a thin lawn over a hard layer.
The half-inch that matters more than the last two
Getting the surface level matters more to how the lawn looks than the last couple of inches of depth. Hollows hold water and the mower scalps the high spots. Rake, walk it, rake again, and check with a long board before you seed.
A well-levelled four inches beats a lumpy eight every time.