What Equipment Do I Need to Compact Subgrade for a Slab?

Subgrade is the native or fill under the base, not the decorative rock on top. A garage slab on fluff is a cracked floor. Proof-roll it. Roll the field, plate the edges, rammer the clay pockets the drum walked over. This is a day you schedule before forms, not during them.

Equipment for compacting slab subgrade

Primary machines

Ride-on roller

Primary

RD12A class.

The field of a garage, shop, or house pad.

Skip it when the slab is a 10×10 shed — plate the whole thing.

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Plate compactor

Primary

WP1550A class.

Edges, thickened perimeters, and small pads.

Skip it when you truly have roller coverage everywhere, including next to the stem wall.

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Optional / nice to have

Rammer

Optional

BS62 class.

Clay pockets and against footings where the drum will not reach.

Skip it when the whole pad is granular and a plate or roller is locking it.

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CTL

Optional

T550 class.

Proof-roll and place undercut/import.

Skip it when you are only vibrating an already-placed pad.

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Typical pairing

Proof-roll with the CTL, undercut with a mini if it pumps, roll the field, plate the edges, rammer the ugly corners. Then place base. Do not pour on a pretty grade that still weaves.

Native vs import

Organics out. Fill in lifts. If the native pumps, you are not compacting — you are stirring. Fix moisture or undercut. A mini can undercut; a roller cannot invent a subgrade.

The proof-roll

Drive the CTL across it. Pumping, rutting, or weaving means stop. More vibration on a wet fat clay is entertainment, not engineering.

Then base

Once subgrade is tight, place base and compact that too — different lifts, same honesty. See compact driveway base if the slab is a garage approach.

FAQ

Do I need a geotech?

Sometimes. A house garage on known fill might be a city spec. A shop on mystery dirt might need testing. We compact. We do not stamp reports.

Winter subgrade in Utah?

Frozen ground will lie to you. Thaw, then compact, or wait. See the winter job-site guide for machines that start at 6 a.m. — density still needs unfrozen material.