How deep to go
| Use | Published figure | Source |
|---|---|---|
| Granular subbase, minimum | 4 in | datcp.wi.gov |
| Thicker subbase, per compacted lift | 4 to 6 in | datcp.wi.gov |
| Coarse clean gravel under a house slab | 4 in minimum | foundationhandbook.ornl.gov |
University of Wisconsin Extension's concrete chapter says that when a subbase is needed it must be at least 4 inches of granular material compacted to near maximum density, placed in 4- to 6-inch lifts if thicker. It adds that going past 4 inches gives only a minor gain in support. Oak Ridge National Laboratory's foundation handbook checklist calls for a layer of coarse, clean gravel at least 4 inches thick under the slab.
Working it out
Cubic yards = area (sq ft) × depth (in) ÷ 12 ÷ 27. Tons = cubic yards × weight per cubic yard.
Example: 24 × 24 ft at 4 inches with 10% extra is 7.82 cubic yards of no. 57 stone, about 10.1 tons at 1.29 tons per cubic yard.
Rochester Concrete Products' install guide adds 10% for compaction and uneven ground.
Common questions
How much gravel do I need for a gravel under a slab?
Measure the area, choose a depth from the table above and enter both in the calculator. It gives cubic yards, tons and bags.
Should I order extra?
Rochester Concrete Products' install guide adds 10% for compaction and uneven ground. Loose stone also compacts, so a compacted layer needs more loose material than its finished depth suggests.
Related calculators
Sources
- https://datcp.wi.gov/Documents/CDSChapter4.pdf
- https://foundationhandbook.ornl.gov/handbook/section4-4.shtml
- https://rochestercp.com/resources/installation-manuals/installation-manual-vintage-6.pdf
- https://www.fhwa.dot.gov/publications/research/infrastructure/structures/bridge/15034/003.cfm