21B in Virginia: The Coarser Base People Order Instead of 21A

Two loads of dense-graded base tipped on a Virginia driveway look near enough identical from the cab of a pickup, and the wrong one of them ends up under a good many patios. The difference is fines, not stone, and it shows up months later when a patio laid on the wrong base has settled at the downhill corner or a pad has stayed damp under the flagstone. 21B is the coarser of the pair, and knowing where that puts it saves a job being rebuilt.

What separates 21B from the finer base sold beside it

Both sizes are dense-graded, both are sold here, and both are described locally by their VDOT designation rather than by a trade name. That designation is how material is specified across the state, on plans and at yards, not something a storefront holds.

Neither is a drainage stone. Both carry enough fine material to hold water if the layer has nowhere to send it, which is why the grade you build into the subgrade matters more here than the choice between the two. If the job needs open voids rather than a hard layer, the answer is a washed size such as #57 Stone, not a change of base.

Paver patio, dry creek bed and fire pit ring, side by side

Three landscape jobs that regularly take this material, with what each one actually asks of it.

• A paver patio. The base carries the pavers and holds the line. Compact 21B in lifts to a flat, firm plane, then set a thin bedding course of Mason Sand on top. The base is structural; the sand is a leveling skim and never a substitute for the base.
• A dry creek bed. The rounded stone people want to see is the top layer only. A compacted course underneath gives the channel a shape that survives a thunderstorm, so the River Rock sits in a formed route rather than migrating into the lawn on the first big rain.
• A fire pit surround. The seating ring has to stay level and stay dry underfoot. A compacted base under the pavers or under Screenings does that job. Keep the base out of the fire bowl itself and follow whatever the pit manufacturer specifies for the burning area.

The common thread: this material goes under the thing people look at, never on top of it.

Clay that holds water, coastal ground that never drains away, and rock near the surface

The same 3/4 inch minus behaves differently depending on what it is laid over, so read the ground before you settle on a depth.

• Heavy Piedmont clay around Richmond, Charlottesville and Lynchburg drains slowly and gives the base no help at all. Compaction and fall have to do everything, and a base laid in a wet spell over saturated clay will pump rather than knit.
• Flat Coastal Plain ground through Hampton Roads, the Eastern Shore and the river necks has water sitting close to the surface and nowhere for it to run. A patio base there wants a separation layer and a route out for water, not extra thickness.
• West of the Blue Ridge, the limestone valleys put bedrock close to the surface in places and soft pockets beside it. A pad can be firm at one corner and soft at the next, so check the whole footprint rather than one test hole.
• Freeze-thaw is moderate across the Piedmont and the north and heavier in the Blue Ridge and the mountains. A tightly compacted layer takes in less water, and water is what lifts a patio over a winter. Frost depth is set locality by locality in this state, so use the figure your own city or county publishes.

Saturated clay after a long wet spell is the usual reason base work waits, and in the mountains frozen ground in midwinter does the same.

Working out the load and clearing a spot the truck can reach

Multiply length by width in feet, multiply by depth in inches divided by 12, then divide by 27 for cubic yards.

• Example, a 12 ft by 16 ft paver patio base at 4 inches: 192 square feet x 0.333 = 64 cubic feet. 64 / 27 = 2.4 cubic yards.
• Example, a fire pit surround 7 ft across with a 3 ft bowl left out, at 4 inches: 0.785 x (49 - 9) = 31.4 square feet. 31.4 x 0.333 = 10.5 cubic feet. 10.5 / 27 = 0.4 cubic yards.
• Example, a dry creek channel 24 ft long and 3 ft wide at 4 inches: 72 square feet x 0.333 = 24 cubic feet. 24 / 27 = 0.9 cubic yards.

This site publishes roughly 3,000 lb per cubic yard for crusher run, about 1.50 tons per yard, and that weight is approximate: the patio base above is 2.4 x 1.50 = roughly 3.6 tons, with the fire pit surround about 0.6 tons and the dry creek channel about 1.4 tons.

Those figures are loose volume before compaction, so order to the next sensible quantity up rather than trimming to the exact number. The 21B calculator turns cubic yards into tons for this material.

On the day, the driver needs a firm approach, a tipping spot within reach of where you are working, and clearance overhead for a raised body. Pick a quantity and a delivery date once the subgrade is cut, since a tipped pile left standing through a wet week is heavier and stickier to move than one you spread the same afternoon. Delivery is arranged at checkout from your ZIP code.