What Good Base Prep Actually Looks Like

Here's the uncomfortable truth about hiring a patio contractor: the part of the job that determines whether it lasts is the part you can't see once it's done.

By the time the pavers are set and the polymeric sand is swept, a heroic base and a garbage base look identical. You find out which one you bought somewhere between year two and year five.

So this article is a spec. Print it, screenshot it, read it out loud at the estimate. It's what we build to, and any competent crew on the Front Range should be able to talk through every line of it without flinching. If someone gets cagey when you ask what depth they're digging to and whether they compact in lifts — that's your answer.

1. Excavation depth

The excavation isn't a number somebody feels out. It's arithmetic:

Excavation depth = compacted base depth + bedding depth + paver/stone thickness

Fill in the terms:

Use	Compacted base	Bedding	Unit	Total dig
Pedestrian patio (stable subgrade)	6 in	1 in	2⅜ in paver	~9⅜ in
Pedestrian patio (expansive clay)	8 in	1 in	2⅜ in paver	~11⅜ in
Dry-laid flagstone	6–8 in	1 in	1½–2 in stone	~9–11 in
Driveway / vehicular	10–12 in	1 in	3⅛ in paver	~14–16 in

Ask the question: "How deep are you digging, and what's the compacted base depth?" If the answer is "about four inches" for a patio, they are quoting you a repair job with a delay fuse.

2. Dig wider than the patio

The excavation should extend about 6 inches past the finished edge on every free side. You need compacted base under and beyond the perimeter row of pavers, and you need room to drive the edge restraint spikes into compacted material.

A base that stops exactly where the pavers stop leaves the perimeter row supported by nothing — which is why so many failing patios fail at the edge first.

3. Deal with the subgrade

Before a single rock goes in:

• Strip the organics. Topsoil, roots, and sod rot and compress. They are not a foundation.
• Proof-roll it. Run the compactor over the subgrade. Soft spots that pump and rut get dug out and replaced with base material, not covered up.
• Lay separation fabric. A woven geotextile between clay subgrade and aggregate. This is cheap, fast, and it's the difference between a base that stays a base and one that slowly turns into clay-contaminated mud. On the Front Range's expansive clay, it's not optional.

4. The base material: Class 6, and nothing else

The workhorse aggregate base course here is CDOT Class 6 — a dense-graded crushed aggregate with a designed gradation. Coarse rock for structure, fines to fill the voids and lock the matrix up under compaction.

What Class 6 is not:

• Not pea gravel. Round, uniform stone can't interlock. It rolls under load forever.
• Not "clean" ¾-inch rock. Open-graded rock drains beautifully and has its uses, but it's not a substitute for a dense-graded base in a standard build.
• Not native soil, "recycled fill," or the dirt from the excavation. If someone proposes backfilling the base with what they dug out, walk away.

5. Compaction in lifts — the one that gets skipped

This is the heart of it.

A plate compactor densifies only a few inches of material. That's a physical fact about how compactive energy dissipates through aggregate. Which means:

Base goes in in lifts of 3 to 4 inches, and every single lift gets fully compacted before the next one goes down.

Dump the full 8 inches and hit it with the plate and you get a firm crust over loose rock. It will look flawless on install day. It will settle for three years.

The details that matter within that:

• A real compactor. A vibratory plate in the 5,000-pounds-of-force class or better for base work — not a hand tamper, not a rented mini plate that skips across the surface.
• Passes in both directions. Overlapping passes, then again at right angles.
• Moisture. Class 6 needs a little water to reach density. Colorado air is bone dry and it wicks the moisture straight out. A crew misting the base is a crew that knows what it's doing, not one wasting time.

Ask the question: "What lift thickness do you compact at?" The right answer is three to four inches. There is no other right answer.

6. Build the slope into the base

About ¼ inch per foot — 2 percent — of fall away from the house and any structure. Over a 16-foot patio, that's 4 inches of drop.

And the slope lives in the base, established with the base, checked with a level or a laser as the base is built.

It does not live in the bedding sand. Sand is not a grading material. If your contractor is fixing plane problems by varying the bedding depth, that sand will consolidate under load and every low spot they buried will come right back.

7. Bedding: one inch. Screeded.

One inch of coarse, washed concrete sand (ASTM C33) or a #8/#9 chip. Screeded off rails to a uniform depth.

Not two inches. Not "however much it takes." One inch, uniform, everywhere. Thicker bedding consolidates more, and it consolidates unevenly, and now you have ruts.

And nobody walks on the screeded bed. Pavers get laid from the field forward, with the crew standing on already-set pavers.

8. Edge restraint — the thing that makes it a pavement

A paver field is a bunch of loose units. What makes it behave as one surface is interlock, and interlock has three parts: vertical (bedding and joint sand), rotational (joint sand), and horizontal (the edge restraint).

Every free edge gets a rigid restraint: spiked polymer or aluminum edging, driven with 10-inch spikes into the compacted base, or a poured concrete toe.

No restraint, and the perimeter units migrate outward under every load, the joints open, sand washes out, and the field goes soft from the edges in. It's the most common failure on cheap installs and it is completely, trivially preventable.

9. Joint sand, final compaction, and cure

• Sweep joint sand in, then compact the field with the plate on a protective pad to seat the units and drive sand down into the joints.
• Sweep again. Joints should be full to the bottom.
• Polymeric sand gets misted per the manufacturer's instructions — and it needs dry weather to cure. A Colorado afternoon thunderstorm on uncured polymeric sand ruins it. A crew that reschedules the joint work because the forecast turned is a crew doing it right.

The one-page version to hold a contractor to

1. Excavated to the full section — 6–8 in of compacted base for a patio, 10–12 in for anything vehicular.
2. Excavation 6 in wider than the finished edge.
3. Organics stripped, subgrade proof-rolled, separation fabric over clay.
4. CDOT Class 6 aggregate base.
5. Compacted in 3–4 in lifts, every lift, both directions.
6. ¼ in per foot of fall, built into the base.
7. 1 in of screeded bedding sand. Uniform.
8. Rigid edge restraint on every free edge.
9. Joints filled, field compacted, polymeric sand cured dry.

That's it. That's the whole thing. Nine lines, and it's the difference between a patio and a project you do twice.

Want to see what happens when it's skipped? Read why Colorado patios heave. Ready for a number? Get an estimate — or see how we build paver patios.