Guides

Paver patio base depth

Almost every failed paver patio failed underneath. The pavers are the part you see and the cheapest part to get right; the base is what decides whether the surface is still flat in five years. Here is how deep to go, why it goes in thin layers, and how much stone that is to order.

What is actually under a paver

Four things, from the bottom up: the subgrade, which is your own soil, compacted; the base, a compacted layer of crushed aggregate that carries the load and spreads it out; the bedding layer, a thin course of coarse sand the pavers are set into; and the pavers themselves, locked in at the perimeter by an edge restraint and with sand swept into the joints.

The base is the structural part. The bedding sand is not — it exists only to let each paver seat level, and making it deeper is one of the most common and most damaging mistakes on a DIY patio. Sand is not a base. Thick sand ruts under load.

How deep the base goes

Depth follows load and drainage. The Interlocking Concrete Pavement Institute puts the minimum compacted base at 4 in. for sidewalks, patios and pedestrian areas over well-drained soil, and 6 in. for residential driveways on the same. In practice most installers go deeper on a driveway, and considerably deeper over clay.

Paver thickness follows the same split. Pedestrian areas and residential driveways are normally 2⅜ in. (60 mm) units; anything carrying regular vehicle traffic wants 3⅛ in. (80 mm). Total excavation is just the three layers added together:

Two things push you toward the deep end of every range. Clay holds water and moves as it wets and dries, so it needs more base above it to spread the load and a geotextile fabric between soil and stone to stop the two mixing. Frost does the same thing seasonally: where the ground freezes hard, a deeper base gives the heave somewhere to go that is not your patio surface.

The bedding layer is 1 inch

Screeded to a nominal 1 in., and it should be coarse concrete sand meeting ASTM C 33 — not play sand, not stone dust, not whatever is cheapest. Fine sand holds water and migrates; the joints and the bedding both want angular, washed, sharp material.

Resist the temptation to use extra sand to smooth out a base that is not flat. Every low spot you fill with sand instead of compacted stone is a spot that will settle. Fix the base.

Compact in thin lifts

This is where DIY patios are won and lost. A walk-behind plate compactor of the sort you rent can only genuinely compact 2 to 3 inches of aggregate at a time. Dumping 4 inches in and running the compactor over the top gives you a firm crust with loose material underneath, which is worse than useless because it looks finished.

So a 4 inch base is two lifts, not one. Spread about 2 inches, compact it, spread the next 2 inches, compact again. A 12 inch driveway base is five or six passes of this. It is tedious and it is the single highest-value thing you will do on the whole job.

Compact the subgrade before any stone goes down, too. And run the compactor over the laid pavers at the end, with joint sand swept in, to seat them into the bedding course.

Slope it, or you have built a pond

A paver surface has to shed water. The minimum is ⅛ in. of fall per foot, and a quarter inch per foot is a common and safer target. Over a 12 foot patio that is 1½ to 3 inches of drop across the whole run — enough to matter, not enough to feel like a slope underfoot.

Slope away from the house, always. Establish it in the base rather than trying to find it in the bedding sand, and check it with a level and a tape as you go, because it is very hard to correct once pavers are down.

Edge restraint is not optional

Pavers work by wedging against each other. Without something holding the perimeter, the outside course creeps outward, the joints open, and the field loses its interlock from the edge inward. Plastic or aluminium edging spiked into the compacted base is the normal answer, and the base has to extend a few inches past the paver edge to give it something to sit on. Plan that extra width into your excavation.

Working out how much stone to order

Base aggregate is sold by the cubic yard or the ton. Volume comes from the same arithmetic as everything else in the yard:

A 12 × 16 patio is 192 square feet. At a 4 inch base that is (192 × 4) ÷ 324 = 2.4 cubic yards of stone, plus (192 × 1) ÷ 324 = 0.6 cubic yards of bedding sand.

Two adjustments before you order. The number above is the finished, compacted volume, and aggregate is delivered loose. Base material with fines in it settles by roughly a fifth under a compactor, so the loose volume you need is about a quarter more:

That takes the 2.4 yard example to about 3 yards. Confirm the allowance with your supplier before a big order, because it moves with the material. Then add the usual margin for an irregular shape and for the base extending past the paver edge.

If your supplier quotes in tons rather than yards, the conversion depends on the stone. Cubic yards versus tons covers that, and why the number moves with material.

Quick checklist

• Compacted base 4 in for a patio, 6 in minimum for a driveway, deeper over clay or where it freezes hard.
• Bedding sand 1 in, coarse concrete sand, screeded and never used to level out a bad base.
• Compact in 2 in lifts, and compact the subgrade first.
• Minimum ⅛ in per foot of fall, sloped away from the house.
• Geotextile between soil and stone on clay or soft ground.
• Edge restraint all the way round, with base extending past the paver edge to carry it.

Lay out your patio in the planner

Draw the patio and the planner gives you the square footage, so the base and bedding volumes above are one multiplication away. Set the gravel depth to whatever base you settled on here and it will work the cubic yards out for you, along with an approximate tonnage.