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The specification

The Base Specification: The Buried Half of a Patio

Almost everything that goes wrong with a patio, a path or a segmental wall went wrong below the surface, before the first unit was laid. This page is the list of numbers that decide it.

Last updated .

The one question that separates two bids. “How deep is the compacted base, and how will the subgrade be compacted before it goes down?” Two quotes for the same patio can differ by a large margin and both be honest — one is building 4 inches of compacted aggregate on a compacted subgrade, the other is laying pavers on a couple of inches of screenings over whatever was there. You cannot tell them apart from a photograph, and you cannot tell them apart in year one either.

What you are actually buying

A segmental pavement is four layers and only the top one is decorative: compacted subgrade, compacted aggregate base, a thin bedding course, then the units themselves. The visible layer is the one shown in every photograph, the one every showroom is organised around, and the one with the least influence over whether the surface is still flat in five winters.

This matters commercially because the three layers that decide the outcome are the three that are cheapest to quietly omit. Nobody sells a thinner base. It simply appears as a lower number on a quote, with no line explaining what was left out, and it is unverifiable the moment the work is covered.

The good news is that the numbers are published. The Interlocking Concrete Pavement Institute’s Tech Spec 2 is the reference the trade works to, and it puts figures on every layer. What follows is that document, quoted, with the retrieval recorded.

How we read this document. Read as the PDF republished by an ICPI member distributor. ICPI has since merged into the Concrete Masonry & Hardscapes Association and icpi.org redirects to masonryandhardscapes.org, so the member mirror is the copy we were able to retrieve on 5 September 2026. The copy is marked © 1995 ICPI, revised February 2020. Every figure on this page is quoted from it; where a number is ours rather than ICPI’s, it is labelled as ours in the same sentence.

Base thickness, by what will be on top of it

ApplicationICPI minimumCondition
Patio, walkway, pedestrian area4 in.Well-drained soils.
Residential driveway6 in.Well-drained soils. A car is a different load from a chair.
Residential street8–10 in.6 in. where there is no freeze.

Read those as floors on good ground rather than as answers. ICPI is explicit about why greater thicknesses turn up in practice: “Greater thicknesses are often used in regions with numerous freeze-thaw cycles, expansive soils, or very cold climates.”

And on cold, wet ground it states an addition:

“In colder climates, continually wet or weak soils will require that bases be at least 2 to 4 in. (50 to 100 mm) thicker.”

That sentence is the reason a national number is not a specification. Two identical patios, one in a dry climate on sand and one on wet clay with a hard freeze, do not share a base depth, and any quote that arrives without anyone having established which one your yard is has assumed the favourable case. The Web Soil Survey from the USDA Natural Resources Conservation Service is a free parcel-level lookup of the mapped soil and its drainage class; it takes a few minutes and it is the cheapest useful thing a homeowner can do before buying hardscape. It is a survey map, not a site investigation of your lot.

The subgrade, which is compacted before any stone arrives

The subgrade is the native soil left after excavation. ICPI specifies compaction to 98% of standard Proctor density, ASTM D698 for pedestrian applications, and modified Proctor density, ASTM D1557 or AASHTO T180 for vehicular ones. Two different tests, because the two applications are not the same problem.

This is the step with no photograph. It produces nothing visible, it takes machine time, and skipping it is undetectable until the surface dishes. A base compacted immaculately over a loose subgrade settles anyway, because the layer underneath is still on its way to its own density with your patio sitting on top of it. Fill is the sharpest version of the same problem: material placed to raise a grade compacts on its own schedule unless someone compacts it on yours.

Lifts, and why base work is slow

Aggregate base is placed in layers, each compacted before the next goes on. ICPI describes lifts of 4 to 6 inches and 7,000 lbf (31 kN) reversible plate compactors for the work.

A plate compactor transmits energy only so far down. Dump 6 inches of stone and run a compactor over the top and you get a firm crust on a loose bottom — a base that tests fine at the surface and consolidates for years underneath. This is why a proper base build takes the time it does, and why “we’ll have it done in a day” on a driveway is a statement about the base rather than about the crew.

Bedding sand is one inch, and it is not a levelling course

“Bedding sand should be spread and screeded to an uncompacted nominal 1 in. (25 mm) thickness.”

The material is specified: ASTM C33 or CSA A23.1 FA1 concrete sand. The exclusions are explicit:

“Masonry sand for mortar should never be used for bedding, nor should limestone screenings or stone dust.”

Screenings and stone dust are the usual substitutions. They screed beautifully, they are frequently already on the truck, and they hold water instead of draining it — which is precisely the wrong property one inch below a surface whose entire job is to shed water.

And the misuse ICPI names directly:

“If there is an uneven base, the bedding sand should not be used to compensate for it. Over time, unevenness in the bedding sand will reflect through to the surface.”

If you watch someone screed a deep wedge of sand into a hollow, you are watching a base problem being buried rather than fixed. Sand consolidates under traffic in proportion to how much of it is there, so the hollow reappears — later, and by then attributed to something else.

Edge restraint: without it, the field spreads

“A minimum of 1 in. (25 mm) vertical restraining surface should be in contact with the side of the paver to adequately restrain it.”

A segmental pavement works by interlock, and interlock requires that the units have nowhere to go. An unrestrained edge lets the field creep outward under load; the perimeter joints open first, the interlock is lost, and the failure then migrates inward. A minimum of 1 inch of vertical restraining surface in contact with the side of the paver is the published requirement, and how the restraint is fixed matters as much as which product it is.

Edge restraint is inexpensive, quick, and completely invisible once the job is finished. That combination is why it is the most frequently omitted item on a cheap hardscaping quote.

Slope: the cheapest specification on this page

“Slopes should be a minimum of 1.5%.”

1.5% is the published minimum, and the direction is away from the house. Fall is set during excavation, it costs nothing extra to build in, and it is the difference between a surface that sheds water and one that sends it through the joints into the bedding and the base every time it rains. A proposal that names a percentage and a direction has thought about water. One that names neither has left the most consequential decision on the project unrecorded.

Paver thickness is part of the specification too

ICPI distinguishes units around 2.375 inches for pedestrian applications from around 3.125 inches for street applications. This one is visible and checkable, which makes it unusual on this page — and it is worth checking, because a pedestrian-thickness unit under vehicle traffic is a substitution that is easy to make and easy to see if you look.

Frozen and saturated ground

“Frozen or saturated sand should not be installed, and frozen material should not be compacted.”

Frozen ground cannot be compacted to a meaningful density, and neither can saturated material. Work built on frozen subgrade looks solid on the day, thaws in spring, and settles as the ice that was carrying it leaves. If work is being scheduled into freezing conditions, put the cold-weather exclusion in the agreement now rather than arguing about it in May.

Quantities, and the one number on this page that is ours

Aggregate is bought loose and ends up compacted, so the volume delivered is larger than the volume in place. Our gravel calculator and paver calculator apply a planning allowance of +20% to loose volume.

That allowance is editorial, and it is ours, not ICPI’s. Compaction is a real volume change and ICPI requires the base to reach 98% of standard Proctor density. The ratio between the loose volume you buy and the compacted volume you end up with depends on gradation, moisture and the compactor, and there is no published national figure for "crushed stone". Twenty percent is our planning allowance, not a measurement of your quarry.

Paste this into your request for a quote

The specification below is the section to print. Send it as written and ask for each line to be answered in the proposal rather than in conversation. A company that builds to the specification can fill this in without effort; one that cannot is telling you something useful.

  1. Excavation depth, in inches, and where the spoil goes.
  2. Subgrade compaction, stated as a method and a target. ICPI’s figure for pedestrian work is 98% of standard Proctor density, ASTM D698; for vehicular work it is modified Proctor density, ASTM D1557 or AASHTO T180.
  3. Base material and compacted thickness, in inches, and the reason for that thickness given my soil and climate.
  4. Lift thickness, in inches, and the compaction equipment. ICPI describes lifts of 4 to 6 inches and 7,000 lbf (31 kN) reversible plate compactors.
  5. Bedding material and depth. ICPI specifies ASTM C33 or CSA A23.1 FA1 concrete sand, screeded to an uncompacted nominal 1 inch. Confirm it is not masonry sand, limestone screenings or stone dust.
  6. Edge restraint: which type, and how it is fixed. ICPI asks for a minimum 1 inch of vertical restraining surface against the paver.
  7. Slope, as a percentage and a direction. ICPI’s minimum is 1.5%, away from the house.
  8. Paver thickness, in inches, and whether it is rated for the traffic this surface will carry.
  9. Jointing material, and whether the field is compacted after laying.
  10. Cold-weather exclusions, in writing. ICPI: “Frozen or saturated sand should not be installed, and frozen material should not be compacted.”
  11. Who calls 811 before digging, and when.
  12. What the settlement guarantee covers specifically, for how long, and what would void it.

How to read the answers

  • Numbers, not adjectives. “Properly compacted base” is not a specification. Inches and a compaction standard are.
  • A depth tied to your ground. If the depth quoted is the same one that would be quoted anywhere in the country, nobody has looked at your soil.
  • Silence on drainage in a wet yard is the answer. Water is the mechanism behind most of the failures on this page, and a proposal that never mentions it has not scoped it.
  • An inch of sand, not three. A deep bedding course is a base problem wearing a disguise.
  • Edge restraint named as a line. If it is not in the proposal, assume it is not in the price.

What HyreYard is, and is not. We do not build patios, walls or paths, we do not excavate, compact or specify anything on your property, and we do not inspect work. We publish what the industry’s own documents say so that the conversation you have with a landscaping company starts from published figures rather than from a price. The specification that governs your project is the one in your agreement, and the authority that inspects it is your local building department.

Related

Sources

Read as the PDF republished by an ICPI member distributor. ICPI has since merged into the Concrete Masonry & Hardscapes Association and icpi.org redirects to masonryandhardscapes.org, so the member mirror is the copy we were able to retrieve on 5 September 2026.

Questions

How deep should the base under a patio be?
ICPI Tech Spec 2 publishes a minimum of 4 inches of compacted aggregate base for patios, walkways and pedestrian areas on well-drained soils, and 6 inches for a residential driveway. Those are minimums on good ground, not targets and not universal answers. The document adds: “In colder climates, continually wet or weak soils will require that bases be at least 2 to 4 in. (50 to 100 mm) thicker.” If your soil holds water or your winter cycles through freezing repeatedly, the published patio minimum is a floor you should expect to build above. The only way to know your own number is for someone to establish what your subgrade actually is before quoting a depth.
Why does a driveway need a thicker base than a patio?
Because the load is different in kind, not only in degree. A chair and a table put a static, trivial load on a surface; a car puts a repeated, concentrated, moving load through four small contact patches. ICPI separates the two explicitly — 4 inches for pedestrian areas, 6 inches for residential driveways, and 8 to 10 inches for residential streets, with 6 inches where there is no freeze. It separates the pavers too: units around 2.375 inches thick for pedestrian use against around 3.125 inches for street applications. A patio base under a driveway is not a saving, it is a rebuild with a delay on it.
What is subgrade compaction and why does it come before everything else?
The subgrade is the native soil left after excavation, and it is the ground everything above it rests on. ICPI specifies compaction to 98% of standard Proctor density, ASTM D698 for pedestrian applications, and modified Proctor density, ASTM D1557 or AASHTO T180 for vehicular ones. If that step is skipped, the base can be placed and compacted perfectly and the surface will still settle, because the layer underneath is still finding its own density. This is the single most invisible line item in hardscaping: it consumes real machine hours, it leaves nothing to photograph, and it is completely undetectable in a finished job until the first winter.
What does "placed in lifts" mean?
That the aggregate base goes down in layers, each one compacted before the next is placed, rather than in one deep dump that gets a pass of the compactor on top. ICPI describes lifts of 4 to 6 inches and 7,000 lbf (31 kN) reversible plate compactors for the work. The physics is simple: a plate compactor transmits energy only so far down, so a single 6-inch placement ends up with a firm crust over a loose bottom. On site, lifts are what makes base work slow, and slow is what you are paying for.
Can bedding sand be used to level out an uneven base?
No, and ICPI says so directly: “If there is an uneven base, the bedding sand should not be used to compensate for it. Over time, unevenness in the bedding sand will reflect through to the surface.” Bedding sand is a thin, uniform setting layer — ICPI: “Bedding sand should be spread and screeded to an uncompacted nominal 1 in. (25 mm) thickness.” Using a deeper patch of sand to bury a low spot in the base moves the problem to the surface on a delay, because sand consolidates under traffic in proportion to how much of it there is. If someone is screeding three inches of sand into a hollow, the base beneath is not finished.
What sand is correct, and which materials are excluded?
ICPI specifies ASTM C33 or CSA A23.1 FA1 concrete sand for bedding. The exclusions are explicit and worth quoting in full: “Masonry sand for mortar should never be used for bedding, nor should limestone screenings or stone dust.” Screenings and stone dust are the common substitutions, partly because they screed beautifully and partly because they are often already on the truck. They hold water and they do not drain, which is exactly the wrong property an inch below the surface of something that has to shed water.
Why does a paved area need edge restraint?
Because a segmental pavement is not a monolith. It is a field of individual units that work by interlocking against each other, and a field with an open edge spreads outwards under load until the joints open and the interlock is lost. ICPI’s requirement is specific: “A minimum of 1 in. (25 mm) vertical restraining surface should be in contact with the side of the paver to adequately restrain it.” Once spread has started it migrates inwards from the perimeter, and nothing done to the surface stops it. Edge restraint is cheap, it is invisible when done, and it is the most commonly omitted line on a low bid.
How much fall does a patio need?
ICPI’s published minimum is unambiguous: “Slopes should be a minimum of 1.5%.” A dead-flat patio built against a house is a drainage problem you have paid to install, and on a segmental pavement the water that does not run off runs in — through the joints, into the bedding, into the base. Fall is also the cheapest thing on this page to specify and the easiest to check: it is established during excavation and it costs nothing extra to build in. A quote that names a direction and a percentage has thought about water; one that names neither has not.
Can hardscaping be built in winter?
ICPI states a hard exclusion: “Frozen or saturated sand should not be installed, and frozen material should not be compacted.” Frozen ground cannot be compacted to any meaningful density, and saturated material cannot either. A base built on frozen subgrade appears solid on the day, thaws, and settles as the ice that was holding it up leaves. If work is being scheduled in freezing conditions, get the cold-weather exclusion written into the agreement rather than argued about in spring.

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