How Deep Should a Patio Base Be in Dane County? (Deeper Than You Think)

The most common patio installation mistake in Madison is an inadequate base. Not the wrong surface material. Not a poor layout. The base.

National installation guides often recommend 4 to 6 inches of gravel base for residential patios. In Dane County, that is not enough. Here is why, what the right depth is, and how to verify that a contractor is doing it correctly.

Why Wisconsin's frost line changes everything

Dane County has a frost line of 48 inches. That means the ground can freeze to 48 inches deep in a severe winter. When soil freezes, it expands. When it thaws, it contracts. This movement is the root cause of most patio failures in our area.

The purpose of a gravel base is to create a stable, well-draining layer between the soil movement below and the patio surface above. A deep, properly compacted gravel base does two things: it drains water away so there is less moisture to freeze, and it provides a stable platform that is less affected by the slight movement of the soil beneath it.

A 4-inch base is adequate in a mild climate with minimal frost penetration. In Madison, it is often not enough.

The right base depth for different site conditions

There is not a single correct depth for all Madison patios. The right depth depends on site conditions:

What proper base materials look like

Not all gravel is equal. The right base materials for a Dane County patio:

• Class 5 crushed aggregate (also called Class 5 or CA6): crushed limestone or granite that compacts firmly and drains well. This is the standard for Madison patio bases.
• Avoid round pea gravel or river rock: these do not compact and shift under load.
• Geotextile fabric: a non-woven landscape fabric placed between the native soil and the gravel base in clay soil areas. Prevents clay from migrating up into the gravel over time, which would compromise drainage.
• Concrete sand: a 1-inch leveling layer of concrete sand on top of the gravel, used for paver installation to allow fine-tuning of the surface level.

How to verify your base depth during installation

You have every right to visit your project site during installation and check the base depth before the surface goes down.

Once the excavation is complete, measure the depth of the hole. After the gravel is placed and compacted, measure again. The compacted gravel should be at the depth specified in your contract.

Some homeowners feel awkward checking on their contractor's work. Do not be. A good contractor welcomes inspection because it protects both of you. A contractor who discourages site visits during installation is a red flag.