How Deep Should a Sub-Base Be for a Concrete Slab?

A flat base course will also allow the slab to slide easily as it shrinks, reducing restraint and the risk of cracks as the concrete contracts after placement (drying shrinkage). The entire subbase and base system should be at least 4 inches thick—thicker if the engineer feels it is needed for proper support.

How much subbase do I need for a concrete slab?

As a rule; your base must be 75mm larger that the finished slab to allow for formwork. Clear the area of stones and plants and excavate to a minimum depth of 100mm (deeper for soft ground). The ground should be levelled and well compacted using a small garden roller or a lump hammer to provide a base for your concrete.

Do I need a sub-base for a concrete slab?

Subbase for Concrete Slab

The subbase placement is optional and it serves as a work platform for construction of the slab and provide more uniform support for the slab. As the thickness of subbase increases, the ultimate load carrying capacity of slab increases.