How Thick Does Concrete Need to Be for a Garage Floor?

Garage Basics

• Most attached or freestanding garages have a foundation, walls, doors, and a roof or ceiling. The walls, roof and doors are designed to protect the interior from the elements as well as from interior damage. In addition, the garage itself -- wood frame, steel or masonry -- is designed to resist wind forces, rot and material deterioration. Diagonal bracing, mass walls and reinforced corners are all elements used to maintain structural rigidity. The framing over the garage door is typically second only to the foundation in terms of structural strength.

Garage Floors

• Garage floors are designed to meet several demands, some of which are conflicting. A garage floor is a long, wide surface that must be of uniform thickness, with a durable, smooth, even surface. Brick pavers and stone over crushed gravel or sand beds generally met paving needs before concrete came into common use over the last 150 years. Floor drains, iron grating and floor fixtures are optional and must be designed prior to installation.

Concrete Background

• Concrete was initially developed by the Romans as cement and refined for several purposes through centuries of trial and error. Hydraulic cement has its roots in aqueducts. Structural cement was developed for pouring into molds. Major advances in cement material technology occurred when cement was rediscovered in early 19th-century Europe. Metal reinforcement, or rebar, substantially improved the structural performance of concrete. Most concrete today is designed with steel rebar to meet specific design requirements for strength and flexibility.

Garage Floor Thicknesses

• Typical garage floors are a slab of concrete on grade construction with construction joints, expansion gaps, rebar attachment to the foundation and a low slope of 1 inch per 10 feet to meet code minimums. Garage floors begin with compacted earth, a layer of crushed gravel 4 to 6 inches thick, a vapor barrier (if the floor is part of an attached garage), welded wire fabric and 4 to 6 inches of concrete. Areas of point loading must be thickened to prevent punching through. For columns, spread footings are required. Consult with a licensed structural engineer or architect to design your concrete garage floor.