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Step-by-Step Guide: Building a Concrete Slab Foundation for a Garage

Author: Odell Complete ConcreteTime: 2024-01-27 02:15:01

Table of Contents

Introduction to Concrete Slab Construction

This blog post will provide a step-by-step guide to building a concrete slab foundation for a metal building garage. We will cover site preparation, building the formwork, installing rebar reinforcement, pouring and finishing the concrete slab, curing, and finalizing the project.

The example project involves constructing a 16 ft by 25 ft concrete slab on a sloped site to support a prefabricated metal garage. The finished slab will be 8 inches thick, heavily reinforced with rebar, and made with high strength 4500 psi concrete to accommodate a car lift that will be installed later.

Project Overview and Tools Needed

An existing smaller concrete slab and wood shed were previously removed from the site to make room for the larger slab and new metal building garage. After removing the old structures, the next steps involved regrading the native soil to create a level pad and digging out the footing area to reach undisturbed soil. Tools used on the project included: laser level, plate compactor, circular saw, rebar bender/cutter, rebar tying tool, screed board, floats, trowels, and an edging tool. Materials included: vapor barrier, gravel base, form boards, rebar, chairs, plastic and metal dowels, high strength concrete, and fiber mesh reinforcement.

Site Layout and Squaring the Formwork

After grading the site, gravel base and a 6 mil plastic vapor barrier were put down before the footing forms were set. String lines were used to lay out the slab dimensions of 16 x 25 ft and confirm the forms were square. The string line measurements diagonally were within 1⁄4” to verify a proper square layout. The form boards consisted of 2x12s and 2x10s anchored with duplex nails and metal corner stakes. Some 2x4s salvaged from the old wood shed were used for underpinning supports.

Installing Vapor Barrier, Gravel Base, and Rebar

A 6 mil plastic vapor barrier was rolled out over the graded and compacted native soil before the gravel base was placed. About 4-6” of gravel fill was compacted on top of the vapor barrier to create a stable base.

With the formwork in place, the rebar grid was tied with 16” spacing transversely and 18” spacing longitudinally. 1⁄2” rebar was used with a lower continuous footing bar and dowel turn-downs to lock the mat in place. Plastic rebar supports were used to hold the proper positioning.

Pouring, Finishing, and Curing the Slab

The formwork was oiled before the pour to allow easy stripping and prevent surface defects. Synthetic motor oil worked well for this. A total of 20 yards of concrete was ordered for the project – 10 yards for the footings and 10 yards for the 8” thick slab.

Various floats and trowels were used to smooth and finish the slab. Unlike most level slab foundations, a slight crown was added to allow moisture to run off. After wet curing the slab for 7 days, the forms were stripped revealing a clean surface finish ready for the new metal building.

Concrete Mix Design and Placement

The concrete specified was high strength 4500 psi mix with fiber mesh added at the truck for secondary reinforcement. Prior to placement, the concrete slump and characteristics were checked after adding the fiber mesh at the job site. The concrete was placed in two stages – initial footings pour, followed by the slab pour when ready.

Finishing Tools and Techniques

Various floats including wood, magnesium, and fiberglass were used to smooth and level the slab. The fiberglass float featured custom notches ground into the bottom to help bring cream to the surface. In addition to floating, an edging tool was used to round all slab edges. The final finish was a hard steel trowel.


Q: How thick should the concrete slab be for a garage?
A: For a garage, the concrete slab should be at least 4 inches thick. An 8 inch thick slab is recommended for extra durability if heavy vehicles will be parked inside.

Q: What size rebar is best for a garage slab?
A: Use a grid of #4 (1/2 inch) rebar placed at 18 inches on center in both directions for a garage slab.

Q: Should garage slabs be reinforced with fibermesh?
A: Yes, adding fibermesh reinforcing to the concrete mix is a good idea for garage slabs to control cracking.

Q: What strength concrete is required for a garage slab?
A: A minimum compressive strength of 4,000 PSI is recommended, but using 5,000 PSI concrete will provide extra durability for a garage slab.

Q: Should control joints be cut into a garage slab?
A: Control joints help control random cracking but are not always necessary with proper rebar reinforcement and concrete mix design.

Q: What should the foundation be made of under a garage?
A: A reinforced concrete slab on grade is the most common and cost-effective foundation type for garages.

Q: How long does it take for a garage slab to fully cure?
A: Allow the concrete to cure for 5-7 days before building on top of it and 28 days to reach full design strength.

Q: What is the best way to cure a new concrete garage slab?
A: Keep the slab continuously wet by flooding or misting for the first 7 days to ensure proper curing.

Q: How do you attach walls to a concrete slab foundation?
A: Use anchor bolts embedded into the concrete slab perimeter to bolt the wall framing.

Q: Can you pour concrete for a garage in cold weather?
A: Yes, but special cold weather concreting precautions must be taken to prevent freezing.