How to Build a Concrete Foundation

Any structure, whether a house, a garage, or workshop, is only as secure as the foundation on which it rests!

Building a foundation is not usually considered a do it yourself project, but it can be done with a little advice from your contractor or a friend who has experience.

But, if you plan ahead and study a little on the specifics involved with building a concrete foundation, you can do the project yourself and save considerable expense.

If you are building a foundation for the first time, a project like an outbuilding, perhaps a detached garage or workshop is a wiser choice than a complex major project like the foundation for a new house.

Level and prepare your foundation area before setting forms. Shallow design foundations are common for structures like a workshop or garage.

Concrete Foundation Designs

Typically, foundations will be of either a shallow or deep design.

A shallow design foundation will be set about three feet into the soil. A spread footing design is typical, with concrete pads extending below the frost line, and transfers load from the walls to the earth.

Another type of shallow foundation used is the slab-on-grade design, which transfers load to the earth through a slab of concrete. Slabs also allow for the use of radiant heating.

A deep foundation is generally more complex and will be used in cases when soil conditions are poor (such as a hillside for example) or the structure must be raised to prevent flooding if the structure is being built in a flood zone.

A deep foundation transfers load from the structure through a weak top soil layer down into a stronger layer. Several variations for deep foundation designs can be used and naming conventions tend to vary, but include caissons, piers, piles, drilled shafts, and earth-stabilized columns.

Deep foundations can be made from steel, wood, reinforced or pre-tensioned concrete. A deep foundation penetrates the bedrock in some cases, so an engineering expert is definitely needed!

This article is concerned primarily with planning and building shallow design foundations used in constructing a garage, workshop or outbuilding.

Planning Your Foundation

Laying a foundation requires good planning, adherence to local code, and careful consideration of how you will be using the finished structure. If you plan on adding features like a filtered drainage system and radiant floor heating, these will need to be installed during construction of the foundation.

Don’t forget you will have to have inspections from city or county officials throughout the process, so be sure to follow your local inspection codes, obtaining the correct permits and any required soils, engineering and/or seismic reports before you begin your project.

A garage foundation is typically constructed of concrete block or a poured concrete wall. First, clear, scrape and level your building site. Next dig trenches below grade for your footings. Be sure to follow local building code regulations.

Boards form the shape for foundation footings, which are typically about 2 feet wide.Setting Footings

Footings should be approximately two feet in width, with an extra two feet on both sides so that you have plenty of work space, which means your trenches will have a total width of six feet.

Use 2×10 boards to build forms for the footings. The forms will be removed after your concrete sets. Be sure to mark center wall lines so that you know where the wall forms will be placed.

The size of your structure will determine the height of foundation walls. A structure like a one story garage needs at least an eight inch tall foundation wall. For a two story structure, you will need at least a 10 inch high foundation wall. Again, check local codes to make sure you will pass inspection!

Steel rebar reinforcement is critical to making sure your foundation can bear wall stress adequately. Mechanical vibration is then needed in order to consolidate the concrete and eliminate air pockets or voids that could compromise or weaken the concrete’s integrity.

Install anchor bolts in your foundation, which will be used to secure walls to the foundation later. The number and location of anchor bolts needed is also set by local building codes. If appropriate to the use of the structure you are building and local climate conditions, insulation can also be added to the exterior of your walls.

Anchors to secure walls will be embedded in the top of the foundation walls while concrete is fresh. The slab will be poured over a vapor barrier between the foundation walls after they are completed.Concrete Slab Pouring

After completing your footings and pouring concrete to form the foundation walls, you will be ready to pour the concrete slab inside the frame of your foundation for the floor of your garage or workshop. A layer of sand or gravel is needed before you pour the slab.

Install a vapor barrier over the gravel or sand before you pour the concrete slab. This will help prevent any moisture from rising into your slab. In some locations where moisture is prevalent, a vapor barrier below the sand or gravel may also be needed.

Place wire mesh in the center of your slab and rebar in the footing areas to increase rigidity and prevent any cracks in the concrete.

Local code sets your specifications, including slab thickness, footing thickness, footing width, what size wire mesh is needed, how thick a layer of sand or gravel is needed, the size and placement of rebar and the placement of foundation anchors or bolts. Again, be absolutely sure to follow your local building code before planning your foundation specifications, setting forms and ordering materials.

Foundation anchors are embedded in the wet concrete to later secure walls firmly to your foundation; you don’t want the walls falling off your foundation in the case of an earthquake or high winds!

Features like a drain or radiant heating system must be planned for since they will be installed during foundation construction.Installing Drainage and Radiant Heating

A nice feature to add to your garage slab is a center drain for washing a car, boat etc.

Drainage pipes and radiant heat tubes should be placed and attached to the wire mesh before pouring your concrete slab. The slab should be pitched from the back towards the front of your garage for proper drainage.
There are some features to your garage or workshop you will not be able to add after your foundation is complete, so decide up front during the planning phase which features like a drain or radiant heating you will include in the foundation.

Radiant heating is a nice, efficient heating system to use in a workshop or garage, since you get heat without any need for blowers or air movement, which helps make for a more dust free environment. Also, the floor will be nice and warm for those occasions when mechanical projects require you to sit, kneel or lie on the floor!

Adding a drain may also be worth considering. In some cases, a garage or workshop drain simply uses drywell drainage; you just dig a hole and fill it with a course stone for water to drain into. Drainage pipe will be placed on the foundation bed and the drain placed before concrete is poured.

Alternatively, a connection to your sewage system may be required for your drainage system. Local codes are usually quite strict about what you can dump in garage drains. A filtering system can also be installed to keep any contaminants from entering the soil or sewer system.

Sill Sealers and Concrete Sealers

You will also want to seal your garage or workshop to inhibit moisture, cold or hot air, and particles. There are two commonly used types of sealers; floor sealers and sill sealers. The sealers are normally applied during the construction to help keep your garage or workshop dry and well insulated.

Sill sealers use adhesive foam and the application will sit between the top of the foundation and sill plate, creating a waterproof barrier.

A sill sealer will conform to the surface of your foundation to prevent any cracks or gaps that could otherwise form between the foundation and sill plate as the concrete shrinks as it cures. Sill sealer applications have a strong adhesive on one side that will stick to your foundation. This does a great job of protecting against moisture, air, rodents, insects, etc.

Typically, concrete sealers are spray on applications designed to prevent moisture and vapor from rising up through concrete, which is porous. Concrete sealers can provide other benefits as well, including preventing radon gas seepage and inhibiting the growth of mildew or molds. Concrete sealers can also help strengthen concrete and prevent cracks over time.

You should always get advice from a professional when planning a foundation to be sure it is all engineered correctly for the intended structure it will support and to be sure it meets all local building codes!

Step by step video on preparing your concrete foundation:

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