The average national cost of building a foundation is $7,697, with most homeowners spending between $4,231 and $11,572. This data is based on actual project costs as reported by HomeAdvisor members.
Any structure, whether a house, a garage, or workshop, is only as strong as its foundation. The better educated you are about types of foundations and how best to plan your foundation, the better you can work with your contractor to create the best foundation for your structure at the best price. Here are a few things that will affect the cost of building a foundation.
Types of Foundation
Most foundations are defined as deep or shallow. A shallow design foundation is set about three feet into the soil, with concrete pads extending below the frost line, transferring load from the walls to the earth. Another type of shallow foundation is called slab-on-grade. This type transfers load to the earth through a slab of concrete.
A deep foundation is used when soil conditions are more difficult, like building on the side of a hill, or if the structure must be raised to prevent flooding. Deep foundations can be made from steel, wood, reinforced or pre-tensioned concrete. A deep foundation sometimes will penetrate the bedrock, so it is good to bring in a structural engineer in these cases.
In most cases a deep foundation will be more costly than a shallow one. It requires more materials, more excavation, and more labor.
Planning the Foundation
Laying a foundation requires a plan. You will have to adhere to local building codes and have inspections from the city or county while you build. So it is important to follow local codes and get the correct permits in advance. You may also need to have soil or seismic reports as well. Even though getting these reports can add cost to your foundation installation, it is essential that you do this and it can save you a lot of money in the long run. If you begin to build a foundation and find out half way through for whatever reason that you cannot continue, the costs could be huge. An experienced contractor will be able to help you with this, or obtain the correct permits for you.
Drainage and Radiant Heat
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. Drainage pipes and radiant heat tubes should be placed before pouring your concrete slab. So planning these features in advance is a necessity! Both these features are expensive but close to impossible to add later on and will add value to your home. So it may well be worth the extra costs upfront.
Sealing Concrete Slab
If you are building a garage or workshop space, you will need to seal your concrete slab 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.
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.
Adding a concrete sealer at the time your foundation is being constructed is relatively low cost compared to the benefits it will give you in the long run.
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