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Monolithic or Floating Slab Foundations for Garages, Sheds and Small Barns

 

Donald J. Berg, AIA

 

 

Most buildings need foundations to transfer the weight of the structure and of the roof and floor loads into the ground.  Small sheds and backyard structures like gazebos and pergolas may not need elaborate foundations because they are so light.  But, for any building over about 150 square feet, a strong foundation is essential.

 

Concrete foundations traditionally consist of three parts.  Footings are wide areas of concrete at the base of foundation walls. They spread the weight of the building evenly into the soil to prevent cracks and sways in the structure above. Foundation walls are usually  8” or thicker and extend from the top of the footings to the base of the building.  A slab, poured inside the foundation walls forms the floor or subfloor of the building and can support interior partitions.

 

The three concrete components: footing, foundation wall and slab, are usually built at three different times. Each part of a traditional concrete foundation needs to be formed, poured and finished, and then needs to dry and set for about a week before the next part can be started. The process adds to the expense of a building project and is a big part of the time involved.

 

Any water that freezes under a traditional concrete foundation will cause damage.  As water freezes, it expands with enough force to lift the entire building.  As that ice melts, it leaves an open pocket of space below the foundation. With each successive freeze/thaw cycle, that pocket expands. The result is a ratchet or jacking action that lifts and lifts the building, eventually cracking walls and windows and opening seams for even more water damage.

 

The solution is to make sure that the underside of the footings are deep enough into the ground that they are beyond the reach of frost. That’s why concrete foundations are often called “frost walls.” The “frost line,” or the depth of maximum penetration of frost is, of course, different for each region. In South Florida and southern areas of California and Arizona the frost line is considered to be at grade. That is, no freezing is expected. In northern Maine and Minnesota the line of maximum frost depth can be more than eight feet into the ground. In areas like that, a lot of digging, a lot of concrete for foundation walls, and a very expensive building project can be expected.

 

In Alaska, and other locations where traditional frost wall foundations are prohibitively deep and prohibitively costly, people have been experimenting with different solutions, with mixed results.  To help builders in these areas, the U.S. Department of Housing and Urban Development sponsored studies foundations that are called Frost-Protected Shallow Foundations or FPSFs. FPSF designs use insulation on the outside of shallow foundations to protect them from frost heaves. The theory is that a heated building will transfer enough of its heat through a shallow foundation to keep water below it from freezing.

 

One of the simplest and most effective designs is a fairly conventional concrete slab thickened to 18” or 20” at the outside edges and insulated on the outside face.  Although based on designs in use in Scandinavia for decades, this came to be known as the “Alaska Slab” foundation.

 

The Alaska slab foundation and other FPSF designs have proven to be effective for heated buildings, but what about unheated storage buildings like garages and barns?  Well, the Alaska slab foundation was the inspiration for a good solution for small storage buildings too. Using a similar thickened slab, either floating on the ground, or at a slightly higher elevation than the ground around it, works well at keeping water away and preventing frost heaves.

 

Besides reducing the depth of trenches and the amount of concrete required, a floating slab foundation can be poured all-at-once or monolithically.  That means just one concrete delivery and one drying period, for savings in both time and cost. 

 

Properly designed monolithic floating slab foundations are approved for use on garages and accessory buildings by many municipalities, north and south, across the U.S.  They need to be reinforced with steel rebars and steel wire mesh to prevent them from cracking under building loads and to help them spread those loads over a wide swatch of ground.  

 

 

Like conventional floor slabs, monolithic floating slabs should be detailed with properly located control joints.  Interior columns should not be supported on these slabs.  Instead, they should be mounted on 8” deep concrete pads that are completely isolated from the slab with 1/2” oiled felt or neoprene expansion joints.

 

The top surface of a monolithic floating slab foundation can be finished just like a conventional slab. It should be sloped at 1/8” per foot toward the largest of the building’s door or openings.  It can be broom, float or smooth finished. It can also be dressed-up with any ad-mixture, color or embossed texture that you’d use on a concrete driveway. 

 

Although some code jurisdictions allow the use of monolithic floating slab foundations on detached garages and accessory buildings of up to 2,000 square feet in area, most restrict them to just 24’x24’ (576 square feet) or less. Make sure that your building department specifically allows floating monolithic slab foundations for buildings of the size that you’re planning.

 

Monolithic floating slabs are not recommended for use on sloping sites and on sites with mucky or soft clay soil. Top soil and all organic material like sod and roots must be removed from the area of the new slab.

 

Is a monolithic floating slab right for your site and your building?  Download and print the details below and then discuss them with the pros at your building department and with your builder, concrete contractor, architect or construction engineer.

 

1.  TYPICAL MONOLITHIC FLOATING SLAB FOUNDATION – These details are for all regions where the maximum frost depth is less than 12” below grade and for colder regions where it is specifically approved for garages and accessory buildings by the local building department.

 

2. PERMEABLE BASE MONOLITHIC FLOATING SLAB FOUNDATION – These details provide additional frost heave protection by under laying the entire monolithic foundation with water-permeable gravel.

 

 

3. TYPICAL FROST WALL FOUNDATION

 

 

 

Architect Don Berg has designed more than a hundred barns, garages and sheds.

 

Photos by Frank DiMaggio

 

 

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Have all plans reviewed by your building department or by an experienced building professional who knows local conditions before you purchase materials or start to build. The design and the plans may have to be modified by a local professional to suit your site's conditions, building and zoning codes and weather requirements.

 

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