Ever feel like things tend to happen in threes? This past week, my three times event has been Hansen Pole Buildings Designers and their clients asking about “Alaska Slabs”.
Concrete foundations, for buildings other than pole buildings, traditionally consist of three parts. Footings are wide areas of concrete at the base of foundation walls. Usually footings will be twice as wide as the foundation walls they support. The footing thickness will be dictated by the number of stories being carried, with a minimum of six inches. Footings spread the weight of the building evenly into the soil to prevent cracks and sways in the structure above. Foundation walls are six inches or thicker and extend from the top of the footings to the base of the building. A concrete slab, poured inside the foundation walls forms the floor or subfloor of the building and can support non-structural interior partitions.
The three concrete components: footing, foundation wall and slab, are usually done in three distinct steps. 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 greatly to the expense of a building project and is a big part of the time involved.
Any water which freezes under a traditional concrete foundation can and will cause damage. As water freezes, it expands with enough force to lift the entire building. As the ice melts, it leaves an open pocket of space below the foundation. With each successive freeze/thaw cycle, the pocket expands. The result is a jacking action which lifts the building, eventually cracking walls, causing doors and windows to not open and close properly and opening seams for even more water damage.
Besides the simple solution of using isolated footings – such as in pole building construction, where pressure preservative treated wood columns are embedded into the ground below the frost line, there may be another solution.
The typical solution is to make sure the bottom of the footings extend deep enough into the ground so as they are beyond the reach of frost. This is 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 the far southern portion of the United States, no freezing is expected, so the frost line is considered to be at grade. Get into the upper Midwest or northern New England and the line of maximum frost depth can be more than eight feet into the ground. In areas such as this, a lot of digging, a lot of concrete for foundation walls, and a very expensive building project can be expected.
Where the Hansen Buildings offices are, along the Minnesota-South Dakota border, I’ve seen the frost reach seven feet deep!
In Alaska, and other locations where traditional frost wall foundations are prohibitively deep and prohibitively costly, different solutions have been experimented with, affording mixed results. To help builders in these areas, the U.S. Department of Housing and Urban Development sponsored studies on foundations which 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 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 used 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, shops 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 concrete slab foundation can be poured monolithically (all at once). This means just one concrete delivery and one drying period, for savings in both time and cost.
Properly designed monolithic floating concrete slab foundations are approved for use on garages and accessory buildings by many municipalities, across the U.S. They need to be heavily reinforced with steel rebar to prevent them from cracking under building loads and to help 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 and interior load bearing walls should not be supported on these slabs. Instead, they should be mounted on 8” deep concrete pads which are completely isolated from the slab with properly designed expansion joints.
Although some code jurisdictions allow the use of monolithic floating concrete 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. Before considering a floating monolithic slab foundation the Building Department should be consulted to confirm they allow the use for buildings of the size being considered.
Pole buildings can be mounted to FPSF designs by use of properly designed and sized wet set brackets. These brackets are going to add to the overall cost of the building project, in some cases significantly.
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 any new concrete slab.
For post frame (pole) building construction, the quickest and least costly solution still remains the traditional embedded column foundation.
