Tag Archives: floating slab foundations

Pouring Concrete into Holes With a High Water Table

Back in my general contractor days we would run into building sites where water would fill up some or all of our hole depth. While this seemed highly problematic then it was actually far less of an issue than originally presumed.

Reader RACHEL in CLARK writes:

“We are looking to put up a 24′ X 32′ pole building in my backyard to be used as a garage/wood shop. We are located in a lower spot in town and have been told our water table is fairly high. I am wondering what type of foundation is going to be the best to use? (Floating Slab vs Sinking Poles vs Sinking Concrete Piers under a slab?). We are hoping to do most of the work ourselves.”

Embedded columns for post frame buildings are almost always both a best and least expensive design solution. Auger holes to depth and diameter indicated on your engineered building plans (always build from engineered plans). If water appears in your hole, it is not a problem, as you can pour concrete into water, professionals do it often. Order pre-mix concrete for your footings and bottom collars with a minimum amount of water content (a W/CM ratio of 0.33 would be ideal).

After about two hours your concrete will have transitioned from a plastic to solid state. Ground water will become your concrete’s friend as it will aid curing processes. Chemical reaction of hydration allows microscopic crystals of Portland cement to grow and interlock as sand and gravel together continues to happen for days, weeks and months after concrete has been poured and it needs water to complete this chemical reaction.

Provided you have available space, you may consider going to a 36 foot length – it takes no greater number of columns, trusses, girts or purlins and will reduce your investment per square foot.

Adding to a Floating Slab Building

Adding to a Floating Slab Building

Most buildings need foundations to transfer the structure’s weight as well as 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.

Any water freezing under a floating concrete slab will cause damage. As water freezes, it expands with enough force to lift the entire building. As this ice melts, it leaves an open pocket of space below the slab. With each successive freeze/thaw cycle, this pocket expands. This results in a ratchet or jacking action repeatedly lifting your building, eventually cracking walls and windows and opening seams for even more water damage.

Properly designed monolithic floating slab foundations are approved for use on stud wall framed garages and accessory buildings by many U.S. municipalities, north and south. 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.

Although some code jurisdictions allow 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. If considering stud wall framing on a building consult with your building department specifically as to floating monolithic slab foundations applicability.

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 new slab area.

Reader WILLIAM in CANDOR writes:

“I have a 24’x24′ stick built floating slab shop. I would like to build a 30’x50′ pole building attached to it. Can I or should I do this.”

Mike the Pole Barn Guru responds:

With this said, your new pole building can be abutted to your existing stick built floating slab shop, however it should not be structurally attached to it, as your new building will be designed to resist frost heave issues, while your existing shop will have up and down movement.