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What Is a Flat Slab?

Flat slabs are typically used in foundations.
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  • Written By: Mary McMahon
  • Edited By: O. Wallace
  • Last Modified Date: 15 June 2014
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A flat slab is a flat section of concrete. These slabs are classically used in foundations, although they can also be used in the construction of roadways, paths, and other structures. Depending on the size and complexity of a flat slab, it may need to be designed by an engineer who is familiar with the limitations and needs of slabs, or it may be possible for a handy do it yourselfer to make one in an afternoon for a simple project.

Typically, a flat slab is made with reinforced concrete, in which rebar is criss-crossed in the forms to provide support and reinforcement once the concrete is poured and hardened. The slab design is designed to be reinforced in several directions so that it can withstand stresses such as shifting ground, earthquakes, frost, and so forth. Failure to fully reinforce a flat slab can cause it to crack or give along weak lines in the concrete, which will in turn cause instability.

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For some sites, a flat slab is poured in situ. In this case, the site is prepared, forms for the concrete are set up, and the reinforcing rebar or other materials are laid down. Then, the concrete is mixed, poured, and allowed to cure before moving on to the next stage of construction. The time required can vary considerably, with size being a major factor; the bigger the slab, the more complex reinforcement needs can get, which in turn adds to the amount of time required for set up. Once poured, the slab also has to be examined and tested to confirm that the pour was good, without air pockets or other problems which could contribute to a decline in quality.

In other cases, a flat slab may be prefabricated off site and transported to a site when it is needed. This may be done when conditions at the site do not facilitate an easy pour, or when the conditions for the slab's construction need to be carefully controlled. Transportation of the slab can be a challenge if it is especially large. Barges, cranes, and flatbed trucks may be required to successfully move it from the fabrication site to the site of the installation.

The flat slab foundation is not without problems. It can settle on uneven ground, allowing the structure to settle as well, for example, and during seismic activity, a slab foundation cannot hold up if the soils are subject to liquefaction. A flat slab can also become a major source of energy inefficiency, as structures tend to lose heat through the concrete.

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Discuss this Article

OeKc05
Post 4

My dad had a flat slab poured on site as a foundation for his new shop. He planned to keep tools and electronics in it and to use it as a workshop, and he wanted a substantial floor that could hold up well to abuse.

I remember helping my dad fence off the area before the concrete was poured. We had to make sure that our dog couldn't get to the slab before it dried, because he really didn't want a pawprint patterned floor or a dog with heavy feet!

The flat slab hardened into a beautiful dark concrete floor. It stays really cool in the summer, and I like to go in the shop barefoot after being out in the hot sun. My dog likes to lie on it to cool off.

John57
Post 3

My husband has done some concrete work, so we took a weekend and poured a flat slab for a dog run and kennel.

We needed a clean, dry place to keep our dogs outside, and a concrete slab was the perfect solution for this.

It doesn't get muddy when it rains, and is easy to clean. This was easily done in a weekend and ready to use by the first of the week.

Before we poured the slab, the area where we kept our dogs was a mess when the weather was bad. We poured the slab next to some trees so they have shade and a clean place to stay when they are outside now.

Mykol
Post 2

When my parents come to visit they have a long, heavy motor home they park in our driveway. They can just plug in to our electric and still have their own place to sleep while they are here.

After a few visits we noticed our driveway started to crack and buckle. This was from the heavy weight of the camper sitting there.

We ended up tearing out the driveway and making it a little bit bigger and much stronger. This included a flat slab exclusively to park the camper on.

There had to be quite a bit of rebar for the concrete reinforcement to support the weight of the motor home. Since we had this slab made, we haven't had any other problems with our driveway.

sunshined
Post 1

We had a concrete flat slab poured to place our hot tub on. We knew we wanted a hot tub outside, so before we even purchased the hot tub, we had a slab made to place this on.

We knew the size of hot tub we wanted, so made sure the slab was large enough to place it on. By the time you fill the tub with water and get a few people in it, that is a lot of weight that needs to be supported.

I didn't think there was much that went into flat slab construction, but the man who did this for us knew exactly how much reinforcement to add underneath the concrete.

I feel much better about having this on a slab than just sitting on the deck somewhere.

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