While not a requirement, a concrete slab foundation provides a solid, clean surface for your steel building and can help it last longer. Since the building will require anchoring to minimize shifting, pouring a concrete slab gives you a chance to pour footings as well.
Step 1 – Site Preparation
- Clear the area where you will construct your steel building of grass and weeds.
- Stake out the slab location. Provide, at a minimum, an additional 2-4 inches per side and 3-6 inches per end outside of the building perimeter.*
- Remove 4 inches of soil from the area and dig the footings.
*The extra length and width of concrete helps mitigate cracking and breaking at the edges of the foundation after the building is anchored into place.
As the site is excavated, footings should also be dug down to 12 inches below the local frost line. The footings must be at least 16 inches wide. In some areas of the country the existing soil is not strong enough to support the loads exerted by the building framing. In these circumstances deeper grade beams or even pier footings may be required. It is strongly suggested you consult a geotechnical engineer if you’re not sure what conditions you have.
Step 2 – Laying the Forms
Forms can be made from scrap lumber or metal and serve to create the outline of the foundation. The forms must be tight, strong, and rigid.
- Tight: Keeps rainwater out and liquid concrete in.
- Rigid: Does not bend, deflect, or deform due to the pressure of the concrete.
- Strong: Bears the weight of the drying concrete.
While laying forms, ensure proper drainage for run-off by leaving troughs and/or placing a 4 inch layer of gravel into the excavation before pouring the cement.
Step 3 – Pouring the Concrete
The concrete must have a rating of at least 2,500 psi and use fiber mesh reinforcement. If you expect to store heavy equipment or vehicles, increase the psi to 4,000 and consider using wire mesh or re-bar reinforcement.. Continue to move and churn the concrete while you pour to keep the aggregates from settling.
- Pour evenly to a depth of 4 inches. Pour 6 inches if heavy vehicles or equipment will be stored on top.
- Fully cover all steel reinforcing bars.
- Eliminate voids and air bubbles.
- Remove any water seeping to the surface.*
*This water is called latinate. Deep foundations are prone to such seepage due to the pressure of the aggregate and sand in the concrete.
Step 4 – Screeding and Finishing
Screeding is a method of leveling the foundation floor by pushing excess concrete away and pushing concrete into areas that are under filled. “Finish” the foundation once it has been leveled.
- Use metal or wood floats to compact the concrete and push large aggregates down into the cement.
- Smooth the surface with trowels.
- Compact the foundation for a harder finish.
- Cut expansion joints with a saw soon after the concrete is poured and leveled and trowel the joints in while finishing.
Now that you have poured a concrete foundation for your metal building, let it cure.
Congratulations! You have poured a concrete foundation that will keep your metal building standing tall for years to come.* Image courtesy of freedigitalphotos.net