Concrete Patio Support

Ron wants to pour a concrete patio and wonders about proper support so it won't crack. He's doing this work in cold Fonthill, Ontario Canada. Here's what he told me:

"We had our basement (block) waterproofed from the outside one year ago and have sandy soil.

The area we are looking to do is 10 x 40. The shed will be moved.

When they excavated they came out 3-4 feet and tapered at the top. 

When are we able to have a concrete patio poured in this area since the soil that was put back up against the foundation was NOT compacted?"

Here's the back of Ron's house. Photo credit: Ron

This view is looking down the back of his house. Photo credit: Ron

Here's my answer:

Ron, you can pour as soon as the frost is out of the ground.

However, I sure wish you would have contacted me BEFORE the waterproofers left the job site.

You had a golden opportunity with the foundation and footer exposed to install concrete block piers that rested on the footer. I would have used ordinary 8 x 8 x 16-inch concrete block.

These block pillars could have been spaced at 6-foot centers and come up so the top of the last block was just below where the bottom of the patio slab would be. Think of them as you would steel columns under a a steel I-beam.

I would have then put #5 (5/8-inch) steel bars in the slab at 2-feet on center projecting out from the house so the steel bars would be centered on the block piers. Additionally, a #5 bar would be tied to the ends at the house with this bar being 4 inches away from the house.

Ideally, you should have a steel grid of bars two feet on center both directions in the slab. The other bars can be #4 that aren't above the block piers.

The slab should be no less than 6 inches thick for about 18 inches out from the house. The remainder of the slab should be 5 inches thick. The thickened part is acting as a beam across the block piers so the slab will never fall.

What I'd do now is all of the above but in lieu of the block piers I'd use a post hole digger and try to extend 8-inch-diameter holes down as far as you can into the soil along the house.

The deeper you go, the better chance you have of the soil being very compact.