Concrete Patio Base

whaas_5a(5A SE WI)July 12, 2011

I don't think this is the right forum but thought I ask in case anyone has any experience with this.

Most sources recommend 4" of compacted gravel for a concrete patio base. I've seen recommendations up to 12", which is certainly overkill.

I have a friend that is pouring a concrete patio as a side job for me. He does this for a living so I trust his experitise. However I checked the base he put in and its only 1" of compacted gravel.

Before I say anything I wanted to get some opinions about the situation.

The house was cut into a slope so this soil under the gravel base is a sandly clay like soil with alot of rock. Figure I share that as it may impact the required depth of crushed gravel.

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You already know that an inch is insufficient, are you sure that is all? As a general rule the base for a concrete slab should go to the frost line (5a is probably 4 1/2 feet). You don't say how thick the concrete will be or what it will be faced with but all this will crack in the winter unless there is sufficient base.

    Bookmark   July 14, 2011 at 5:53PM
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karinl(BC Z8)

Doesn't it also matter how well drained it is? If well drained, and it sounds like it is, maybe you can do OK with less. Does anyone really go down 4 1/2 feet??? I've never heard that even for base under flagstone... you could ask why he put that amount of base down. But before you do, what are you going to do if you don't like his answer? If you won't change contractors anyway, then maybe no point in risking the friendship?

NB: friends doing things as a side job may not be the best way to get top quality! And I guess you choice here is: potentially a cracked patio for free, or pay in full for the quality job?


    Bookmark   July 14, 2011 at 7:37PM
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whaas, we lived in central Wisconsin for many years (zone 4). When we poured concrete we used a 6-8 inch base. Our soil was clay. At our current location (zone 4/5) when we poured our concrete driveway I believe they did about the same. The concrete was poured 6-8" deep along the edge and then came up to only 4-5 inches deep. We have had the driveway in for 2 winters, and they were tough ones. So far no problems. He did score the concrete, but even there it hasn't cracked. At least not yet.

    Bookmark   July 14, 2011 at 8:20PM
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4.5' is crazy - that's the depth of footings or maybe a thermal mass for a passive solar home. 1" is not deep enough, though. Concrete isn't as flexible as asphalt, so cracking will be a concern. Did he score the top to alleviate that issue? If not, you might ask him to score the top a LOT - like every 2' so it looks like tile. That would give the concrete a better chance of cracking in the scored lines which won't look as bad if/when a frost heave hits it.

If you have him cut the score lines on a diagonal it could even become a design element to the patio.

    Bookmark   July 14, 2011 at 10:48PM
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The depth of foundation or base for a patio is one of the most frequently asked questions here and on the stone forum. How deep is also the easiest to disguise by an unscrupulous contractor as it is hard to tell once the concrete is down. The correct answer can be found but is less likely to be found here where participants live in the whole range of zones and such amateur information as 'scoring' is freely given.The biggest problem, however is that the advice is often asked for at the eleventh hour long after important decisions have already been made.

While on the face of it this seems like a simple question, is compacted gravel even the best base, regardless of how deep and is concrete the best solution to a problem we are not privy too etc. etc. if answers are offered without much details being given they will range from 4 1/2 feet to 4 1/2 inches each one could be right depending on unknown details.

    Bookmark   July 15, 2011 at 8:26AM
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whaas_5a(5A SE WI)

What other details would be needed to determine the proper type and depth of a concrete patio base?

    Bookmark   July 18, 2011 at 2:45PM
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In all seriousness I think a 4" slab on a compacted 4" sub-base will be adequate in most situations. The area to be covered is relevant as is the location, drainage and whether the concrete will be the final surface or not. Large areas will need reinforcing for instance . The point I was trying to make in a fairly un-amusing way was that it is the sub base that provides the stability and this should not be skimped and should be compacted solid not made from loose material.

    Bookmark   July 18, 2011 at 6:46PM
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whaas_5a(5A SE WI)

I ended up adding more stone myself. Below are the projects pics. Scroll all the way to the bottom for the concrete pics.

Here is a link that might be useful: Project Pics

    Bookmark   July 19, 2011 at 1:07PM
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We live in Northeast Pennsylvania and want to put in a patio. Frost is a big issue here so I am concerned on how thick of a base to put in. I have asked a lot of people and get varying answers. We would like to put in a flagstone patio, or a similar material (maybe 20' x 20' but possibly larger). I would rather do some "overkill" to ensure that we won't have problems in the future than skimp. Any advice or suggestions would be greatly appreciated.

    Bookmark   July 24, 2012 at 5:40PM
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