Concrete porch slab on grade - detailing

[Ben29]

I am designing a custom home. The owner wants a 10ft wide, wrap-around covered porch. Porch to be concrete slab on grade. He wants the porch slab to be at the same elevation as the first floor.

The soil is sandy loam with pea gravel.

I'm worried about frost heave where the porch slab meets the house. The frost depth in our area is 30 inches.

I am also worried about water infiltration. Keep in mind that the porch does have a roof, but I am still worried about it.

Here is my detail so far. What should I change?

 

[dauwerda]

Here is a typical detail I see used.

 

[hokie66]

The fill behind the wall will settle with time. You need to reinforce the slab to span onto the wall.

Regardless of there being a roof, failure to provide a weathering setdown is not best practice.

 

[Enable]

For structural slabs on-grade or those that cant be allowed to move for whatever reason we would usually use 6" of EPS insulation (our frost depth is 4' or so). Alternatively you can use void forms, but I dislike them as a contractor since the window from install to pour is tight (rain gets in and wreaks havoc - dik likes them though). In both cases the slab should be designed to span from stem to building wall. If you don't want to do that you can use 4" of XPS but then the slab will go for a ride and all you are doing is mitigating the amount.

I find dauwerda's detail to be a poor one for colder climates since heave underneath the slab at the building face would do something awful to the veneer.

 

[jerseyshore]

I'm with hokie, if the slab is going to be exposed you will get a crack right at the edge of the wall if it's not properly reinforced.

I do no like the other detail with the slab sandwiched between the veneer. There will be a bad winter that causes frost heave eventually and you have to allow for that slab to move up a bit.

 

[dauwerda]

To the comments about frost heave - I feel like that is not much of a concern. To get frost heave you need three things, freezing weather, a water source, and frost susceptible soil. A water source does not mean soil with moisture in it, it means enough water that where you have ice lensing happening more water can be drawn to that area through capillary action in the soil. I'm having a very hard time seeing a water source for this. Any surface water is going to drain away very well between the roof and the slope on the slab. You have a footing going down to frost on the outside which will help prevent water from migrating under the slab. But, all of the soil is going to be removed when they excavate for the basement anyway - backfill the porch area with non frost susceptible soil and there are no issues.

In an ideal world, it certainly wouldn't hurt to have enough reinforcement for the slab to span - in a residential world I don't think you will find that happening all that often. With proper compaction it shouldn't be an issue (also hard to find in the residential world). That said, I have seen lots of houses done with the detail (or a similar one) above (not mine, nor do I do residential engineering) that 10+ years later don't have any issues.