Grill Island on Flagstone in dust patio?

[norseman]

Good afternoon- Thanks to Ground Trades Exchange and its contributors for an extremely informative site.

Does anybody have fact (or sound opinion) on the feasibility of putting a grill island directly on a flagstone set in dust patio? Or must it be set on a poured concrete foundation? Pertinent facts:

- Island will be steel stud/fiber reinforced cement construction (not brick/block)
- Fully loaded island will weigh 500-700 lbs
- Island will be roughly L-shaped, the weight will be born by a roughly 8' x 12' area
- Location is Zone 7 (frost/freeze)
- Gas/water will not be piped in.

Thank you!

-Ben

[Bill Schwab]

If you have a freeze/thaw area, absoultley, you should do the very minimum and dig post holes on each corner, just below the frost line, and bell flare the bottoms. Then pour the concrete and tie your island walls into the caison style footing you made. Run latteral rebar through the island every 18".

As far as a dust set patio, I am not at all a fan of dust set, we typically pour a slab then mortar the flagstone to the slab. Alot more work intially, but little to no work replacing grout, shifting stones, etc.

[Lanelle]

For a Zone 7 construction, you need a two foot deep concrete footer to get below the frost line. Also, if you are building this next to a new(er) home, the backfill may not have enough compaction at that depth so you really should field test the soil compaction before pouring the footer.

[ianc]

I would pour a strip footing and auger some sonotubes below the frost line, set the footing on 2" stryofoam SM (per pic)

this will provide the support and by using the SM the frost should not grab the footing

~ian

[Stonehenge]

Ian, I don't think I've ever seen an installation like that - does the rigid insulation really keep the footings/slab warm enough?

[norseman]

Thanks all. Flagstone in dust was suggested by our LA for cost savings (most of the project budget is going towards retaining slopes). We're using boulders - many, many boulders for that. Though I can't wait to see the finished construction, sometimes I wish I lived on a nice flat piece of land in a more temperate climate!
The grill island is a nice-to-have, and likely won't fit the budget given the additional site prep work, not to mention the island and outfitting it. I'll ask my LA how much of a budget impact the slab will have.

[ianc]

Yup- rigid stryofoamSM gives 1 foot of frost protection per inch of material.

used SM many times instead of footing underpinning when doing walkout basements and the wall opening created does not have a full 4' of depth

[Stonehenge]

Norseman - it's those changes in grade that provide the opportunity for truly breathtaking landscapes. Kudos for using an LA to put the project together.

Ian - I learn something new every day. :)