Foundation - matching existing house

[Barold]

Hi -

I have a two-story home with a concrete slab on grade floor with radiant heat. The foundation is a four feet deep (upstate New York) frost line footer. I am putting on a one-story addition on the back of the house and would like a concrete floor with radiant heat in that part of the house as well.

The way I see it, my options and concerns are as follows:

An addition with a four foot frost wall foundation and concrete slab like the rest of the house. I think this would be most solid, but most costly.

A monolithic slab. I'm concerned that this slab butting up against a slab on a frost wall foundation will move up and down during freeze/ thaw periods and cause problems where the roof and walls anchor to the existing house.

A pole framed addition with a concrete slab floor poured separately. Similar concerns to those above, but the cheapest of the three options.

Any thoughts would be appreciated. Thanks.

Barry

[marson]

I wouldn't be comfortable with anything but the walls below frost depth. Even with frost protection, it seems possible that a floating slab with move a bit. A pole framed addition seems more appropriate for a barn or a shed than a house.

[Dave in Houston]

Common practice is to drill multiple holes into the existing slab in order to epoxy rebar in order to tie the new foundation pour to the existing slab. I'm in Texas but would guess you're still going to need foundation walls below frost line.

[jloomis]

[Barold]

I see your point. what is you thought about it more as a concrete porch that is screened in. If I were to build something like that, pole construction would be more appropriate, yes?

[marson]

Well, some half formed thoughts....

If what you want is an addition to your house, then it feels to me like a pole building is a hillbilly approach. If you want to side, insulate, sheetrock in a pole building, a lot of the cost saving from not having to put in a foundation are going to be offset. Also, how is the roof going to work? I'm having a hard time picturing how a pole barn roof would tie into an existing roof.

Course, I can't see your house so I don't know what it is worth. Maybe resale absolutely doesn't matter to you, but if you do sell it, what will potential buyers think? I know I would not like to buy a house with a pole addition on it.

I guess my second choice after frost footings would be a shallow frost protected slab on grade. I'd talk to an engineer about it, though.

[feng94]

=46rom a geotech viewpoint, transferring the building load to the same level of the exisitng building footing should be most appropriate. It is based on a consolidation behavior of the soil at the existing footing. If a differential settlement is inevitable, the careful selection of the founding elevation is very important.

For your case, it is not necessary to construct a perimeter footing. The most economical way, based on the soil condition at the frost line (4'), may be concrete/timber piers, plus grade beam and on top of that floor slab on grade. However, the cost from a local contractor should be quoted.

Hope this helps.

Bill