General foundation question – is it really necessary to step down footings below the frost line for a walk-out basement, if the floor slab will have radiant heating installed? I am of the (maybe uninformed) opinion that the residual heat from the slab will prevent the frost from getting underneath the footing and slab and heaving it. Frost levels in my area are between 42-48″ deep, and I’d rather not have to drop the footings if I don’t have to.
The one bit of information I have that backs me up on this, is that the house I live in, and two others I know of, have walkout basements without dropped footings. There have never been problems with heaving or cracking in any of the three.
Replies
This is a little different take on a similar situation. I have a prefab concrete foundation. (Superior Walls) They have no footing. When I looked into it, I found it hard to believe but it works very well. What they do is go down 4 ft. and put pea stone tamped and lay the concrete vertical slabs on that. It takes care of underground water also. I ahve been in the house two years and no cracks in the walls yet. The walls are also guarrenteed water proof for 15 years. We had a very wet summer last year and no water in the basement. I love the walls and taalking to the Superior Wall people, the cost is about equal to or less than a block wall. Bottom line, do you need a footing at all? I have a walk out basement and my garage is in the basement. No cracks in the garage floor either, it seems to work.
Thinking "out of the box"
Dennis
Consider the possibility of a power outage with a higher than normal water content in the soil close to the concrete. The temperature drops. Now what? Are you willing to live with that risk. Most of us are not.
So you are willing to pay through a higher than needed heat lose under the slab, just to avoid a few bucks up front to step the footing?
If your frost depth is 48", what kind a insulation is spected for the raddiant slab?
Lots of reasons to step the footing, insulate the slab, provide a thermal break at the slab edge..... all up front design and construction issues,.... not to mention getting a footing inspection
You know, pay now or pay later...
Dave
that a good idea but you forget one main point, code says you have to.
BB, I was just trying to point out that there are reasons to step the footing and make it continuous around the house. Not the least of which was the building code reguirement. I doubt any inspector would pass a slab on grade edge to support a building load w.o a footing beneath that edge.
A footing and stem wall with the slab inside is the correct way. A thermal break at the edge of the radiant floor slab, to seperate it from the stem wall will decrease his heat loss at the slab edge.
I can't remember the numbers, but heat loss at the edge of a slab is consideralby greater than in the field of the slab. Seems like it would be a waste of the radiant slab efficiency to use it to create a frost free zone beneath the slab edge. It would be some big bucks over the life of the house, even if he could get it approved.
Dave
A friend of mine had a house built near me where they had exactly the scenario you describe. He had a radiant slab in his walkout basement. They were using ICF's for the outside walls. I stopped by one Sunday afternoon to check it out before the backfilling happened and I noticed that the footer on the walkout side ended up 12" below grade at bottom of footer. Unfortunately, this was on the north side of the house on a damn windy hill. I told him about it and he was pretty upset with the builder, but I don't know what the ultimate fix was.
Our frost depth here in PA is 36".
No, I would never build it without the step. The fix for the broken footer would be 10 times the cost of doing it right now.
carpenter in transition
Look at using a frost protected shallow foundation system. Basically all that requires is some gravel for good draingage and some insulation both vertically against the found and horizaontally way from the foundation. The insulation gives you "artifical depth".
Don't know how that would in combination with deep foundation on other parts of the house.
Doing a little research, reading, and I think I've come up with a solution. I'm going to have the excavator overdig the walkout side by three feet, and fill the trench with stone. The bottom of the trench will have a drain to daylight, and the sides will have a layer of rigid insulation installed before the stone goes in. That way, the drained stone is insulated and can't heave. I'll add a little extra reinforcing in the footing, but if the stone is put in properly, I don't think it will settle. It also seems like it will provide a place for water to go during the construction process, if we get a lot of rain or muddy conditions, the deeper drained trench provides somewhat of a drywell, in essence, to allow for the entire footing to drain to daylight.
In case anyone is wondering what this looks like in section view, I can send along a PDF of a sketch if you e-mail me.
I've seen a similar concept in a couple of books, I think they call it a rubble-trench foundation. The guy who works for me designed his house foundation around the concept.
i've done rubble trench especially where we were building on old fill.. dug down thru the fill with a narrow bucket and filled the trench with 4" tailings from the screener..
worked great .. the building stood like a rock.. the sidewalk next to it sank 3 inches... hmmmmmMike Smith Rhode Island : Design / Build / Repair / Restore