Adding and addition with basement to house on slab

[Shorelunch]

We have a slab house and are in the process of getting bids for a family room (28 wide x 30 deep) with a basement underneath. We're in the process of getting three bids. After touching base with all three builders, two of the builders want a minimum 6' buffer from the slab foundation before they dig the basement (leaving the basement roughly 28 wide x 26 deep instead of 28 x 30). Family room would remain 28x30. The other builder, who built our house, said the buffer isn't necessary and would add re-enforcement under the slab and tie the slab and basement together (I forget the technical terms, sorry).

Any thoughts on this?

[King Canada]

We have a slab house and are in the process of getting bids for a family room (28 wide x 30 deep) with a basement underneath. We're in the process of getting three bids. After touching base with all three builders, two of the builders want a minimum 6' buffer from the slab foundation before they dig the basement (leaving the basement roughly 28 wide x 26 deep instead of 28 x 30). Family room would remain 28x30. The other builder, who built our house, said the buffer isn't necessary and would add re-enforcement under the slab and tie the slab and basement together (I forget the technical terms, sorry).

Any thoughts on this?

You must still have a frost footing that supports the slab. Since that must be the case believe the original builder. If it is floating slab you shouldn't tie them together at all.

[cavalierowner]

I believe that if you talk to the building inspector, you will find that if its attached to the house it has to have conventional frost footings just like King Canada said. I don't care how much a builder reinforces a floating slab, frost will get under it and it will move, maybe a couple inches depending on the soil type and moisture content. You do NOT want that kind of movement attached to your house that doesn't move. It would literally tear your house foundation apart. I would recommend you run as fast as possible from the guy that wants to attach a floating slab to your house. That guy doesn't know the code and doesn't understand basic soils engineering relating to construction. I would wonder what else he doesn't know. Sorry to be so vehement, but you are spending many thousands of dollars and you really want it done right. The first time.

I see you're from Bemidji. You know the MnDOT building that is across the highway from the airport? That is a building I designed and followed through construction. That's a big building, about 120,000 SF, the contractor was Christianson Construction (Don Berg), in case you happen to know any of those folks. CC was one of 2 of the best contractors I worked with in my career. I hear from the MnDOT people here in the Cities that the buinding is aging very well.

[Surface Tension]

I'm guessing here that the builder knows there are frost footings but you have to ask.

Could be the other contractors know this as well and the reason for the separation between the slab and basement is a simple as it'll be easier to do it that way. No worries about dirt from under the slab falling in and tying in the slab to the block wall.

If that is the case I'd go with the builder that wants to tie them together. Just remember that his bid could be higher on a account that your basement it bigger and he has to tie in the slab that will equal more labor and materials. Not to mention that barrier is nothing more then a good place for pests to live.

As said a floating slab and basement won't work together. You have to ask if there are frost footings,

[Shorelunch]

Thank you for your feedback so far. Of note so far:

* Cavalier, the house we live in is a slab, and we would be adding the addition with the basement and family room. Does this make any difference in your thoughts? I will call Christianson's.

* ST, yes I'm sure he mentioned the footings and that's why I believe he feels we can have the basement right next to the house - instead of a 6-10 foot buffer. The bid with the basement attached/next to the house is considerably less than the buffer bid.

I'm going to get four bids and do my best that I'm comparing apples to apples.

[cavalierowner]

If the house was built to code, even though it's a slab on grade, it will have frost footings. To my knowledge, houses have never been able to be built on a floating slab. So if the garage is attached to the house, it would still have to be on frost footings.

I believe Christianson Construction does only commercial work or they did at the time. Although with todays economy, you never know. I think it's worth a call. If you talk to Don, say Bruce Hassig says HI!

[Shorelunch]

If the house was built to code, even though it's a slab on grade, it will have frost footings. To my knowledge, houses have never been able to be built on a floating slab. So if the garage is attached to the house, it would still have to be on frost footings.

I believe Christianson Construction does only commercial work or they did at the time. Although with todays economy, you never know. I think it's worth a call. If you talk to Don, say Bruce Hassig says HI!

Cav, yep, I called and they just do commercial. House is slab on grade with the footings, so you think it would be OK to have the basement right next to/tied to the house instead of having a 6-10 foot buffer?

Thanks again.

[cavalierowner]

To make sure I understand this, the house is existing with frost footings and the basement would be new? The tough part of attaching a house with a basement to an existing house with a slab on grade & frost footings is digging the new basement without undermining the existing frost footings. The safe and right way to do it is to use a method called underpinning. With underpinning you actually dig a little bit at a time and build a new wall under the existing higher footing and the new wall goes down to a new footing at the basement depth. The other possibility is to place the new wall several feet away from the existing house and then step the new footing up to the existing frost footing. Then your new addition would have a partial basement and partial slab on grade (or crawl space) next to it. In fact, the light above my head just went on; this may be what the contractor was talking about.

The underpinning will be much more expensive than building a wall away from the existing house.

Here is a HSOforum that shows underpinning abd sort of the method of building a wall some distance away http://www.draincom.com/page.php?aid=11&cid=3

[Shorelunch]

We're on the same page now Cav. I'm getting three quotes, and they all are approaching it differently.

Quote #1 (from the original builder) would dig the basement right next to the existing house (that's on a slab).

Quote #2 wants a 6-8' "buffer" away from the house before he digs - but still dig a full size basement (28 x 30) and have a larger family room to accomodate the buffer. This quote is about $15k more than #1.

Quote #3 want to do what you're talking about. Have a few foot buffer, then partially dig down, go out about about 4 feet and then dig down fully - so it would look like a big stair from the side. Could have storage above the "stair" or crawl space. This would cost us about 10 feet of basement. Haven't got his quote yet.

Not sure which direction is best - and best for the $.

[cavalierowner]

I hope quote #1 is planning on underpinning. That's the only way it can be done safely. I can't imagine how quote #1 is cheaper, unless it's because he's building less house above than #2. If #1 plans on doing it without underpinning, make sure he has really good insurance!

[LightningBG]

This is a really tricky situation.

The goal is to isolate those frost footings that are present now.

Could use piers or pilings. Could use underpinned (which piers and pilings technically are.)

I think if it were me. I'd look into hiring a special company to do just the footings/basement floor, concrete walls, etc. Maybe Knoblauch Masonry Inc.

Then I would hire the contractor to do the rest of the construction.

I just wouldn't want someone winging it based on what they think "should" work.

I'd also talk to an inspector and get the plan Ok'ed

[SkunkedAgain]

I'm not in residential construction, so I'm a bit out of my element but this is why you've got architects and engineers for......to know the proper way to make such additions and tell the contractor how it should be done. When commercial projects are done design/build like you're doing here, you don't have the separation of duties that should ensure that the right steps are taken at each stage.

All that said, it may be worth paying a few bucks to a residential engineer to assess the situation and provide a professional recommendation.

[Tom7227]

I am not a builder, just a retired lawyer. My first impression after reading this thread is that there's a good chance that things won't go bad while the work is still under any type of warranty, and if it does you could have a whale of a time getting any relief - assuming a contractor is still doing business under the same name X years from now. I don't know how you protect yourself against those risks. From the little that I’ve read it would seem that the less risky solution is the step out deal where you end up with only a partial basement under some of the new addition.

Does the original contractor have unique information about soil types and how far down the original house footing go? I would be wary of anyone who was going to take this on that didn't have those pieces of info. Maybe the original footings go down the full 8 or 9 feet and so there isn't really an issue. Maybe the soil is all sand and it's going to collapse no matter what is done. If it's wet soil there could be other things to consider.

Also, assume that the footings go down 48 inches or whatever. Isn't there going to be a poured cement footing under the block? What's going to happen with that? Assuming it has to stay in place that's going to impact that wall in your basement no matter what.

Finally, I find it odd that the original builder is proposing what would seem to be the most risky solution and yet he has a price that is considerably lower than the one you have already. Why is that? Assuming you live someplace where there are building officials I think I would spend some time with them and get the scoop on the various builders and any info they can provide about the soils. If you’re really lucky there’s some sort of documentation about how the house was originally built and you can get some background info to help sort things out.

Good luck.

Tom

[hanson]

Using SkunkedAgain's post (which is good and I agree with) as a jump off point, your situation is why you shouldn't go to a handful of contractors and have them give you bids without some sort of plan in place. You aren't getting apples to apples bids and there is no way to compare them or tell who is giving you the best value.

Even if you developed an outline spec of parameters and some basic plans, with some engineered details of the new to existing connection, you'll be alot better off. I'm guessing a building official is going to require some sort of engineered details on that connection regardless, which your contractor would probably hire to have done.

The 2 solutions that are being proposed are both acceptable but you'll have to determine which one you want. There is no doubt building right next to the existing building is going to be more expensive, your overall square footage in the basement is more and you have the existing footing/foundation underpinning issue to deal with. Building away from the building protects the integrity of the existing foundations. There is a slope ratio that is generally followed that will tell you distance away and down from your existing foundation that can be maintained.

I hate to say it but I'm wondering if your contractor is going to get you with an extra for dealing with the soils (underpinning) of the existing foundation when they get to it. You'll get a phone call saying we have a problem and they'll explain their was an unforeseen condition with the soils. Or maybe not, find out if its included.

I've worked for an architecture firm on commercial buildings for the last 10 years now so this situation isn't unusual at all. Where it becomes unusual is with different soil conditions. You can excavate almost straight down in clay but not in sand, sand or loose gravel will cave in. If you have a loose soil condition and still want to excavate under an existing building, there is a process of injecting watery concrete into the soil called soil solidification. Its a pretty expensive process and it doesn't always work on the first attempt either.

The person you'll want to talk to is a structural engineer, they'll probably give you some structural recommendations and details to give to your contractor to follow. You can call an architect as well but we'll end up consulting with our engineers anyway.

I really think it would be good due diligence to work with an architect or consulting engineer on this part of the project. With a design in place, you'll get better bids and protect yourself from big change orders.

Just my 2 cents...