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Building a basement under an existing home

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Building a basement under an existing home

Unread postby farmingengineer » Tue 18 Sep 2007, 11:52:15

Several weeks ago I started excavating dirt from under my 1,450 sq ft ranch style home with the plan to eventually build a full basement. I had always planned to build a root/wine cellar under my home but after visiting a friend of the family that had built a 900 sq foot basement I became convinced I could and should do the same thing.
My family of four, wife and two young children, could definitely use the added space as our parents on both sides are not getting any younger and I definitely forsee either them or some other family members needing a place to stay during the coming hard times. Were on two acres that I use extensively for producing food and firewood, so unless things go mad max like I plan on staying here long term.
I’m pretty confident that I’d end up paying a contractor at least $100 per sq foot for an above ground addition (assuming I’d have someone else do it), but that I will be able to build a roughly 1,000 sq foot basement myself for around $15 per sq foot. A basement will be cool in the summer and easy to heat in the winter. I basically grew up in one in a climate with much cooler winters and hotter summers than here near Seattle. Anyway, here is my plan.

I’m leaving about 18 inches of dirt between the existing foundation and the new walls I’m building out of rock from the property, free concrete from around the area and cement I’ll be mixing myself. The walls will extend at least one foot underground (planning on eight foot ceiling) and will have rebar apropiately spaced in it. These new interior walls will obviously not be carrying any vertical stress from the house. I will then build a cap from the top of the new walls to the existing foundation. After this is finished I’ll pay for a load from a cement mixer to lay a five inch floor. I’m currently placing the dirt in five gallon buckets and transporting them to the back of my property and using it to build the sides for a future irrigation pond. I’ve read some about the best ways to avoid moisture and radon exposure but would appreciate any advice from people with knowledge on this.
Has anyone on the board done something similar? And if so please advise me to any possible pitfalls. Obviously I’m trying to do this with the least possible expense without endangering myself, my family or the structural integrity of my home. Thanks....

Jim
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Re: Building a basement under an existing home

Unread postby RdSnt » Tue 18 Sep 2007, 13:02:12

There's a few things I would suggest you consider.

I'm not sure why you are bringing your new walls in so far. I would put the outside of the new wall flush with the existing footings.
You need to be careful that you don't weaken the dirt under your existing footings.
You need to waterproof the outside of the new walls.
Does the original footings have weeping tile?
Don't mechanically tie your new walls to the footings. The new walls will settle and break at the connection. Seal the joint with tar and plastic.
Your new walls will need footings just like the original.

I'd recommend a heavy sheet plastic barrier for the whole floor, with rigid styrofoam insulation on top of that.

I'd rent a conveyor belt to lift the dirt through a window out of the basement. Doing it with buckets will take too long.
You need to dig down below your cement level and then top up with good gravel and sand , then compact to the level you want. Don't try pouring just on the dirt floor.

Where's the main sewer lines? Are you digging below them? You'll need a sewage sump if that's the case.

Have you accounted for the expense of re-positioning all your existing mechanic stuff? Do you have a forced air furnace? You'll need to extend your ducting, or hang the furnace.
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Re: Building a basement under an existing home

Unread postby frankthetank » Tue 18 Sep 2007, 14:11:19

I'd insulate the outside of them walls with pink foam board if your climate is cold like ours. All new constructions of seen around here do that and while my basement walls aren't, i wish they were because some of the heat from the basement is moving through those cold walls (and with no snowcover, the soil temp can get awfully cold). For comparison, my basement was down into the upper 30's last winter (no heat) during a cold spell! (good for a root cellar, bad for a living space!).
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Re: Building a basement under an existing home

Unread postby Laughs_Last » Tue 18 Sep 2007, 18:38:05

As mentioned above, don’t screw up your plumbing lines or furnace.

From this end of the internet, I couldn’t tell you if 18” in from the existing footing is enough to prevent undermining, or if it is overkill. It depends on how much deeper you are going than the existing, and also on soil conditions that shouldn’t be speculated upon.

In Seattle, you should spend a lot of time thinking about moisture, as RdSnt mentioned. How high is the water table on your property? Also, if your walls are cool, will humidity condense on them in the summer?

Radon is normally vented with a pvc pipe (4 inch?) that runs from below the basement slab out through the roof. The polyethylene vapor barrier should be below the slab and above 6 inches of compacted gravel. The air spaces in the gravel allow for radon to reach the vent, (and also reduce the amount of ground water that wicks up from below). You can use landscaping fabric below the gravel to prevent it from getting filled with mud over time. To prevent the concrete from settling, borrow a vibrating compactor to use on the earth before laying the gravel, then again on the gravel. As for cost, where I live gravel is almost as cheap as dirt, and is hauled by the same companies.

I think you can do the concrete slab yourself, in batches. Just plan on having some kind of pattern to accommodate many smaller pours. You could do a 4” slab, if you keep the wire mesh in the center and have a joints less than 16 feet apart, and if your concrete isn’t junk.

When you are done, test for radon. If it is high, add an electric fan to the radon vent.

Oh, and, get a building permit if it's required.
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Re: Building a basement under an existing home

Unread postby Laughs_Last » Tue 18 Sep 2007, 18:46:01

frankthetank wrote:I'd insulate the outside of them walls with pink foam board if your climate is cold like ours. All new constructions of seen around here do that...

Make sure to only use the kind rated for ground contact, if it is contacting the ground. And beware of creating an easy entry path for termites, wood ants, et cetera.
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Re: Building a basement under an existing home

Unread postby Fiddlerdave » Tue 18 Sep 2007, 23:12:05

I'd rent a conveyor belt to lift the dirt through a window out of the basement. Doing it with buckets will take too long.
Buy a used one if you are going to take a while and renting would cost more than buying, you can resell it when you are done, although I have used them elsewhere too I helped dig some basements in my youth, and a small conveyor would be worth every dime.

The only problem we had was a brick wall come apart from the bottom when it turned out the house "foundation" had apparently been excavated wiith a hand hoe, and had maybe 3 to 5 inches depth of concrete in it, slightly wider than the bricks. Financial corners are cut in amazing ways by builders.
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Re: Building a basement under an existing home

Unread postby Pops » Wed 19 Sep 2007, 11:19:05

I think if you look back in this forum there was a similar thread not too long ago with lots of good advice.

The main thing to remember is the load on your footings spreads at a 45* angle on both sides. Imagine a long pile of sand; that is how the load is distributed from your footing underground.

I’m not real clear how you are going to get 8ft of headroom with only a 1ft excavation but I probably misunderstood. But at any rate, if you are going to hold the new wall in 18in. you shouldn’t dig more than 18in. down. And I’m no mason, but if your soil is expansive clay or loose sand for example, a rubble wall, even one with some rebar probably doesn’t have enough lateral strength at that distance.

I’m a pretty good DIYer; in fact a former General Contractor, but no engineer, so I would certainly pay one to eyeball a project that could potentially do as much structural damage as this one. Sort of a pay him now or pay him and a bunch of others later kind of thing.

FWIW…
Plans are nothing; planning is everything.
--Dwight Eisenhower

We don't see things like they are, we see them as we are.
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Re: Building a basement under an existing home

Unread postby Pops » Wed 19 Sep 2007, 11:34:46

Hereis the thread I was thinking of, not much new info tho…
Plans are nothing; planning is everything.
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We don't see things like they are, we see them as we are.
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Re: Building a basement under an existing home

Unread postby farmingengineer » Thu 20 Sep 2007, 10:09:48

Thanks for the advice everyone.

Originally my biggest concern was with the load distribution being essentially at a 45 degree angle from the original footing. But after doing some research on building interior walls for existing basements and seeing an example with walls at about the same height as mine are planned to be, in similar soil, I have decided on the interior wall plan rather than going right under the existing foundation. At any rate I’ve got a lot of time to plan this out and search for the best answers while I’m excavating and I may actually end up building columns under the existing foundation and building right under it. After getting out the first seven yards I have decided for now to build this under only half of the house (for at least a 500 sq foot addition) for now and see how it goes. At my rate of excavation I should be able to be finished by mid- summer of 08. If all goes well I’ll then start on the other half, hopefully with a few lessons learned.

Jim
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