Addition Foundation question

Have a small house that we're thinking of adding on to. Have talked to the city about what I can do on my lot as our situation is a bit unique.

City tells me where I can build, but notes that I'll need a frost-depth foundation.

Pic below gives an idea of what I want to do.




So, my question is this: How do I install a foundation like that if it's as deep or deeper than the current foundation without disturbing the existing footings? Do I only put a foundation on 3 sides of the addition and tie in one side to the house framing? If so, I still have 2 ends that would butt up against the existing perimeter foundation to worry about.

Just wondering out something like this is normally done.

Crawlspace, and most likely 4', or lower depending on where you live in the country.

You would be better off pouring all four walls, would end up with a much stronger foundation for the addition. Then when you lay the block just leave an access on both ends of the new addition and knock out a section in the old crawl where the new opening is.

Ok on doing the 4 walls.

I guess I wasn't clear enough.

My current foundation is a perimeter foundation which creates a "crawlspace" of about 6". (Yes, I have joists hung over 6" of air, then dirt. Some of the house has a slab under the joists but not touching them, some is just dirt.)

The new foundation would be a slab floor with footings to frost depth.

I guess I'm questioning the issue in the picture below:


The pink lines are usually the area you must leave undisturbed near a footing so as to not compromise it, correct? Gray lines represent current footing, brown line is ground level. So how can I put a new footing (The blue one) next to it safely?

Ahhh, gotcha. I have seen builders pour footers in the wrong place and then pour right next to them to correct it but this is before the house is bearing on it. I'd give an engineer a call and ask their opinion.

Hmm, I'll probably have to ask an engineer. I just figured someone here may have added on to their house, and I thus figured this would be a relatively common thing to encounter.

We'll see if anyone knows.

Although, if I'm reading my state's building code correctly, I can just dig an 8"wide trench 4 ft deep around the perimeter and do a monolithic slab in one pour instead of footings and slab separately. That would probably be easier to deal with any existing footing as well.

foundation

The most common way of doing it would be 3 walls and pin the new foundation to the old where they butt together. Otherwise pour the fourth wall on top of the existing footing, rather than put a new wall 8" or more away from it. Even if you had to reinforce the existing footing somewhat.

Nice drawing, Nate. You have a very clear understanding of the issue. As plumbers , we are more than happty to completely ignore structural issues and just put that footer anywhere!!!! Just kidding. This situation must come up, and many of us are not builders, and in my case, never heard of a frost line (!!! Southern California ), so there must be a proper way to deal with it. An engineer/builder will have good answer. I was thinking put the new footer out away from the original, then cantilever the joists out over that to meet up with existing. This may be a totally impractical idea.....what do I know! Let's hope to hear from an experienced builder on this.

hj said:

The most common way of doing it would be 3 walls ...

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That is what I did. One end of the addition went up against a basement wall, but the end behind the slab-floor garage had a situation similar to yours plus some repair work to do first. Overall, we dug a U-shaped trench behind the house and poured foundation walls to a little below the frost line.

You mentioned a monolithic pour, but I believe you will still need footers underneath that.

Yeah, I'd rather not cantilever joists over it if the addition would have a slab floor anyway.

There is a rule about footings in my state's construction standards that reads as follows: Trench Footings: Footings poured integrally with the wall may be used when soil conditions permit. The minimum width shall be at least 8 inches nominal.

Found another site (A concrete place in Michigan) with this on their site: Our concrete division consists of trench footings, all interior and exterior concrete for commercial and residential buidings, concrete removal, and concrete road repairs. Trench footings are the concrete foundation that the building stands on. The footing is usually approximately 12" wide and 42" deep or where stable ground is reached during excavation

So, if I understand correctly what a trench footing IS, then I think I could do a foundation as the picture below shows. This would be 8" thick walls down to 4 feet. Or is my understanding of a trench footing wrong? Obviously an engineer would have to approve my soil for this.

Just seems to me this would be much easier to dig/pour if the soil is stable enough. (Our soil should be.) And since it's only a 1 story addition, I don't think the loads would be very heavy. This is what I meant by monolithic pour. It's done in one pour, but still has below frost depth foundation.

Nate R said:

There is a rule about footings in my state's construction standards that reads as follows: Trench Footings: Footings poured integrally with the wall may be used when soil conditions permit. The minimum width shall be at least 8 inches nominal.
...
So, if I understand correctly what a trench footing IS, then I think I could do a foundation as the picture below shows. This would be 8" thick walls down to 4 feet. Or is my understanding of a trench footing wrong? Obviously an engineer would have to approve my soil for this ...
This is what I meant by monolithic pour. It's done in one pour, but still has below frost depth foundation.

Click to expand...

Understood, but I will be surprised if even a short (4') wall is considered "footed" unless the portion of it that is being called the footing is actually wider than the wall. I could be wrong here, but I believe the idea of a monolithic footing and wall (and even slab, if possible) would still ultimately result in something like this:

In your picture the pink lines can go from the bottom of the footing rather than the top.

A monolithic pour of floor with footings and foundation wall will almost certainly result in nasty cracks in the floor where you least want them. It is better to have a joint that will allow some movement of the foundation without wrecking the slab. http://www.thisoldhouse.com/toh/article/0,,559048,00.html

One thing you want to do is keep a good supply of any tile that you put on the floor so you can replace the ones that may crack after a couple of years.

If the soil is cohesive you can probably dig down adjacent to the existing footing because the loads are generally far below the design loads. If it is sand you need to do something to keep the soil from falling away from the footing.

The "monolithic" design involves leaving forms inside. It is usually a bad idea to leave wood in the soil under the house.

Bob NH said:

A monolithic pour of floor with footings and foundation wall will almost certainly result in nasty cracks in the floor where you least want them. It is better to have a joint that will allow some movement of the foundation without wrecking the slab. http://www.thisoldhouse.com/toh/article/0,,559048,00.html

Click to expand...

Monolithic pours are usually a bad idea where there are freeze thaw cycles.

Bob NH said:

The "monolithic" design involves leaving forms inside. It is usually a bad idea to leave wood in the soil under the house.

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No they don't. You set the form on the outer perimeter and no form inside.

The footing against the other footing depends on your roof load. Is your new roof bearing on the footing against the other footing?

If not, form a turn-down and pin it to the existing footing. Then pour footings for the other 3 walls.

Gable end would be up against the existing footing, so there shouldn't be any roof load on that end.

Anyone know of any resources to learn more about how to pour/dig foundations? Books? Sites? Love to see/understand how more of this is done, and find out if there's any other options. (Piers under grade beam, etc)

I don't understand what you mean by a turn-down. Anyone have a pic? This is why I obviously need to read up more on foundation/footing stuff.

I just want the least expensive option, really. (Who doesn't!)

I have found some helpful information here:
http://www.concretenetwork.com/concrete/footing_fundamentals/index.html
http://www.huduser.org/search/search_site.asp?qt=foundation

Excellent! Thank you! I did find one of those links in my searching, but had quickly skimmed it. You're posting of it made me go back and reread.

But the lightest buildings on the strongest soil require footings as narrow as 7 or 8 inches. Under an 8-inch-thick wall, that's the same as saying you have no footing.

Click to expand...

That combined with this from my state's code:

Trench Footings: Footings poured integrally with the wall may be used when soil conditions permit. The minimum width shall be at least 8 inches nominal.

Click to expand...

Indicates to me that my picture above may be possible. (foundation wall/footing same width, meaning a simple trench for excavation)

What I'm really trying to avoid is having to excavate out the entire area under where the addition would be. To be able to do an 8" wide trench 4 ft deep and make sure the bottom edges are square would be ideal. Could do it by hand, and wouldn't take too much time/labor.

I looked at pier/post foundations, but I'm stuck with 2 things with them: They either put the building 2+ feet off the ground, or one has no access to the framing underneath. I don't like either of those options.

I don't know the bearing capacity of my soil, but I would think it's fairly high from the charts I've seen.

I made some general calculations of soil load. Assuming and 8" footing/wall size on all 4 sides, and that all the roof load is on the eave sides, coming up w/ less than 2000 lbs PSF on the soil on gable sides, and ~2600 PSF on eave sides. Not too outrageous, and could make the eave sides 12" wall/footing if I needed less than 2000 PSF.

This calculation was rough, but did take into account dead/snow load of the roof, live/dead load of the floor, and the weight of the foundation walls and the slab.

I don't think I can go much further without the help of a structural engineer and a soil engineeer. Wish I knew what the depth of my current foundation was. I'll have to dig in spring, I guess.

Still wondering what a turn-down is.

Well, wife and I figured out another possible layout that would take care of 90% of my questions/problems by seperating almost all of the addition from the rest of the house. We've been working on possibilities for about a month now. I like the one we came up with today. Doesn't require tying into the existing roof and dormer face, and I don't have to deal with/worry about how deep my existing 1920s perimter foundation is, or have nightmares of underpinning.

Still will have to have the soil tested probably and consult a structural engineer to do the foundation I'd like to.

It will cost a bit more since it won't share a whole side with the house but I'm more than happy to deal.

Newest plan is pictured below. The little square between the 2 is an existing porch that I'll have to reconstruct foundation on up, but shouldn't be a problem.


I will update this when I find out what I can do. First thing is to double check with the city that this would be approved as far as setbacks and such.