Foundation questions

skidoo · Mar 22, 2009

I have a daylight basement where the back of the house is concrete up to the second level and the front of the house has maybe 4 feet where there is probably at least 2-3 feet of dirt.

I am re-doing the deck for the second level and I would like more space under the deck. There will be a new full height retaining wall at the back and along half the side. I want the new space underneath to be clean, so I'll put concrete there too for the floor. Always hated the dirt under the deck! So this leads me to the questions below.

Is it practical to excavate the 2-3 feet at front and all of it in the back (to essentially the bottom of the basement), add new concrete floor and retaining walls? Since the back retaining wall will be poured to the existing foundation, there may not be a good seal. If it can be done like this and work well in a cold climate, what is the proper method?

Should I just leave the floor level at the 2-3ft and be satisfied with a 6ft high wall height under the deck? Non-110TLB Tractors can still be parked there...

yooperdave · Mar 23, 2009

When you excavate, make sure you maintain the minimum frost depth of the footings.

Footings are usually lowered at the location of an excavated basement for this reason.

Otherwise, your foundation may heave due to frost.

Good Luck

Yooper Dave

ScottK · Mar 26, 2009

To make a good connection to the existing foundation drill holes at least 4 inches into the foundation and epoxy short sections of rebar into them. Tie that rebar into rebar in the new slab.

skidoo · Mar 26, 2009

Thanks for the replies.

But, what I am asking is related to the floor level under the new space. Can I make that floor level the same as the level inside the daylight basement and allow for an 8ft high space, or do I keep it filled at about 2 to 2.5 ft against the existing foundation and allow only about 5.5 to 6 ft of wall height. I plan to pour concrete for that floor too with a new retaining wall on the back and side.

WVBill · Mar 26, 2009

The footers for the enclosed space need to be a certain depth below the exterior finished grade. That depth is the frost line depth in your area and is usually specified by the building dept.

So if the new slab is open to the weather then the current wall's footers will need to be frost depth below the slab. If the new slab is fully enclosed then it's footers will need to be below the frost depth.

skidoo · Mar 26, 2009

The first picture is what was there. Since, the deck has been torn down. Couldn't believe how poorly it was constructed. The nails came out and it warped. The posts were set on concrete rounds only 6 to 8" deep.

Anyway, the next picture shows the level of the floor vs fill in the front of the house. This wall is about 15" high and goes level to the other end of the house. So, as you can see in the picture, the fill slopes higher towards the back of the house and under the deck there is less and less height. I plan to extend that back wall by 15 feet and add a full retaining wall along the first half of the deck from the back, and a lower height wall for the rest. I want to fill under the deck with concrete. So, I have been thinking it would be nice to have it full height for all the space to the back retaining wall. I wasn't planning to enclose it in front, but may decide to later.

So, after measuring the existing floor height vs front fill, I guess I am looking at a 15" difference. Keep the fill at 15" and live with a 6.5ft hight to the ceiling beams, or dig down 15 more inches for the full 8ft height? If I did this, then it could become a second small garage. Not that I need it for a full garage because I am planning on a 40 x 80 building about 250 feet from the house. But, for convenience items such as the garden tractors and other hand power equipment.

I am in an unzoned rural area and don't require permits. Regardless, I want it done the best possible way. Is it too risky to go down the additional 15"? I don't yet know how much lower the existing footers are deep. Our minimum lows typically do not go below -20F for not longer than a week or so.

rimshot · Mar 26, 2009

Well,what the previous posts are saying still remains a potential problem assuming you proceed with your plan. Picture this scenario; When the house was initially designed and built, the masons probably poured a continuous footing at one level based on the premise that the front of the house would receive at least two feet of backfill. They should and most likely did pour the footing level at a certain distance below grade for protection from frost and to meet code.

Now here you come maybe 30 years later with this plan to excavate that two feet of fill away from the front and install a stilted deck and you are wanting 8' of headroom with a concrete base none the less. Being it is going to be concrete you will probably scrape what little snow gets under a deck (slim to none) over the Winter. So you have not only taken one thermal blanket consisting of 24" of backfill but will also keep or not receive any insulation quality due to the lack of any snow.

What we are trying to say is that it looks like you are creating a vulnerability for your house due to frost penetration in your cold climate. Make sure if that footing should be exposed with this plan that you are aware of this potential problem before you commit to such a project.

Frost levels for footings in areas where Winter lows can gt down to -20* F are probably at the very least 42"from top of grade to bottom of footing. You might be able to use a layer of Styrofoam placed on top of existing house footing and left in place beneath any potential flatwork to reduce frost depth codes. Another consideration to think about after looking at your current elevations now. If you lower that grade will you be creating a situation where water has to run back toward your new concrete floor. You sure don't need that to ruin your day. Good luck and I hope this helps.

rimshot

skidoo · Mar 26, 2009

Thanks for the input!

It sounds like I should not go deeper than it is in front. It is lower on the other side of the house for a 1 car garage and there is concrete at that level out in front with another 15" retaining wall from the edge of the house. If I was able to do that on this side, obviously, I would need another short retaining wall there too. I am not talking about reducing the level at the front of the house, but only along the side and create a similar pad to the existing garage on the other side of the house.

We are re-building the deck into an enclosed sun/screen-room with a lot of glass. The space underneath is bonus, but I would like as much available as practical.

How likely is it that the back of the house footers are higher than in the front? Could I have the same issue if I level out the space under the sun-room to that same 15" level, maybe sloping back just slightly.

If that is risky, how about at the back 4 feet, I create another wall 3 ft higher so that there would be a shelf at the back wall. That would still give me a lot of space underneath and still have the back footer well covered. Is this a good idea or anyone have any better idea?

rimshot · Mar 26, 2009

<<How likely is it that the back of the house footers are higher than in the front?>> (skidoo)
===================

Good point. For the sake of argument let's assume the footing was built according to code and that frost code was 42'' backfill grade to bottom of footing. That would tell us that the bottom of the footing was at least 42'' below the point at which the backfill grade intersects the front wall.

If all our assumptions prove correct (which only seems to happen in Indiana Jones movies) the footing very likely stepped deeper as it ran down the front wall. if this were so, perhaps you would end up with a diminished need for backfill height or a the very least could provide a 2" thick layer of a good sheet foam such as Dow (blue, r-11 ?)foam because removal of that berm has created less protection than needed however you would still likely be deviating from code.

I don't advocate for your shelf suggestion unless you insulate and make it at least 4' or more in width out from building. I still would be concerned with creating a negative grade out from the building which would invite surface drainage towards the structure.

rimshot

ScottK · Mar 27, 2009

Sounds to me like you need to excavate however deep you want (15" I think you said) and then dig a new deeper footing adjacent to the existing footing and tie them together, Then you have effectively deepened the original footing and you will be fine.