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Footer Question

Krico | Posted in Construction Techniques on February 7, 2008 04:27am

I am about to start an addition on my house. I am sure that I will be here with many questions as we progress. We got the permits and I hope to start the excavator in about 10 days. In a former career I was a framer, but whenever we built, the foundation was already there. We are planning to do an ICF foundation and SIPS for the walls.

Since I have never done a foundation, I have a lot of questions. From what I have read, my typical footer for this application should be 8″ deep and 16″ wide, given the 8″ ICF block I am going to use.

My question is, since this is an addition over a full basement and I want the finished grade of the floor to be even with the current basement, how far below the finished grade should the top of my footer be? Also, I read that I should use a plate compactor over the soil before my footer goes in. Do many people do that? I don’t mind renting the compactor but the thought of getting that 400 lb machine in and out of that 8- 10′ hole is not very exciting. Thanks

– Kevin


Edited 2/6/2008 8:28 pm ET by Krico

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  1. mike4244 | Feb 07, 2008 04:52am | #1

    The bottom of the footing will be 12" below the existing finished floor if the slab is 4" thick.I always make the footings wider and deeper than called for.Generally 10"x 20" is what I put in ,depending on soil conditions.8x16" is the bare minimum.

    If you do make the footing deeper, then just add the slab thickness ( assuming 4") to the depth of footing ( 10") . Now you have 14"from the floor to the bootom of the footing.

    You can hand tamp the footing,takes longer to do but not a big deal. When hand tamping, keep pounding in a small area, about 2 or three lineal feet of footing until the soil will not compact anymore.If the soil is sandy ,this won't take more than three or four minutes in that small area.

    mike

  2. Piffin | Feb 07, 2008 05:30am | #2

    You are asking questions that should be detailed on plans. It sounds like you are in an area that does not require too much detail on plans to issue a permit. It that right? Or have you not studied the plans yet?

     

     

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    1. Krico | Feb 08, 2008 07:38am | #11

      Thanks for all the replies. To answer Piffin, I did have the plans stamped by an engineer, but he did not specify that aspect of the drawings.

  3. Riversong | Feb 07, 2008 05:56am | #3

    how far below the finished grade should the top of my footer be?

    Depends.  Are you going to insulate under the slab and over the footings?  Then add the thickness of the foam board to the thickness you want for the slab to determine the depth of footing below existing basement floor.

    And, by the way, 8"x16" is the industry standard for typical soils and for an 8" thick foundation wall - it is NOT the minimum as another poster claimed.  I just built a two-storey house on a 10" wide footing/foundation.  The extra width of the standard footing is not so much for soil bearing capacity as for overturning moment of an imbalanced-fill foundation.

    Also, I read that I should use a plate compactor over the soil before my footer goes in.

    It's always a good idea to compact the soil after excavation.  Ask your excavator - he might be willing to lend you his and lower it into the pit with his backhoe.  Or he might do the compacting as part of the service.

     
    Riversong HouseWright
    Design *  * Build *  * Renovate *  * Consult
    Solar & Super-Insulated Healthy Homes

  4. bridge_dog | Feb 07, 2008 06:04am | #4

     Shouldn't the new foundation be tied into the old one to help  prevent setteling? How would you do this with ICF's?

    1. Riversong | Feb 07, 2008 06:16am | #5

      I wouldn't use ICFs.  But if you do, you'd tie it the same way you'd tie any other foundation - by drilling into the existing foundation and inserting rebar ties with epoxy (easiest method is to use glass vials with 2-part epoxy inside like these:) 

      http://www.stampersstore.com/maintenance/encapsulated_anchor_bolts.htm

      and make sure they're located where they'll be well embedded in the concrete cores.

      The footings should also be tied into existing footings, and the new footings tied with steel to the ICF walls.

       

      Riversong HouseWright

      Design *  * Build *  * Renovate *  * ConsultSolar & Super-Insulated Healthy Homes

      Edited 2/6/2008 10:18 pm ET by Riversong

  5. Marson | Feb 07, 2008 06:23am | #6

    The width of the footing is totally dependent on the bearing capacity of your soil. I build in clay, and we use a 24" footing. We never compact under footings--footing are always specified to go on undisturbed soil. Only exception if slab on grade houses which are sitting on fill.

    1. snap pea | Feb 07, 2008 09:40pm | #7

      Is a soils engineer usually brought out to test the soil under a footing before it is poured? Or is that unnecessary as long as the footing is being poured on undisturbed soil? In other words, how do you "know" what size footer to use. One poster mentioned industry standards, another indicated that he plays it safe by building a larger footer that the norm.
      I know there are quick tests you can do on the soil such as squeezing a ball of dirt in your hand, putting dirt in a jar of water to watch it settle, etc. but these tests don't seem like enough to go on when building something as permanent as a foundation. So what steps do those of you with experience take before pouring a footing and/or foundation?
      Thanks

      1. bridge_dog | Feb 07, 2008 10:10pm | #8

        It is usually known to a specific area what kind of ground there is. The rule of thumb for footings is twice the the width of foundation. Going over spec. does nothing but add to the cost. Bad ground is bad ground increasing the width usually does nothing because the point load is still the same. If the ground is undesirable you need different options such as pilings or some other system. That being said sometimes a wider footer is used because of lateral pressure on the foundation wall or retaining wall. The use of the wider footing is used as a counter balance. EX. Using an 8" wall on top of a 20" footer. You would put the wall 4" in with 8" exposed on the dirt side, the down force would help hold the wall from tipping.

      2. dovetail97128 | Feb 07, 2008 10:16pm | #9

        Local Building Officials should be able to tell you the bearing of the soils in your area. If they cannot a Soils Engineer and many structural engineers can. Most codes are based on assumed soils conditions and bearing capacities. Each area has soils maps that show what the assumed capacity is for that area.
        They can't get your Goat if you don't tell them where it is hidden.

      3. bridge_dog | Feb 07, 2008 10:37pm | #10

        The basic soil test you are referring to is usually used to test stability rather than bearing. In heavy construction when we excavate we use that method to determine what should be used to avoid a collapse. I.E. trench shoring, proper cut back or ledging, or sheet pilings with tie backs.

      4. IdahoDon | Feb 08, 2008 09:56am | #13

        Is a soils engineer usually brought out to test the soil under a footing before it is poured?

        That depends on a few things.  In high risk areas that have water issues, especially involving clay and side hills, a soils engineer is good insurance, and a good building department will keep on top of this as a requirement.

        On a run of the mill foundation with "average" soil, you either need to know how much the soil will support or you have to design for the worst case (1500 lbs/sqft).

        Something few people realize is that any building, new or addition, will settle a bit in the first year.  If you are tying into an existing building it makes sense to minimize this by oversizing the footer and paying close attention to the details.  Again, if it's important enough we'll have an engineer drill some bore holes and look at what's under there. 

        Another case of CYA comes into play when working for others.  By having an engineer look at the soil it passes a little of the risk to them.  Most engineers will also state if they want a drainage system installed to keep the footer dry if they anticipate problems.

        If it's your personal foundation it's probably better to simply size the footer for the worst case and not worry about it.  The extra concrete isn't that expensive.

        Once upon a time we had a building department that required a soil test for our build site that was a solid granite knob.  "Er, uh, Mr. Plan Reviewer...we, uh, don't actually have any soil."  *chuckle*

        Good building! 

        Beer was created so carpenters wouldn't rule the world.

      5. Marson | Feb 09, 2008 01:31am | #14

        No, we don't get a soils engineer for every project. However, an engineer will stamp the blueprints and has assumed a soil bearing capacity which is probably conservative. Every residential project I have ever seen around here has used 24" footings, so If I was doing my own design, that's what I personally would go with. If I was building a house with exceptional point loads, or on a very steep site or a site that was once a wetland or something, some professional advice would be in order.

    2. Riversong | Feb 08, 2008 08:48am | #12

      The width of the footing is totally dependent on the bearing capacity of your soil.

      It's dependent on the soil bearing capacity (tons/sf) and the unit weight of the structure (all dead and live loads) on the foundation (tons/lf).  Soil bearing capacity divided by unit weight x 12 = inch width of footing.

      Bearing Capacity of Soils

      Tons/Sq Ft                        Type of Soil<!----><!----><!---->

                 1       soft clay, sandy loam, firm clay, slay/sand, loose fine sand<!----><!---->

                 2       hard clay, compact fine sand<!----><!---->

                 3       sand/gravel, loose coarse sand<!----><!---->

                 4       compact coarse sand, loose gravel<!----><!---->

                 6       gravel, compact sand/gravel<!----><!---->

                 8       soft rock<!----><!---->

               10       very compact gravel & sand<!----><!---->

               15       hard pan, hard shale, sandstone<!----><!---->

               25       medium hard rock<!----><!---->

               40       sound hard rock<!----><!---->

             100       bedrock: granite, gneiss            <!----><!---->

      I build in clay, and we use a 24" footing.

      That's extreme.  Even soft clay has a bearing capacity of 1 ton/sf which is more than ample to accept a 2-storey wood-frame building with 16" footings.

      We never compact under footings--footing are always specified to go on undisturbed soil. Only exception if slab on grade houses which are sitting on fill.

      The specification might be "undisturbed soil" but that's often not possible.  Sloped sites might require a "cut and fill" and unless the excavator is using a finishing bucket and a very light hand on the controls, there's always some hand levelling and grading to be done.

       

      Riversong HouseWright

      Design *  * Build *  * Renovate *  * ConsultSolar & Super-Insulated Healthy Homes

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