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Sizing footers

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  • Sizing footers

    Hello, I am hoping to get some guidelines in sizing footers; here are the specifics:

    Location: Western Oregon (7" frost line) (clay dirt)
    Structure: 60 year old house, single story
    Framing: 11 foot joist spans, 2"x8" (Douglas Fir), 16" o.c.

    As you can guess, I have a little sagging in one spot. My remedy for the time being will be to place a 4"x6"x12' beam (#2 or better Douglas Fir) underneath the saggy area, with a beam span of 4'.

    I have the beam, and some pier blocks (12"x12" base, 8"x8" top) already under the house ready to go.

    My original plan was to dig down an inch or two (under the gravel) and place the pier blocks directly on the ground, but I am starting to realize that I might have to use some type of footer underneath the pier blocks.

    My question is basically to confirm that I really do need footers, and what size/type. Can I use a crushed rock (3/4 minus) footer, or will I need to use cement with a rebar "X" in the middle?

    Also, what is the smallest size footer I can get away with using? There is only about 20" of vertical space in the crawl space, so digging/placing material is going to be a bit of a job.

    Thanks for your suggestions.
    Last edited by jonnycat; 04-28-2012, 07:50 PM.

  • #2
    Re: Sizing footers

    Structural questions such as this are best answered by someone local. You can always read up on footing designs, frost cover, etc., in your adopted state building code. You could also discuss this with the AHJ, who could guide you to a safe remedy, unless it is determined your conditions require a professional determination of the proper solution.
    Take Care



    • #3
      Re: Sizing footers

      My first thought on this is since it is not new construction or an addition you don't legally need permits for this project. You are simply adding to what is (or presumably has been) permitted and passed a while ago.
      That said I would make 2500 PSI conc. footings 12" deep with 2-#4 re-bar both ways, 3" clear from earth. Set the piers in the concrete while still wet. Level off as best you can, set your PT posts, blah blah know the rest.

      Was a GC, doing drafting & design now.


      • #4
        Re: Sizing footers

        Thanks Andy, I appreciate it. I had seen some diagrams with 16"x16" footers that were 8" deep, but I'll try to see if I can dig it down a bit more. Great idea about levellng the pier block on top of the concrete.

        Will I be wanting to connect the two pieces of rebar forming the "X", say with bailing wire, or is it enough to just put one on top of the other halfway in the pour?

        As to permitting and regulations and such, I didn't include that topic in my post as that is beyond the scope of my main concern right now, which is just trying to understand more about the properties of footers and their construction, especially in an application such as mine.

        Last edited by jonnycat; 04-29-2012, 05:25 PM.


        • #5
          Re: Sizing footers

          Holy cow - pouring footers for a mid-span support?! We aren't over engineering this to pass an inspection, but just need enough extra support to get rid of the sagging floors.

          You don't have a frost problem so there's no need to dig under any existing gravel - the gravel helps support weight.

          In these situations in my location there is nothing wrong with putting in pier supports as long as they are at least 16" square. That means just 2" outside the 12" pier bases and I'll usually do that with a 16" square precast concrete block 2" thick. Remember weight is transfered at a 45 degree angle downward so the 16" block only needs to be as thick as the projection beyond the 12" block.

          Adjustable post bases and short 4x4s up to the bottom of the beam make it easy to adjust out any sag you may have. In your case I'd put them every 4' and would feel confident that will take care of your problem. Even if your soil is only going to rate at 1500 psi, you'll be able to support 2500 lbs with each base - I'll buy lunch if 10,000 lbs won't support a simple midspan sag!

          If an inspector won't allow simple precast concrete bases because it's not load rated from the manufacturer, you'll have to pour your footer half as deep as it is wide to use the threaded adjustable post bases (16"x16"x8"). Often you'll be asked to include 4 pieces of rebar (2 each direction) in the pour, but for this purpose I don't think it does anything useful if you use decent concrete.


          • #6
            Re: Sizing footers

            Hi Don, thanks for the reality check. I don't get the chance to see any "real world" applications of this, so I greatly appreciate your experience.

            I think my biggest concern is with the expansive qualities of the clay in the local dirt, which is quite noticeable near the sidewalk outside between summer and winter. It is for this reason alone that I think I will end up going with a poured footing, most likely of the size you suggested.

            As far as concrete goes, I'll probably get the 60 pound bags of Sakrete from the locally owned Ace dealer in town. I like your idea of arranging the rebar like the pound sign (or a tic-tac-toe framework); that sounds like it would give the footer a lot of additional strength.

            Speaking of concrete, since I will need three bags per footer, I'll probably end up pouring a bag at a time. During the five or 10 minutes after I pour a bag and have the next bag ready to pour, should I cover the mix in the footer with plastic, or will it be okay to wait the short period until the next bag gets mixed up and placed?
            Last edited by jonnycat; 04-30-2012, 01:11 AM.


            • #7
              Re: Sizing footers

              You size footings (ftgs.) as a function of the loads on them, the allowable soil bearing pressure, and how much differential settlement you can tolerate. Without any knowledge or discussion of these values it’s a crap shoot. With what you call clay dirt, but being well inside the bldg. perimeter, how much does the moisture change in the soil under your ftgs.? Without changing moisture or freezing and thawing you shouldn’t have much trouble with expansive soils, should you? You want to dig down at least deep enough to remove all organic soils and materials. And, then cast the footing on undisturbed virgin soil, level tops. Otherwise, I agree with Don’s suggestions and some adjustability in final height.
              Dick Hackbarth, PE
              RWH&AI, Consulting Engineers


              • #8
                Re: Sizing footers

                Concrete will wait long enough for your multiple pours - make sure you get the new "pour" mixed into the surface of the last to prevent a cold joint (separate layers of unbonded concrete).

                It's a good point Dick makes regarding making sure there isn't organic soil under the old layer of gravel.

                Once you get everything in place and are ready to start cranking on things, depending on the severity of the sag you may not want to take the sag out all at once. The worst case when forcing a joist back into place is that it will split or an end will pull free and lift the floor in that area. Just go slow and watch what is happening on the ends of the joists - most likely it won't be anything on a house as young as yours.

                If your floor develops a squeek after all this it will be on a joist end that has lifted slightly and a half dozen big deck screws in the right places will normally be all it takes to anchor it down.

                If you have tile above it's a special case and I'd go slow and not take all the sag out at once - tile cracks are much easier to prevent than to fix in this situation - those adjustable post bases are the bees knees for this.

                When correcting old houses that have settled or sagged a great deal it helps to have a feel for how much stress to put on things - if in doubt I let time help me out and I'll jack a little and let it sit for a few weeks then jack a little more and let it sit, repeating until it's where we want to be. In the worst situations it would take years to ease bananna shaped joists back to something straight without cracking them - in those cases I'd plan on cutting them out, or cutting in multiple places and sistering new joists next to them (if it's advantagous to keep the old joists for their connection to the subfloor as is the case with finished floors that aren't to be replaced)


                • #9
                  Re: Sizing footers

                  Originally posted by Dick Hackbarth View Post
                  You want to dig down at least deep enough to remove all organic soils and materials. And, then cast the footing on undisturbed virgin soil, level tops. Otherwise, I agree with Don’s suggestions and some adjustability in final height.
                  Thanks Dick, fortunately I won't have any trouble with organics, as once I got below the rock it was straight clay, like the kind you could form and put into a kiln (well, pretty close anyway).

                  Originally posted by Don_in_Idaho
                  Once you get everything in place and are ready to start cranking on things, depending on the severity of the sag you may not want to take the sag out all at once.
                  Thanks Don, I have been thinking about this project for a few months now, and I will be taking my time lifting things once everything is in place. I'll probably do the "once every week or two" like you suggested between turning the screws.


                  So I went under the house today to begin excavating, and I discovered a few things:

                  1. The "gravel" isn't crushed gravel but a round rock, about an inch and smaller, and it is at a depth of about four to six inches until I hit the clay, which is wet (this is Oregon after all). I think the round rock was used to help facilitate drainage to the sump.

                  2. It's a lot of damn work digging material out of the ground in that crawl space and hauling it out in buckets, and then you come back and it looks like you barely scratched at the surface!

                  3. My local greystone dealer has cement blocks 16"x16"x4" thick.

                  So..... I'm starting to rethink if I really want to pour footings for what is just mid-span support. I was thinking that perhaps one of the 16x16x4 blocks (thanks for that suggestion Don) would be a lot easier to place than pouring footings, and would be a good compromise between just putting down the pier blocks and pouring footers.

                  If I go this route, I'm guessing that I should dig out all of the round rock and place the 16x16x4 block on the clay surface?

                  Thanks again for all of the help, and encouragement.
                  Last edited by jonnycat; 05-03-2012, 09:35 PM.


                  • #10
                    Re: Sizing footers

                    Of course you can do as you wish but it o cures to me that this is not a basement that will be heated during the winter months, so the frost depth will be seven inches as stated above.
                    Concrete pads on the ground might possibly heave in the winter. Conc. footings wherein the bottoms of are 12" bellow grade will not heave. I would amend my previous suggestion of 12" thick footing pads to 6" thick with two #2 rebars each way.
                    Well, just one man's opinion.

                    Was a GC, doing drafting & design now.