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Adding a second story to a garage

GibsonGuy | Posted in General Questions on

I would like to add a second story to a garage and convert the entire structure to living space.  The garage is in zone 6 and currently has no insulation.  It’s on a 4″ slab with a thickened edge of 8″ for a total of 12″.  In a brief conversation with an engineer, he stated I have to go down to 42″, frost line, to be able to do so.  I feel very confident that I can negate any potential frost heave.  On two sides of the structure there are 40″ retaining walls holding back the earth that are not in the best of shape, they lean slightly towards the structure.  There is a clear walkway around these two walls.  My son recently purchased the property and I think the retaining walls are something we’ll have to deal with at some point in time.  The garage has 8′ walls, the bottom half is made of block and the top half is framed.  Because it would be very difficult to get large quantities of backfill in the walk way, I looked into using geofoam to fill the void and was pleasingly surprised that I could use 2′ thick pieces of the foam and cover the entire  bottom half of the wall at a reasonable cost.  So the foam blocks would be 2′ thick and 4′ tall. I would then back fill with gravel between the block and earth the remaining small voids.  So that would be more than adequate for frost protection.  On the two other remaining walls I would be adding on more structures using a FPSF, so the all four walls would have excellent frost protection.  The remaining walls would become interior walls.  So since I have eliminated the frost heave potential, my question is.  Will the modest thickened edge of the slab support a second story?  The soil conditions seem very good there.  The garage slab has a few hairline cracks but it is 35 years old.  There are also a few hairline cracks in the mortar of the blocks, but one really has to be looking for them to find them.  I will add that the garage floor of my house has more cracks, it was built to code and half the age.

This potential house is for my son and we are trying to build it as inexpensively as possible, but will be using very green materials with  insulation  values and windows being beyond code.  The price of Zip System has more than doubled in our area, so we are working hard so we don’t have to compromise on building materials.., insulation and windows.

Your input would be greatly appreciated.  Thank you.

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  1. user-6623302 | | #1

    Put down posts outside the current building and build a the second floor supported on top of the posts. Envision a beach house with a blowout first level only more perminate.

    1. GibsonGuy | | #3

      I had considered that, but pole structures in our area require significant pole depths and large pads due to snow loads as we are in a snowbelt. The back corner, where the retaining walls meet is completely inaccessible for machinery. The walkway is narrow and there is a significant rise to the land and lots of mature trees on the hillside. By code, the poles are to be 8’ o.c. so at least five of the poles would be very difficult to place.

      1. Expert Member
        MALCOLM TAYLOR | | #5


        I don't think Jonathan was proposing a pole building, just taking the vertical loads on new posts located outside the existing structure. They wouldn't have to continue into the ground as the existing building would supply the horizontal support. You could use helical piles as support for most of them and only have to hand dig where it was inaccessible for machinery.

        Before contemplating anything like that I'd get the engineer back and divorce the question of frost-heave from that of whether the existing thickened footings can support other storey. There is no way we can make that call over the internet. If you get the okay for that, then I'm sure you can deal with the frost protection.

  2. gusfhb | | #2

    How did they build a structure in Zone 6 without a frost wall? I think the building inspector is going to have the last word

    1. GibsonGuy | | #4

      It is very common for accessory buildings to be built on floating slabs in our area.

      1. gusfhb | | #7

        But there you have it, they allow accessory buildings to not be up to code for a dwelling, and you hope to make it a dwelling and add a floor. And the area around it is not 100 percent stable.

        1. GibsonGuy | | #9

          You have definitely given me second thoughts, but this is not the first I have had them which is why I posed the question in the first place. Once again, floating slabs or raft foundations are very common in northern climates. Their use is not in violation of the building code. Some lower budget homes/cottages also used floating slabs. Sorry to say, like the poorly insulated homes of the past, most did not any insulation around the slab and if they did it was probably an inch or two of vertical XPS a couple of feet haphazardly placed against the thickened slab and none had it under the slab.

          So my question that I was posing, is there really much difference between a floating slab and a FPSF? I'm sure it goes beyond just the addition of insulation. But how much? The floating slab probably has steel reinforcement, but they probably did not use rebar in the thickened portion. Also, in my case, it is evident that this was a two pour procedure where many, but not all use a mono slab with a thickened edge.

          As for the area around the structure the retaining wall which is made up of railroad ties. There was an area excavated for the placement of the garage, but I do not consider the land unstable. The railroad ties were used without deadman anchors, so a slight movement at the very top of the wall is not surprising. The earth on the other side of the railroad ties has not been disturbed. I don't think it will be much of a challenge to improve upon this situation. I was looking at the use geofoam killing two birds with one stone.

  3. user-6623302 | | #6

    After some additional though, I do not think the time and money you will spend on reworking this building is worth the cost or the effort. There are just too many issues. Blow some insulation in the attic and wood walls, put in a big furnace and camp in it while you build something that is efficient and well designed. Something were the whole structure is not a compromise.

  4. gusfhb | | #8

    double post

  5. gusfhb | | #10

    I live in zone 6 and absolutely no one builds anything with a poured foundation that is not 4 foot deep.

    Sheds on blocks, sure.
    In 1986 when I pulled my first building permit, it was 4 foot.

    In 1975 when they poured the foundation for my family's house, it was 4 foot

    In 1970 when they built the house I live in now it was four foot.

    So pretty sure for 50 years it has been code in zone 6.

    My grandfathers house was built in get the picture.

    The fact that people build all sorts of stuff that isn't code is neither here nor there, your problem is you apparently have a building inspector now, and no one can come up with any convincing argument that might sway them.

    A frost protected foundation is built from the beginning on a well drained bed of stone. That is the fundamental difference, being on top of stone it will not hold water which expands when frozen and heaves the foundation and the house.

    Now if your garage happened to need a roof, and you happened to decide to replace the roof with attic room roof trusses because, well, it seemed like a good idea at the time....................

    1. GibsonGuy | | #11

      I actually meant to put the stone component in there, you are correct.

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