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Raising the grade above the foundation

jhausch | Posted in Construction Techniques on April 24, 2003 05:11am

I figured I’d have to put a “head scratcher” title on this thread to get some people to look.

My neighbor has a sloped lot where the previous owner built with no attention to drainage. 

One part of the driveway passes by the side of the garage, 4 feet away and 3 feet above.  I suggested that he improve the slope and put a gravel bed with some drainage tile in the “one-foot-or-so” gap between the bottom of the new slope and the garage wall.  Then trench and steer the tile/water to the back of the garage where it can continue its run downhill.

He would rather build a 3 foot high block wall parallel to the existing garage wall and make the grade level with the top of the wall. The plan would be to leave a 4 inch gap to the existing garage wall (the garage wall is vinyl sided, stud construction).  He is suggesting a waterproofing barrier on the side of the wall facing the dirt.  Some sort of flashing to steer water running down the wall to the top of the new grade.

Neither solution is perfect – anybody have any ideas?

As I typed this out I thought of the following:  The existing garage is on a slab only.  Could we replace the bottom of the existing wall with block?  Sort of support the wall, cut off the bottom, build the block wall, add a new sill plate.

What would we need to do to the slab if we were to put 3 or 4 courses of 6 or 8 inch block on top of it?

Once again, any ideas?

Steelkilt Lives!
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Replies

  1. wallyo | Apr 24, 2003 09:08am | #1

    Could you post some photos or a drawing?

    1. User avater
      jhausch | Apr 24, 2003 01:01pm | #2

      Hope this helpsSteelkilt Lives!

      1. wallyo | Apr 24, 2003 07:13pm | #4

        I like your Idea too but it is probably not the cheaper route.  Block alone may not be enough unless it has rebar and grouting in it. Can the garage slab carry the block grouting and weight of the soil depends on a lot of factors, footing size, rebar in the footing, etc.   With hill homes in my area they do as you say a step foundation that acts as a retaing wall too.  Friends of mine have a garage where the up hill side is level with the ceiling of the garage so it is an 8' concrete wall on that side. They have a door in the rake wall so they can walk into the attic space of the garage from ground level.

        How much rain do you get in your area?  Also it seems that there is no soulder on the driveway as it is now. I would worry more about some one misjudging and going off into the garage or the road bed giving way.

        1. User avater
          jhausch | Apr 25, 2003 01:22am | #5

          Stocking it with Trout, *BIG GRIN*

          How about this option:

          Dig a 8 inch deep trench along the wall, one side of the trench would be the edge of the slab.

          Strip the siding up 3 feet, remove the plywood, exposing the studs

          Put 3/4 treated ply along the studs in place of the ply, put 2" rigid foam over that, tape the seams and cover with a membrane that extends down to the bottom of the trench.

          Lay drainage tile in the trench, cover with 3/4" gravel

          Backfill the area with sand or more gravel (to guarantee that expansive clay is not part of the backfill)

          Cover with a layer of top soil and seed it.

          Re-inforce the studwall from the backside.  Add second parallel, adjacent 2x4 sill plate and sister 2x8's to the existing studs.

          This sounds good to me and would be lot easier than laying block.

          Whachyall think of that?

          Steelkilt Lives!

          Edited 4/24/2003 10:10:34 PM ET by Jim

  2. canuckguy | Apr 24, 2003 02:38pm | #3

    "He would rather build a 3 foot high block wall parallel to the existing garage wall and make the grade level with the top of the wall. The plan would be to leave a 4 inch gap to the existing garage wall (the garage wall is vinyl sided, stud construction).  He is suggesting a waterproofing barrier on the side of the wall facing the dirt.  Some sort of flashing to steer water running down the wall to the top of the new grade."

    No way no how is the above going to do anything to mitigate water damage, actually he will be making it worse by building a pool, great place for mold, moisquitos and rats to congregate.

     I like your suggestion much, much better. Gutters along that garage wall if not already present will help move water away as well. He has a real problem there.  If he decides to build that wall he can consider stocking it with trout.

  3. migraine | Apr 25, 2003 10:47pm | #6

    Here's my input.  Your picture was not too clear.

    Build a retaining wall at the edge of the drive way and have it about a foot higher than than the height of the slope so that any water from the drive area is diverted away from the garage.  This wall give toy a 4' wide area between the garage and thew drive.  This would make a nice walkway and you would still be able to add additional drainage for water that comes off the floor and surrounding garage area.  Does this work?  I don't know if the drive is already paved with concrete or something else and I'm assuming the drive is your own an not a public road or easement. The retaining wall would need to be pretty substantial to support the drive and retain the slope

  4. clevispin | Apr 26, 2003 01:16am | #7

    Beating back the water with new garage wall construction may or may not work. It seems like an expensive risk. The new retaining wall is not too bad an idea as it may reduce a good bit of the volume headed to the garage. It might also improve the look of the place (the retaining wall must be properly engineered for stability and for resistance to being undermined by water action). But, I think the first step ought to be to build a deep trench. The trench ought to be as deep as can be sustained to find daylight at the end of the run. Nest drainage tile on a little gravel and cover the tile with gravel up to the halfway point and top with geo fabric. Then continue with gravel to within a few inches of grade. Top with a pretty landscaping stone. Try to maximize the depth and width of the trench. If the trench comes close to the side wall of the slab then you might want to expose the slab wall and apply a waterproofing membrane as deep as possible and maybe 4-6 inches above grade.

    Topping with a heavy stone will allow the little alley to be blown free of accumulated debris a couple of times a year.

    This approach is fairly inexpensive and could be done by oneself. You would not constrict the existing pathway and access to the garage wall would remain as-is. One caveat is that you'd need to be careful about the trench maintaining positive slope along the whole run to daylight.

    1. User avater
      jhausch | Apr 26, 2003 02:02am | #8

      I hope the revised attachment helps to clarify things.  For lack of a better description, one wall of the garage will have its lowest 3' below grade. 

      Does anyone see a problem with this? 

      Remember, the drawing has no scale, do not interpret the width of the driveway as representative.

      By the way, thanks to all for any and all input.Steelkilt Lives!

      1. RalphWicklund | Apr 26, 2003 08:48am | #9

        Elevations in your drawings would be most helpful. It might even be possible to bring in a dozer and just regrade the area so that you don't have to deal with all the uncertainties of trenches, foundation drainage, block walls, waterproofing, fish ponds and mosquitoes. You could end up with several BIG truckloads of saleable fill if you could lower the finished grade to below the slab height within a reasonable area of the garage. Then, if you needed a retaining wall on the other side of the driveway because you cut into the slope it would appear to be an easier build and make water runoff more manageable.

        1. User avater
          jhausch | Apr 26, 2003 05:52pm | #10

          I apologize for the less than perfect illustrations.  I agree with the efficacy of your suggestion, but the distance/space available makes proper grading difficult.  The best solution would be to knock down the garage put a proper foundation in under a new one.    Steelkilt Lives!

          1. RussellAssoc | Apr 26, 2003 07:07pm | #11

               I'm afraid this fellow has a significant problem.  Retaining walls, trenching, etc will provide some "Rube Goldberg" relief, a little bit like layering up the roofing pitch around a failed chimney flashing.  He needs to re-grade.  I realize this may not be economically practical or desirable, but it is the correct solution. 

              If he can't afford re-grading, try the other "fix's", I would if possible try to get the water to drain into a sump pit and get some mechanical pumping help.  I'm afraid just relying on gravity may not relieve the additional potential problems mentioned above.

          2. Piffin | Apr 26, 2003 09:13pm | #13

            Here it is . The upper ( which with your slab on grade would be the only) drain line is set in 3/4" washed gravel about 16" out from the house and same 16" deep..

            Excellence is its own reward!

      2. Piffin | Apr 26, 2003 08:59pm | #12

        The attached photo shows a sitruation much like what you describe except that the raised portion grows trees and flowers instead of carrying vehicular traffic. I can testify that it has performed well for thirty years.

        I'll make a schematic section drawing of how it is detailed - easier than text.

        Later gator.

        Excellence is its own reward!

      3. clevispin | May 04, 2003 05:11am | #14

        Jim,

        This is a late response but maybe it might help.

        You might have two camps on this-one being the let nature take it course camp and the other being the lets build something camp. I tend towards the first way but increasingly in my field of architecture (mostly renovations) I see people are more willing to take the "risk" of 'back-building' for lack of a better term.

        I looked at your illustration and the idea seems like it would work. One source of information worth looking into would be your local building code manual where it addresses below grade wood foundation systems. These are pressure treated assemblies whose details might be applicable in your situation.

        I don't recall what your siding material is but if its a clap then the lower courses of your modified exterior siding could be a Hardieboard type cementious siding. You can bury this stuff in mud for years and it will still hold up. It comes in various sizes and styles and could even be modified on-site to perhaps match your existing siding. You could also treat the lower courses of siding as a "watertable" that is distinctly different than the upper part of the wall.

        Hope this helps.

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