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Question DetailsAsked on 8/6/2014

The slab of m1928 garage is far below the ground level, allowing wet to soak the lower walls and floor. What to do

1928 garage sits at base of 65 ft dirt and pebble driveway that's at about a 5% grade. With any significant rainfall the floor and lower walls --hard to tell how much-- get wet and STAY wet and dank for days! The soil has built up around the slab so much that the slab is only found by digging. To repeat with emphasis: GROUND LEVEL HIGHER THAN SLAB!!! As I see it: water from the surrounding soaked soil seeps into garage and stays through many days of dry weather. HOT, HUMID Baltimore MD.

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1 Answer


Depends a lot on your ground configuration and soil type, so without an onsite inspection one can't really say - talking with a few Basement Waterproofing and Landscaping contractors might give you a better idea. Or a Civil Engineering or Landscape Architecture firm that does residential grading and drainage improvement designs.

Possible solutions that come right to mind:

1) Put in a collector swale or slotted drain in front of the garage (near the foundation say at the or a bit outside the drip line so not too close to garage slab, and intercepting water coming down the drive, then carrying that into a buried french drain around the side of the garage to the back - assuming ground level is low enough nearby in the back to allow the drain to free drain to the ground surface. Obviously, the shallower the frost penetration is in your area (hence shallow drain), the easier to do this - french drain would have to be installed to be at least at or below the bottom of the slab to work. French drain might handle it by itself, but if digging down to install one to the bottom of the foundation wall, you might as well put waterproofing on the outside of the foundation wall, though from your descrioption I am not sure if you have one or if you have a slab-on-grade situation.

2) IF a slab on grade, then digging back the slope at the edge of the slab down to below top of slab level (and putting in concrete block retaining wall or even maybe just cobbles or shot rock cover to hold the slope) might solve the problem, assuming you have somewhere to drain away from the slab perimeter ditch thus formed to free ground draining away from the garage and house.

3) Put collector swales on the surface all around the higher sides of the garage but say about 5 feet or so away fromthe garage, using compacted low permeability soil like a silty or clayey gravel, to catch the majority of the runoff and direct it away from the house and garage. This will take care of runoff water, but NOT solvethe problem if the water soaks into the ground and then percolates on top of an impervious silty or clayey or bedrock layer to the garage slab.

4) Make sure your roof drainage is not coming back toward the house/garage - that is commonly the main source of this sort of problem, so gutters and downspouots with swales leading the water away fromthe house can solve the issue in many cases

5) put in a sump pump at the main "incoming" source of the water side of the garage. If you have relatively free draining soil under the slab then a 2-3 foot deep sump with surrounding gravel bedding pack might work without interceptor pipes, or you might have to cut slab along sides to put in interceptor drains under the edges of the slab on the sides where the dirt is higher. Would have to be combined with pulling the dirt back to beyond the edge of slab to work

If looking for a contractor for this, the probably more expensive under-slab or full perimeter deep french drain solution would be by a Basement Waterproofing contractor in most cases. For excavation/swales and cutting back the slope from the slab to lower the ground level below slab level and provide slope retention as necessary, a Landscaping contractor would be the normal one to talk to. You could get several ideas and weigh the relative costs and certainty of each before deciding on a solution.

My gut feeling - since only a 5% slope for 65 feet, that is only 3-4 feet of hill in 65' - so cutting back the dirt around the garage and resloping it and stabilizing it; or more expensively cutting back and putting in a retaining structure (concrete blocks, railroad ties, shot rock berm, etc) to provide a free-draining swale below slab level (and a few feet outside it) from in front of the garage (so an interceptor swale maybe 5 feet or so in front of garage, sloped to it from garage door also to intercept water hitting there) then sloped drainage swale around the garage to free surface drainage away from garage sounds likely to be the cheapest solution.

The excavation and swale solution you might be able to DIY or use a handyman if not up to doing it yourself - at least cutting back the dirt on the slab and sey a couple of feet clear of the slab and a few inches below top of slab - then if that does not solve the problem you could go to the next step with professional help. Unless you are totally down in a hole - surrounded on all sides with hillside above the garage slab, I think you are talking some hundreds of $ to solve this, rather than the several thousands that basement waterproofing commonly costs.

Answered 5 years ago by LCD

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