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I have an old carriage house on my property, probably over a century old, which sits on what is now the lowest part of my property. As a result, water runs back into it along the driveway (which also has grade issues). The shed currently has a dirt floor because of all the topsoil that has washed back into it.

I want to rehab this shed to make it more usable, but I don't have the money to do what really needs to be done, which is to tear it all down, do all the dirtwork to get drainage headed out to the street and away from my structures, and build a new building. In lieu of that, I want to make the best use of the building with the minimal restoration I can get away with.

To that end, I have shoveled all the dirt out of one side of the shed to reveal the concrete underneath and assess what I can do with it.

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In the photo, you can observe the depth at which the concrete blocks were revealed. I would guess (I didn't measure) that at the lowest point, there was 10-12 inches of sediment on top of the concrete. In other places, not nearly so much. Clearly, the floor is very uneven.

What I'd like to do is build additional walls on along that support beam in the middle and across the front on this side where I'm working to create a closed-in storage room that would be safer from the elements and my dog and thieves. What can I do to make a suitable floor that would allow me to build a couple of walls and make this a secured barn-like room?

My current thought is to use gravel to create a level underlayment up to the highest point of the concrete, and then use 8x16" concrete blocks on top as the actual floor. In my thinking, that would allow water to infiltrate without causing further damage, but would also provide a strong support for a floor.

Is that a terrible idea? Is there anything better I can do at a low cost?

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    A trench to divert the water. Doesn't need to be very deep at all, just enough to guide the flow around the area. Address the grading if you can. With that much dirt inside, either this has been building up for a very very long time or you have a bit of an erosion issue. A french drain could also help if you have a place to empty the water, but you said this is already at the lowest point in the yard. – Phaelax z Dec 29 '20 at 13:57
  • Thanks, @Phaelaxz. The problem is that this carriage house sits on the lowest part of the property, so I'm not sure where a trench could go. – Ben Collins Dec 30 '20 at 3:56
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It seems a sump in the lowest corner with an electric pump would keep water at a minimum . Maybe two sumps . Break /cut through the concrete and put in a cement or even a plastic tub to set the pump into.

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  • Yeah, I think this turned out to be my best option without doing something far more substantial. I ended up building a pretty conventional wood floor truss and laying down a plywood floor. It rained really heavily while I was working on it, which sounds unfortunate but it turned out to be very instructive as I got to watch where the water was infiltrating in realtime, as well as how high it would get (answer: too high!). I found a spot between joists to set a portable pump, and it seems to be keeping up. – Ben Collins Jan 1 at 3:40
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I would start by digging a trench and filling with gravel / small stones to direct the water away as much as possible.

Did that at the back of my property - started at 6" deep and went to 12" as it needs a small slope. directed the water into an existing drain and it worked a treat. Only took 2 afternoons of digging and measuring the slope.

Edit: if the property is the lowest part of the surrounding land then another possibility is to dig a soakaway to help the drainage. Any method of reducing the amount of water to deal with at the building will help.

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    yeah, I was also about to suggest a french drain outside the shed. – Jasen Dec 29 '20 at 8:10
  • Unfortunately the driveway leads directly into this carriage house, so in order to dig a trench across the front of it I would have to dig through concrete. Not impossible, but that would represent a pretty significant scope (and cost) increase from what I was planning. More importantly though, the shed is in the lowest part of the property, so there's not really anywhere to go. – Ben Collins Dec 30 '20 at 3:53
  • @BenCollins is the proerty the lowest part of the land in the area? – Solar Mike Dec 30 '20 at 5:45
  • @SolarMike no, not particularly. The lots adjacent to mine are more-or-less at the same level. – Ben Collins Jan 1 at 3:46
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Building a higher floor inside is not solving the problem of water getting into your shed. Your plan to to level gravel for a cement floor sounds smart and feasible. The gravel under the block would allow water to drain if the existing concrete slab were pitched properly.

You did state there was a high point to the floor, but unless that high point is located were the water is entering (and sloping in the opposite direction) the water won't drain away.

What concerns me is that water will still be able to enter your barn even after the new floor is constructed. Won't other sections of the structure still be getting wet? Every wood framing member below your new floor will still be saturated.

Your plan is admirable and you will be investing alot of time and effort to complete it. I would suggest you first make the effort to prevent the water from entering the building. This can be done in several ways; one was mentioned by another writer. Also consider constructing burns or continuous mounds of dirt on the upside of the building to divert water. Also use in conjunction with drainage channels and perforated pipes. The higher up the slope you begin with whatever solution you choose continue so down the slope adding another diversion or channel so rather than one trench at the bottom you have several small swails descending down the slope.

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  • Thanks for your answer. Unfortunately I don't have the resources at present to really solve the problem of the water draining back into the building, so my goal is merely to work around it with a solution that need only last for 3-4 years. – Ben Collins Dec 30 '20 at 3:55

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