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Foundation for small shed on wet slope

 
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Location: Western Massachusetts (USDA zone 5a, heating zone 5, 40"+)
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Western MA, zone 5, on a slope.  I want to build an 8x12 shed as a home office (probably super insulated) in our back yard.  The site slopes 18” from the highest to the lowest corner.  What is my best bet for a foundation?  Gravel held in place with 4x4s? I want to minimize the amount of impervious surface - our yard gets seriously soggy as it is - so I was hoping a gravel base (extending further out than the footprint of the shed) might help with infiltration.
 
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Would stumps be worth looking at?
 
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Things I'd be considering, allowing that I don't know your ecosystem, budget, or over-all needs:

1. How deep is your frost line?
2. Could a green roof slow runoff to help with the sogginess?
3. Could you plan a "rain garden" short term pond to help with water infiltration?
4. Would you benefit from a great big rainwater collection tank to delay infiltration to a time of year when nature could put it to better use?

Generally, people don't want water underneath buildings due to the risk of freeze/thaw cycles and humidity rotting things. Every ecosystem needs to be evaluated from its point of view, as the approach in my climate could be quite different.
 
Steven Kovacs
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Theoretically the frost depth is 48”.  No way I am digging that deep for a shed foundation.  In practice the climate has warmed a lot here so 48” is not realistic anyway.  Some frost heave below a shed is ok though I am not sure how electrical conduit will like it.

Stumps will rot far too fast with the amount of water in the soil (rich loam on top of clay on top of bedrock, at the bottom of a hill).  We have some 12” stumps in the yard that have turned to mush in 3 years.

Green roof is a nice idea but would require a more robust structure than I am planning for.

Water storage is also a nice idea but my experience on this property is that we need to get water off it as fast as possible except in the summer - we get at least 54” of liquid water equivalent per year, increasingly in rain rather than snow, and we are getting deluges of 2-6” in 48 hours sometimes (a town nearby got 10” in a day recently).

Concrete blocks seem the cheapest option but don’t keep critters from nesting underneath.
 
Jay Angler
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Steven, have you looked up any height restrictions?

With the answers you've given me, I'd be thinking digging holes for concrete sono-tube pillars and actually raising the building a good few feet to avoid the critter issues - ie high enough off the ground for air flow to discourage mold and to decrease any risk of the building flooding if the weather keeps getting high volume storms.

I've had huge issues with rats in my area (water attracts rats - they're not going away no matter what anyone does). Being able to get under a building to inspect it and set traps is huge for me.

My only other idea was to capitalize on the building, dig and pour a (rat-proof) basement, and turn it into a cold cellar/storm cellar. "Cool" food storage could be useful in a future with higher electricity costs and back up for longer power outages caused by more frequent and larger storms.
 
John C Daley
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I had in mind concrete stumps
 
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I would call concrete "stumps" piers. Those would likely work fine if the OP wants to have open space under the floor to avoid damp. A wood-framed structure on the piers could overhang two feet on each end and use just four piers while having the floor as stiff as possible. Just a couple of steps up from the high end of the ground would give adequate airspace and inspection access without much climbing of stairs.
 
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Steven Kovacs wrote:Concrete blocks seem the cheapest option but don’t keep critters from nesting underneath.



Concrete blocks is the way we have built in the past.

You can keep critters out with skirting.

There are many skirting options from boards to welded wire.

I liked Jay suggestion to do rainwater catchment.  That will help with getting water off the property as quickly as possible.

During hard times of drought that rainwater will come in handy.
 
Steven Kovacs
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Thanks, all.  I think I will do concrete blocks, which worked for my 5x12 wood shed.

We have three rain barrels to ensure adequate water during droughts; the two droughts we have had in the last decade only required about one and a half barrels of water so I think we are good on that front.
 
John C Daley
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In Australia, 'stumps' are generally made of concrete, have a steel bar through the middle are 5 inch x 5 inch and what ever length you want.
To install, you dig a hole, drop a 4 inch thick pad of a dryish concrete mix, lower and level the height of the stump and backfill with concrete or cement stabilised soil.
Piers are built up from a shallow concrete pad using bricks or blocks.
 
Glenn Herbert
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Cool. "Stump" is not a term used in the US in this context.
 
John C Daley
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OK, what do you call stumps?
 
Jay Angler
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John C Daley wrote:OK, what do you call stumps?

A "stump" is the bottom of a tree that is still in the ground, when the top of the tree has been cut off or broken off. There are ecosystems where using this "stump" as a support for a house has been done and can last for many decades. There are other ecosystems, where this would be a very bad idea.

I don't know what the North American term for what you call a "stump" is, other than "some version of a pylon". Because my area has a lot of glacial till pockets, and earthquake risks, one foundation technique is to ram some version of a pylon into the ground until it hits bedrock. This would be a commercial technique, so I don't know more - only that the noise from pounding can last a week or more and travels a fair distance!
 
Glenn Herbert
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A piling or pile is generally driven into the ground deep enough to firmly support its load (sometimes to bedrock). I don't know of a distinct term for a thin concrete post set in a hole and  backfilled.
 
Steven Kovacs
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Thanks, all.  Ultimately I think I will go with concrete blocks and skirting, not least because it is the cheapest and easier option.  It also has worked well for my wood shed.
 
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