Reusing Shed Foundation on Hill

[garageuser]

My house came with an old 8x10 wood shed which I'm tearing down and replacing with a 8x12.5 resin shed. The shed will be used for lawn tools, push lawnmower, bikes and junk valuable garage overflow. It will not be used for anything heavy or as a workshop. No frost or snow and only rains a few times a year here.

The foundation has four concrete round piers (three front, one middle) and two concrete blocks in the back corner, all forming a 8' x 8' area (see pic below). On top of these were three front-to-back 8' 4x6 PT beams then 10' 2x4 joists running side-to-side on the beams. OSB for the floor (Not visible because already removed)



I'm planning to reuse all the concrete footings to save time and effort and avoid pouring concrete. I will add some post anchors to those concrete piers since the old shed just had beams sitting on them without any attachments

I was thinking the platform will be rebuilt with the same set up. Three 8' 4x6 beams front-to-back and on top of that 12' 2x6 PT joists running side-to-side. Plywood for the floor. I might put an extra concrete footing in the middle rear but there's a tree stump in the way so I'm assuming that's why the original builder didn't either. See below (green = beam, blue = joist, circle = concrete pier, square = concrete block)



I will also be building a small retaining wall behind the shed to prevent dirt and vegetation runoff from collecting on the side of the shed.

Questions

1. The old shed stool many years, as evident by some of the wood rot, so presumably the foundation is stable and reusable. However, I also want to correct any major issues before putting up the new shed. Anything of concern?
2. How can I make the rear concrete blocks more stable? This is on a hill so I'm concerned about soil erosion. Perhaps filling in the ground in front of the concrete blocks with all the extra earth from the retaining wall?

 

[Zeke]

That's a weird layout. I don't understand the center pier except for your explanation of an existing stump. Still, it a silly design.

If it was me, I'd see what the precast blocks are sitting on. If nothing of not much, I'd dig footings and add more poured piers or reset the precast ones on a concrete base. Which is the entry side?

The precast block piers are likely there because of the stump. At the time it was tough digging there. Might be easier now the the stump has begun to rot.

Probably don't need much of a retaining wall if you can ease the grade and create swales for the water to follow. Just enough of a wall to hold back any mud when the winter comes.

 

[garageuser]

If it was me, I'd see what the precast blocks are sitting on. If nothing of not much, I'd dig footings and add more poured piers or reset the precast ones on a concrete base. Which is the entry side?

The entry will be on the left 8' side.

Both of the rear blocks are sitting on dirt and roots. There's a tall pine tree about 10 ft up the hill so it's possible the roots are from that tree which is very much alive. I'll dig around and see.

What about digging a hole, filling it with crushed gravel then set the blocks on top of there? Similar to what's described here: https://www.finehomebuilding.com/2003/04/01/a-firm-foundation-for-a-backyard-shed

Probably don't need much of a retaining wall if you can ease the grade and create swales for the water to follow.

Here's a side profile of the grade. The green line represents level.


All of the light brown material are pine needles and dirt that have piled up along the back of the old shed. What do you mean by "ease the grade"? If I flatten the slope behind the shed then the ground will end up extending farther forward and will cover the rear of the shed.

 

[Zeke]

Well, from the first pics the grade didn't look that steep. You could move the shed forward using the existing piers for the back or even use the center one for part of the back installing additional blocks at either end. The ones in front would be the center and could hold a girder and then put the joists in the shorter direction. With some side hangers on the round piers you could keep the same floor height.

A hundred dollar stick welder and you can make anything.

There's a dozen ways to do this, but a good solid foundation and drainage are the most important to get started. You might still be able to clear the area behind the new shed, put in gravel and a drain and even the retaining wall you suggest. Me, I'd find a way to avoid the wall unless I could dry stack something with a drain behind that so there's no mud/debris flow under the shed. Keeping that dry and airy will be the key to how well your equipement fares in the shed. The floor should be moisture proofed as discussed many times here WRT decks.

Will you need a ramp to the doors? Seems like a good idea.

 

[garageuser]

There's a dozen ways to do this, but a good solid foundation and drainage are the most important to get started.

Yup, I've read about the whole range of shed foundations from just putting skids directly on the ground to building posts that rival decks and houses. I'm hoping this is more of the former in cost and effort.

You might still be able to clear the area behind the new shed, put in gravel and a drain and even the retaining wall you suggest. Me, I'd find a way to avoid the wall unless I could dry stack something with a drain behind that so there's no mud/debris flow under the shed.

I was thinking something like the picture below with gravel under the rear blocks and cut out some of the dirt behind the shed so there's some room (red arrow).

I'm 50/50 on the retaining wall at the moment.

If I built a wall, it would go to the left of the arrow and have a drain pipe behind the wall.

If I don't build a wall, you're recommending a french drain of some sort that would go where the red arrow points?

Is mud and debris under the shed an issue since it's off the ground? It's essentially a pier foundation so as long as all four sides are relatively clear, it should be ok with respect to moisture and vermin?

The floor should be moisture proofed as discussed many times here WRT decks.

It would be pressure treated plywood.

 

[Old Moparz]

This should be absolutely fine. I have an 8' x 8' wooden shed I built in 1990 that is still sitting on a foundation almost exactly like this without any issues. At all four corners I dug down 6 to 18 inches, dumped in & tamped 4 inches of gravel, stacked standard sized, concrete blocks & built a flooring system similar to what you sketched.

 

[garageuser]

This should be absolutely fine. I have an 8' x 8' wooden shed I built in 1990 that is still sitting on a foundation almost exactly like this without any issues. At all four corners I dug down 6 to 18 inches, dumped in & tamped 4 inches of gravel, stacked standard sized, concrete blocks & built a flooring system similar to what you sketched.

Awesome to hear! What did you do with the ground under the shed between the blocks? Leave as bare ground or put down landscape fabric and gravel?

 

[Old Moparz]

Awesome to hear! What did you do with the ground under the shed between the blocks? Leave as bare ground or put down landscape fabric and gravel?

I didn't use anything underneath & I believe all I did was rake it to make sure I kept the slope for drainage. It's on a slightly sloped area of the yard, not as steep as yours.