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Greenhouses & Garden Structures

Replace Shed on Concrete - Tying to foundation and drainage questions

7 years ago

Our house came with an old metal shed on a concrete pad with a ramp. The pad is 11x9 feet, but the shed is less than 10x8, with an edge over 6 inches of the pad showing on each side. The front with doors is on the long side. There is some metal track at the base of the walls, and there has never been water inside the shed. It is at least 25 years old now and is rusting and in need of replacement. We are on the edge of the Northeast Ohio Snow Belt and get a lot of rain and snow.

I would like to build a new wood shed on the whole pad. The ground around the shed slopes down from the back to the front. The concrete pad top is level with the ground in the back and 5 inches above ground at the front. The ground extending away in back is relatively level with a large wooded area on the next property behind the shed.

The first question is with building a wood wall on the concrete foundation. Every instruction for a wood shed states to overlap the wall siding over the edge of the pad. However, this pad is level with the ground in the back. There is no practical way to overlap the concrete pad. If the metal shed can be built in the middle of the concrete pad without going to the edges and not have water problems, there has to be a way to do the same with a wood shed. Anyone know how to reliably seal a wood shed wall to concrete without overlapping the foundation?

A second question is with drainage. The old metal shed has a gambrel roof which slopes off to the sides. I would like to build a gable roof the other way with and let it slope to the front and back for access to loft storage for 10-foot long poles that I use in the yard. This will result in plenty of rain and snow coming off the roof onto the rear uphill side. Is it safe to rely on sealing a wood wall to a concrete deck, or will something else have to be done to redirect water from the rear to the sides?

Thanks,

---H

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Comments (3)

kudzu9
7 years ago
I'm not sure why you have never had moisture inside the existing shed given those issues, and changing the roof line so that it increases the amount of water and snow on the uphill side sounds like a recipe for rotting of a wood structure over time. It may be that the existing roof design is a big factor in preventing water damage to date.

Moisture will cause expansion and contraction of wood, which means that a caulk/adhesive bond between wood and concrete will likely fail over time, so it's hard to recommend this type of connection.

I have three thoughts on these challenges:

1): excavate around the pad so you can lower the soil level to several inches below the top of the pad, and trench down and install 4" drains all around the perimeter. Then you should be able to overlap the sides of the existing pad; or
2): pour a new, slightly smaller, 4" pad on top of the existing one: an 8'X10'X4" pad will take about one yard of concrete. Then you can overlap the siding; or
3): Lay a cinder block perimeter foundation in mortar all around the outer edge of the existing pad and build your shed on top of this.

With any of these approaches you should also do two things:
1): Securely anchor the shed to the foundation; and
2): Install gutters and downspouts.
hillel_s
Original Author
7 years ago
The original shed is metal - not wood - and clearly designed to be placed on top of a foundation without overlapping its edges. Why is it surprising that it works as designed? The question is if a similar design can be pulled off with wood.

Excavating behind the pad would just pool water up against it. I might be able to slope the ground behind it to drain off to the side. An interceptor drain is an interesting idea.

With ground sloping away, the front already requires a ramp to wheel in a lawnmower and similar tools. There is a tree which prevent the ramp from being longer, so a taller floor would need a steeper ramp. It would be preferred to not have the floor move up well above the existing floor if at all possible.

Thanks,

---H
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kudzu9
7 years ago
I agree that excavating alone might not resolve the problem, which is why I suggested adding a drain.
If you don't want to raise the floor, the cinder block perimeter could still work: you would just not lay block where the doorway was, and you could put in a metal threshold.
It sounds like this is a case of choosing the least worst option since you are starting from a non-ideal point.

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