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Small Cabin Forum / Cabin Construction / first build - existing concrete slab
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karladaune
Member
# Posted: 2 Jan 2021 23:01
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Hi All -

I am starting to plan my very first build. It's not a cabin, but I am a complete novice so I am starting with this to build my skills (and confidence.) I want to build and 8'x8' lean-to style shed on my existing concrete driveway for my motorcycles. The concrete is in decent shape, but it's really old and I have no idea how it was poured...it's also not 100% level, but not far off. I really want to keep it as low to the ground as possible for ease of getting the bikes in and out of it - what is the best way to do this? Can a pressure treated floor base be put directly on the concrete, or should I put it on a skid made out of pressure treated 4x4's? Any input would be greatly appreciated!

Karla

BobW
Member
# Posted: 3 Jan 2021 17:55
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Why not just use the existing concrete floor? Bikes don't get cold feet from it. Also it is closer to ground level than a raised wood floor. I can either push or guide the sidecar rig up a small ramp to my shop. If it does not crank, it takes two people to get it into the shop.

Nobadays
Member
# Posted: 4 Jan 2021 09:57
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If you are wanting to build directly off the he existing slab, you could drill the slab and insert anchors to hold your bottom plate. Yes, use PT wood for the bottom plate and a strip of tar paper between the wood and concrete.

Anchors LIKE THIS should work.

A word... or two of caution about building off a slab. First, being off level is going to multiply, and complicate itself. Say it is off by 1/4" per foot, over 8' that is 2" difference front to back. If you put a door in that side, everything will be on an angle... door won't fit, or you have to do a lot of custom framing around the door... and put the same slant on the door bottom.

Another consideration is water intrusion under your plate. That may or may not bother you but will eventually rot out the bottom plate, even PT. On a conventional slab built building the bottom plate is flush with the outside of the slab and flashed to prevent any water from getting under the wood. Building on an oversized slab you won't have that option. You may be able to seal the perimeter but I'm not sure how to do that.

If possible do your best to do that custom work of the bottom plate you bolt down. Get it level even if you have to cut larger dimension lumber on a slant to bring it up level. If you spend the time and level it well on the bottom, the rest of the build will be a breeze.

snobdds
Member
# Posted: 5 Jan 2021 00:40
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Large overhangs are your friend here. Keep that water away from the on slab wood.

If it was me, I would upsize the bottom plate to a 4x6 scribed to the concrete for level. Then you still have enough board to last a long time before rotting. This coupled with large overhangs, you are pretty set to build.

Brettny
Member
# Posted: 5 Jan 2021 09:32
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If your floor is concrete and its bigger than the building no matter if sealing will keep water from getting in. I would suggest you build its floor like a shed on skids.

Irrigation Guy
Member
# Posted: 5 Jan 2021 10:14
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I have a slab that I have been thinking of doing the same. I have thought of building like others have said on a larger bottom plate anchored in but with smaller potentially removable trex material sandwiched between the concrete and the real bottom plate. That way wood would never contact the ground and even if the trex degraded you could loosen the anchors and replace the trex.

gcrank1
Member
# Posted: 5 Jan 2021 10:59
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2x that
Use a sacrificial bottom plate, and make it the scribed/cut/etc to level then the actual shed bottom plate atop that. Power screw down from the shed plate into the sacrificial plate for ease of future issues.
If/when the sacrificial plate goes bad you can replace or if you move the shed you just make a new base as needed.
Ime having a ramp (which I currently do) to go up for mc storage, or even a 2x4 to bump over, is fraught with danger of losing the pushing balance of the bike; I have almost lost the one I was pushing in over/into the other. Almost. Murphy says that if it can happen it will.
So, Id make the doorway have nothing to keep from rolling right in/out. A sensible gap under the door will allow the shed to breath some too (if you vent the roof), not a bad idea, you dont want to trap moisture in with the bikes.

karladaune
Member
# Posted: 5 Jan 2021 18:20
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Thanks, all...I like the term ‘sacrificial plate’ - totally makes sense to me. I’m a novice at all of this, so I appreciate the confirmation that the way I’m thinking about it makes sense

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