Pour concrete under exsisting structure?

928ram · 03-08-2005, 06:59 AM

Maybe someone here can shoot me some ideas on how to accomplish this project. It's something I'd like to do before we move to our new home in June, but at 600 miles away it may have to wait until we relocate.

In the backyard of the new place there's a 24x12 shed/workshop that's of all wood construction and situated on a series of poured-concrete piers. What I'd like to do is maintain the current elevation (or close to it) and somehow replace the wood floor/piers with a poured floor. Since the current setting leaves a gap that varies from about 4-10", I'm a bit worried about wood rotting, vermin moving in, and trying to keep plantlife from growing under the edges.
The structure is of 2x4 construction (probably bigger under the floor), plywood sheathing, vinyl siding, shingled roof. At first I was thinking of taking it out of there, but it's wired and roomy so I'm thinking that it would be a great woodworking shop plus keeping the yard and garden stuff out of the garage.

Ideas?

turbo6bar · 03-08-2005, 07:09 AM

Can you reach the structure with a concrete truck?

Sounds like a major pain, because you would need to support the structure from the exterior walls, form the walls for the slab, and then pour the concrete into the center. All-in-all, it would be a lot of work for the perceived benefits.
Jurgen

Z-man · Z-man's Garage

You should be able to pour a floor. When I re-did my shed a few years ago (one of those aluminum jobs) my wife and I poured a concrete floor. The old shed was knocked down, so we didn't have to worry about working around the current structure.

But you should be able to remove the wood, and pour the floor. Make sure that you put down a water barrier underneath the floor! The area isn't that large, so you should be able to mix the cement yourself easily. Make sure you have rebar / chicken coop wire in the floor to have something for the cement to adhere to. You're probably not going to put a lot of heavy things in the shed, so I wouldn't go crazy with reinforcement. But make sure the floor is 3-4 inches thick to prevent cracking. Allow the floor to sufficiently dry before putting things on it!

Hope this helps,
-Z-man.

PS: Yes, I said my wife and I poured the floor - she did a ton of the mixing by hand!!! She must really love me!

juanbenae · juanbenae's Garage

you might try forming right around the perimeter of the building and pouring a VERY wet concrete. maybe a NO SLUMP mix. you will need conrete vibrators to get the mud to flow and elevate up to the bottom of the existing floor. you will also need multiple locations to introduce the concrete and vibrators in.

is the existing floor supported by joist, or more of a post and beam affair? if joisted my suggestion above won't work.

turbo6bar · 03-08-2005, 07:21 AM

Z-Mon, the aluminum shed is supported at the edges. A wood frame shed of this type has a superstructure built on top of the floor. In essence, you build the floor first, and then build the walls on top. Removing the floor without removing the structure is not a trivial task.

BTW, I would be really hating life if I had to mix enough concrete to fill a 24x12 slab with depths reaching 10" or greater. I'm guestimating 4 yards of concrete for a 4" slab. Sackrete is in 1/3 CF bags, I think, so you'd need 300 bags??? 928's wife would need the biceps of Ah-nold.

vash · vash's Garage

are the pier/piles configured around the perimeter? concrete ones, and do you know how deep (well doesnt really matter i guess). since it is a framed workshed, couldnt you rent some beams, and jacks and pick it up a tiny bit, actually support it, demo out the floor and any foundation, drill dowels and epoxy them into the existing piles, lay down a mesh, or if you get fancy, a simple rebar grid, maybe a gravel pad prior to rebar, do some forming and pour the new floor and a raise foundation, set the woodshop back down, anchor it, and away you go. i wouldnt mix the concrete. get it delivered and hire a concrete pump company. but i dont know your budget.

928ram · 03-08-2005, 08:33 AM

I don't think getting a truck in would be any problem, but there's the risk of cracking the drive or rutting up the lawn and damaging pipes or such that may be under.

My basic idea was to brace the shed on the inside, support it in place, pour the floor as high as I could get away with, cut the floor out from under the shed, somehow lower the shed onto the new slab and add anchors after the fact.

The home isn't exactly rural but there's enough farm and woods with running streams in the area that I worry about snakes or raccoons and such finding the shade under there a nice hiding place; not to mention fire ants and bees.

Perhaps putting a layer of gravel or something under there would do the trick??

id10t · 03-08-2005, 08:39 AM

If you just want to keep critters out, have you thought about blowing/pumping some sort of liquid foam underneath to take up all the airspace?

competentone · 03-08-2005, 08:39 AM

Quote:

Originally posted by turbo6bar
Z-Mon, the aluminum shed is supported at the edges. A wood frame shed of this type has a superstructure built on top of the floor. In essence, you build the floor first, and then build the walls on top. Removing the floor without removing the structure is not a trivial task.

BTW, I would be really hating life if I had to mix enough concrete to fill a 24x12 slab with depths reaching 10" or greater. I'm guestimating 4 yards of concrete for a 4" slab. Sackrete is in 1/3 CF bags, I think, so you'd need 300 bags??? 928's wife would need the biceps of Ah-nold.

Exactly what I'm thinking -- especially the part about mixing the 4 yards of concrete!

I'd really have to look at the structure to determine what is best, but for a relatively small building such as that you describe, attaching beams to the existing walls and carrying the wall and structure's load to the outside of the building's footprint should not be to difficult of a task -- this would allow you to do a continuous pour for your floor.

If you're not overly concerned about seams in your slab, you can notch out sections of your floor under your walls and pour footing sections to support the wall partially, then remove the rest of the wood floor and pour the rest of your slab (use re-bar extended past/through the forms of your first footing sections so they can be tied into the main slab).

In either situation, don't forget to install an effective tie-down system so the walls are tied into the slab. You're not just dealing with down-load/weight forces that need support; there is the issue of wind side loads and in extreme cases even "lift" -- your walls need effective anchoring at their base.

vash · vash's Garage

sounds like an excellent project. looks like some of us are on the same page. i agree with competene, put some holes strategically in the shed, run some steel beams thru and jack up the thing outside the footprint. then take the floor out first, do work and lower and anchor. i helped a friend do an entire house that way, in san fran. he redid the entire garage that the house sits on (like most SF homes). when he was done, his garage had no post, so you could fit six or seven cars in there.

competentone · 03-08-2005, 08:51 AM

Quote:

Originally posted by 928ram
.My basic idea was to brace the shed on the inside, support it in place, pour the floor as high as I could get away with, cut the floor out from under the shed, somehow lower the shed onto the new slab and add anchors after the fact.

Is there a crawl space under the shed now?

Your original post said, "...a 24x12 shed/workshop that's of all wood construction and situated on a series of poured-concrete piers. What I'd like to do is maintain the current elevation (or close to it) and somehow replace the wood floor/piers with a poured floor. "

How high are these concrete piers? Is the wood on top of the piers something like 2x6s?

The distance between the ground and thickness of your slab can be made up with sand and gravel -- if the height is significant you could need to pour a retaining wall.

928ram · 03-08-2005, 08:52 AM

Vash, the piers looked a bit random to me, about 12-16 diameter but no idea of the depth. I was thinking I'd just clean them off and pour on top of them using a combo re-bar/mesh in the slab; it's actually a pretty level lot so settling and erosion aren't a concern.

This building didn't figure into the price of the house for us and I would've built one anyway (smaller), plus it's only 4 yrs old, has 40 or 60 amp service, and survived the recent hurricanes. I figure with all that going, it's worth saving and fixing up a bit since it'll still be easier/cheaper than replacing it.

competentone · 03-08-2005, 08:56 AM

"hurricane..." "fire ants..."

It sounds like you're moving to the Southeast! Where is this?

928ram · 03-08-2005, 09:03 AM

Here's a small pic of one corner (had to crop out people); the space beneath actual looks higher than I remembered:

Z-man · Z-man's Garage

Quote:

Originally posted by turbo6bar
Z-Mon, the aluminum shed is supported at the edges. A wood frame shed of this type has a superstructure built on top of the floor. In essence, you build the floor first, and then build the walls on top. Removing the floor without removing the structure is not a trivial task.

Understood.

Quote:

BTW, I would be really hating life if I had to mix enough concrete to fill a 24x12 slab with depths reaching 10" or greater. I'm guestimating 4 yards of concrete for a 4" slab. Sackrete is in 1/3 CF bags, I think, so you'd need 300 bags??? 928's wife would need the biceps of Ah-nold.

We poured a 12x12 slab, about 4-5 inches thick. IIRC, we went through 150 bags of quickcrete. Hand mixed. Took a lot of time, but I'm proud that we did it! Actually, the floor was the easy part - the 1,000,000 nuts and bolts required to put the shed together was a pain in the butt!!!

If you can't pour the concrete without removing the floor than the notching idea sounds good, as long as you can deal with seams.

Yeah, my wife really loves me!

-Z.

928ram · 03-08-2005, 09:14 AM

Quote:

It sounds like you're moving to the Southeast! Where is this

The new house is in the Mobile, AL area on the east side of the bay, and currently I'm right down the road from you by NWS Charleston.

competentone · 03-08-2005, 09:19 AM

Quote:

Originally posted by 928ram

Was that structure build on site?

It looks like it might be a pre-fab shed delivered to the site. If that's the case, your best bet is probably to keep it treated for pest (same treatment as the house would get) and just install some sort of skirting to keep the bigger critters from getting under the building.

competentone · 03-08-2005, 09:22 AM

Quote:

Originally posted by 928ram
I'm right down the road from you by NWS Charleston.

Hi neighbor!

--Joe

Moneyguy1 · 03-08-2005, 09:39 AM

If the wooden floor is in good shape, and the piers are concrete, there are a number of things you could do to reduce the probability of critters/rot/etc, especially since you live in a relatively wet area.

Sealing around the perimeter would not be a good idea, since there has to be air circulation in order to reduce the probability of mold, etc.

You could dig a trench around the area from pier to pier and pour a footer. I don't think you have much to worry about re: frost heaving. If there is enough space, you can roll heavy plastic sheet from one side to the other under the structure to reduce moisture infiltration. Then, install a concrete block "wall" from pier to pier. This might take some fussy measuring, since the dimension between the footing and the floor joists will be determined by the height of the block(s).

If you still wish to eliminate the wooden floor structure, this can be done now. The floor can be cut out, leaving of course the rim joists, the rim joists covered with plastic (after a hefty coat or two of copper based preservative), a dump truck of gravel shovelled in and a cap of 3 inches or so poured on top of the compacted gravel.

When we moved four years ago, I had a similar situation: a very neglected but sturdy storage shed (aluminum sheet with a wood frame), sitting on blocks and a rotten wooden floor. Except for paying for the gravel delivery, everything I needed including the redi-mix I brought home in the trunk of a Passat (multiple trips). My shed is, unfortunately for me, about half the size of yours (Lucky you!!) So, if my figures are anywhere close to correct, at 1/2 cu ft per bag and 3" thick, you would still need 150 bags.

Come to think of it, that is a heck of a lot of concrete!!

Better call in the truck!!

928ram · 03-08-2005, 09:42 AM

Quote:

Was that structure build on site?

This I don't know; it may even be possible with as wide as this thing is that it's built mobile home style and has a place for a hitch and axle. I didn't see any signs of such but it was a bit wet out so I didn't get a real good look underneath.
The fact that the house is a big beautiful custom built brick house and the expense that he must've put out to bring electric out to the shed, leads me to believe that it was a pre-fab as it really is out of place being so plain.