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Laying a foundation for a tool shed or children's playhouse

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Picture of Laying a foundation for a tool shed or children's playhouse
This tutorial shows how to lay a strong 6' x 3' x 4" concrete slab a foundation for a small structure such as a toolshed or children's playhouse.  What's different from this foundation and a simple concrete slab is the inclusion of steel bolts embedded in the concrete that a 2x4 frame plate can attach to.  If you just want a concrete slab for other uses (grill, trash cans, etc.) just leave the bolts out in the last step.

I made it in TechShop.  You just need to take the WOD101: Woodshop SBU class in order to be qualified to use all the tools in the TechShop to make the concrete mold frame and stakes.  The really nice thing about TechShop is that you can buy the lumber next door at Lowes and walk right into the woodshop and cut it before taking it home.
 
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Step 1: Tools & Materials

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Tools you'll need:
- flat head shovel
- float or cement trowel
- curved cement trowel for creating a smooth rounded edge around the slab
- large flat bucket for mixing the mortar
- 2x4 about 2 or 3 feet long for tamping the concrete
- (2) 6' 2x4s and
- (2) 3' 4" 2x4s for the concrete mold frame
- several 1x2 stakes to secure the mold frame to the ground while pouring
- gallon bucket
- long (3' ) level
- (optional) bolt cutter to cut the wire mesh to fit inside the frame

Materials you'll need
- (6) 80 lb bags of Maximizer quick setting concrete mix (or the equivalent that makes a 3' x 6' x 4" slab)
- 14 gallons of water (2.33 galllons of water for each 80 lb bag) (or the equivalent for your concrete mix)
- (optional) 3' x 6' piece of steel wire mesh
- (11) 3" long 1/4" wide hex bolts
- (not needed right now for the foundation but for when you attach your frame) (11) 1/4" washers
- (not needed right now for the foundation but for when you attach your frame) (11) 1/4" nuts

Step 2: Prep the site

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First you'll need to scrape a 3' x 6' section of ground with the flat shovel to level the ground while leaving the ground firmly packed.  If you need to build up the ground rather than scraping down, you'll need to use a tamper to pack the ground before proceeding.  Use the level to check your progress.

Then build a frame with internal dimensions of 3' x6' (i.e. the 6' 2x4's screwed or nailed to the 3'4" 2x4s).

Level the frame by digging down or using stones and dirt to raise the frame so it's level in both X and Y directions.  Pack dirt under any gaps under the frame so the concrete doesn't leak out while pouring.

The concrete is very heavy and will force the frame to move sideways and even straight up as you pour it, so you'll need to stake the frame securely to the ground.  I just cut the ends of some leftover 1x2s into stake-like shapes.  After the frame is leveled, pound the stakes into the ground up against the frame then fasten them to the frame with screws or nails.

Finally, use a bolt cutter to cut the wire mesh to fit inside the frame.  Lay the wire mesh propped up on some rocks to keep it about 1" above the ground.

Step 3: Mix and pour the concrete

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Finally, we get to mix the concrete and pour it into the frame.  You'll have to move quickly as the quickrete sets in an hour and we need this to be a monolithic slab meaning you have to mix and pour all 6 bags within an hour.  Be mindful of your back.  Tighten up your abs and lift with your legs...remember, it's easier to replace knees than spine disks.

Mix one bag at time in the large flat mixing trough.  I cut open the bag in the trough, spread it out with the shovel, and added all 2.33 gallons at once, spreading it out over the whole surface with the shovel.  After letting it soak in for a few seconds, I mixed it by shoveling it over and over within the trough.

After it looks like chunky ice cream, I pushed the trough up against the frame and shoveled the concrete into the frame.

After it's all in, pull up on the wire mesh a little in a few places to make sure it didn't fall off the rock props and so the wire mesh sits up in the concrete rather than on the ground.

Step 4: Level and tamp the slab

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Level the slab with the scrap piece of 2x4 and the level.

Then, using the long side of the scrap 2x4, tamp down the entire surface of the concrete to knock down the aggregate (i.e. the gravel) so you have a nice shiny surface of cement.  This will leave a smooth rather than rough surface on the slab after it dries.

Smooth the surface with the float/trowel.

Make nice rounded edges around the slab by sliding the rounding tool along the edges.

Step 5: Add the frame bolts

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After the concrete has stiffened (about 1/2 hour to an hour), tap the hex bolts, thread side up, down into the concrete about 1" in from the edges every 1.5'.  Leave enough thread on top to allow it to pass through a 2x4 and bolt to a washer and nut (about 2.5" above the surface of the slab).  Use your fingers to squeeze the concrete back flush up around the bolt.

I am planning on having a single door to this tool shed, so I only added 2 bolts on the right front besides the bolts on each corner.  These 2 bolts will fasten a short piece of 2x4 to the floor to attach the frame for the hinge side of the door.

Step 6: Cover the slab (if necessary) and cleanup!

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If there's a downpour, the pounding rain could wash off the cement from the surface of the concrete, leaving a rough surface, so if there's danger of rain, cover the slab with a tarp.  The tarp also keeps off leaves, stray dogs, bicycle traffic, marauding children, etc.  I just tied strings through the tarp grommets to the fence and draped the tarp over some stakes.

Hose off all your tools (and your clothes) and you're done!  The concrete should set strong enough to start framing within a few days, then it's safe to pull up the stakes and remove the slab frame.

In the last two photos you can see where I worked ahead and attached the bottom foundation frame over the embedded hex bolts.  I'll cover more in a future Instructable, but be sure to use water resistant lumber (I used pressure-treated chemical infused 2'x4's).

Thank you for sharing your hard work.
The great thing about an effort such as yours (even if imperfect), is it does two things. First, it allows knowledgeable people who don't have the time or knowhow to make an Instructable to make improvement instructions.
And second, it encourages the rest of us to have the courage to attempt what you have done.

Bravo.

I meant improvement suggestions. Oops.

I see you were following the instructions, but just to give you a heads-up. Rather than scraping the ground you want to dig at least 4 to 6 inches , compact the soil, and put 4 inches of gravel also compacted before pouring the concrete. You want to do this to get the proper support. This will also let water drain around it and into the soil without causing the slab to upheave during heavy rain or freezes. I read what papa2 said and he is correct, but you can fix that with pressure treated wood, painting the wood, or putting water sealent on it. No matter what you do though eventually wood will rot. duration and speed reduction is all you can truly hope for. Good luck, don't know what your climate is but if you get a heavy freeze you may see some cracks or even movement if this step was omitted.
papa21 year ago
Missing one important part !!! Between concrete and wood are no INSULATION ! The wood will start to rotten. In my country we use simple rubberoid. Otherwise good work !
chuckyd1 year ago
A few suggestions:

After going to all that trouble, you may as well have removed the organic soil and replaced it with compacted fill dirt. The organic soil will continue to move and settle for years to come.

Even with the temperature steel, the 3 1/2" slab seems awfully thin.

The anchor bolts should have an "L" on the bottom side to resist pullout. You can pull those bolt heads right out by simply tightening the nuts on the plates.

Wood in contact with concrete and within 18" of the soil should be preservative treated, else you will have a collapse in a few years.

I would have gone for a modular size, say 2' by 4', to minimize waste.

Great work, though, and thanks for paying attention to the amount of water to use in the concrete.
In a area that is subject to freezing for days or weeks on end a footing below the frost line and a layer of gravel will keep it from lifting then settling all wonky. It is also less likely to crack.
darwincam1 year ago
Nice job. It's difficult to tell what kind of wood you are using for your plate, but it's worth noting that one should use something rot resistance as the concrete will continuously wick moisture to the wood and cause rot. ps - concrete cures.
sircorvus (author)  darwincam1 year ago
That's a great point, I'll call that out in the last step and in the next Instructable I write which will cover the frame.
And...the anchor bolts should be of an "L" or "I" type. If the concrete begins to fail, the small hex heads will pull out IF there is sufficient force. That said with a small shed or storage area, most likely it would be fine given the number you have :-)
Agreed - the normal method here is to use much longer bolts and wire them to the reinforcing mesh before the pour. You can use a scrap of tape to keep the threads clean of splatter.

Plus, depending on your local authority, a fixed structure like this may require some kind of permit, and roof drainage may be necessary.

Be aware of potential low spots too - don't allow the top of your floor to be below a puddle.

And if you have a young helper, get them to sign your work. Writing the year in a corner is a good idea too.
kinderdm1 year ago
Just curious as I don't see any in the pictures, but is any sort of underlay needed on this? Like say a bed of gravel for support or anything. I've never poured a slab before but I am looking to do a lot in the near future as we just tore out a deck and want to replace with a slab patio, in addition to a new shed and pouring a new pad for an existing closet sized shed.
enginepaul1 year ago
Nice; simple and efficient.
Not criticizing, just from my experience, I use 3" or 3"1/2 regular concrete. On larger sheds I thicken the edges, but have used a plain slab and then 2x6 pressure treated fir a\'footings' to spread the load with no failures here in California.
(the area freezes, but no snow) I always bolt for wind resistance.
sircorvus (author)  enginepaul1 year ago
Good point, I don't have to worry about earthquakes but good to know for those who do.

Wind resistance is a big problem here, but I bolted the plate mostly just to protect against marauding children :-)
Good job, dont want to look like a smarty, but think that you had to leave some space between the shed and the fence, so that water and snow dont get trapped, other then that, good job )
sircorvus (author)  Serega_Canada1 year ago
Good catch, I don't have to worry about lots of snow in Austin but it's worth noting for those who do. Leaves also might be a concern. In the case of my shed, I'm planning for the roof to hang even with the fence which should keep out most of the leaves.