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Please help -- Building a shed over an existing concrete slab

Posted by andrewindc (My Page) on
Thu, Jul 13, 06 at 16:22

I am posting this in the garage/workshop forum but perhaps it belongs in the remodeling forum so please let me know.

We have just bought a house and despite my desires, I am not up to many of the projects the house needs so I am relegated to reach my glory with a new shed.

The shed will go over a concrete slab that once housed a metal shed. The concrete is in good shape and is 11x8. Here is my would I attach a wood floor to the shed? Do I need a power hamnmer to connect the wood to the concrete? And, does my new shed have to fit exactly over the slab or could it be bigger?

I have some shed plans that I will probably use (god help me when i get to the roof) but my first question relates to the floor. Please help if you can add anything to this.

Follow-Up Postings:

RE: Please help -- Building a shed over an existing concrete slab

The walls could be on the edge of the concrete slab (assuming slab is square) but not larger. Why do you want a wood floor over a perfectly good concrete surface? It's only extra work and expense. Anyway you can attach the walls of the shed to the concrete using 3/8" anchors. They come in different lengths. You will need a masonry bit, drill the required size hole and then pound the stud into the hole. Once tightened it spreads to hold the stud and therefore the wall in place. A very strong method of fasteneing and it's what I used.

RE: Please help -- Building a shed over an existing concrete slab

Thanks Cruise. That makes that the same concept of using a hammer jack? That's what a neighbor recommended. So, without a floor the door frame would allow for a seemless entry into the shed (without having to go over the frame. Conceptually I am a little challenged but I want to give this a go.


RE: Please help -- Building a shed over an existing concrete slab

I'll try and post pics of mine tomorrow. Maybe that will help.

RE: Please help -- Building a shed over an existing concrete slab

You could build you shed right on top of the concrete slab, just like they do with a house on a slab. You didn't say want you wanted to use this shed for but you might want to consider "the lay of the land". Sometimes when slabs are poured for sheds they aren't quit as high above grade as they should be. You don't want to find that the floor of your sheds ends up under water after a heavy rain.

To connect the base plate of the wall to the concrete youd need to use an expansion type anchor. There are a couple of variations a wedge type and a sleeve type but they all do the same thing expand in the hole to make a secure connection to the concrete. Generally speaking things like powder actuated fasteners and "Tapcon" type screws arent sufficient. A hammer drill makes drilling holes in concrete much easier. You can rent one for $20 or so. Any wood that comes in contact with the concrete must be pressure treated. Id also run a heavy bead of construction adhesive under the wall base plate to form a seal.

A "jack hammer" is generally used for destruction, not construction.

You didnt mention anything about permits. Many communities require permits for this kind of construction and you local building department can be a useful resource if youre less than sure of your carpentry skills. Certainly the inspector isnt going to help you drive nails but he can make sure you dont get into too much trouble.

Good luck!


RE: Please help -- Building a shed over an existing concrete slab


As a tip once you have the base plate and wall studs anchored to the concrete slab, put a bead of polyurethane caulk around the primeter between the base plate and concrete. This will seal the shed from water coming across the floor from underneath the base plate. Also, be sure and use treated lumber for that base plate. I used Tapcons to secure the base plate to the concrete slab at 16" intervals centered between the studs. Use the 1/4" Tapcons with at least a 2 1/4" length and the hex head so you can use a ratchet and socket to set them.


RE: Please help -- Building a shed over an existing concrete slab

Thank you very much for this advice. I had gotten the hammer drill advice (and offer to borrow one) from a neighbor but this is the first I've heard about the caulking. In Arlington, VA you can build up to 100 square feet without a permit. The slab is, if I recall correctly, about 11x8.

Now I need to find shed plans that actually make sense to a yahoo like me ;)


RE: Please help -- Building a shed over an existing concrete slab

Hi Andrew,

I just came across this discussion thread when searching for this information. I am looking to do the same thing. I'm guessing since this is a few years old that you might have some advice for how you went about building your shed. Let me know if you have any helpful tips.


RE: Please help -- Building a shed over an existing concrete slab

Sheds - the bigger the better..
I've seen many people with 2 and 3 sheds; one large one is better..
100 sq ft is tiny.. Mine is 12 by 24, and its full..
The use of chalking is an "overbuild", IMO, and its the best way of doing things..How much does chalk cost? In the long run, doing things the best possible way is the least expensive.

RE: Please help -- Building a shed over an existing concrete slab

what size shed are you building?
is there an existing foundation/concrete slab? if there is, outline your plan/drawing on the slab.
what size shed do you want?
if & where do you want windows.
do you need to have an unobstructed path thru the doorway? or can you have a 2x4 across the bottom of the doorway.
what type of door will you use?
do you get alot of snow?
what pitch do you want the roof (how steep)
are you going to have a bunch of shelves?
are you going to use the "attic" space?
these are some questions to ask yourself to help make your plan.

RE: Please help -- Building a shed over an existing concrete slab

Depending on the style of shed you're building, the size, and how you intend to use the shed, another thing to consider is using pre-built trusses for the roof. I built a 12X15 foot shed and ended up using pre-built trusses made at a nearby factory (instead of building all the rafters myself). I ended up paying about a 15% premium over what the materials alone would have cost me from Home Depot. But the convenience was incredible. I was able to build the entire shed in a day and a half (not including the cement foundation). If I had cut all of the rafters myself, the roof alone would have taken two days, although I will readily admit that the angles involved in custom rafter construction can baffle me.

One bad thing about pre-built trusses is that you might not end up with any attic space. This depends on the specifics of the truss (I actually have minimal amount of usable storage).

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