Shed foundation

[clemnd86]

I am planning to build a shed for my BX 2200. I was planning on building a 12' X 20' gambrel shed with a second floor used for light storage. I was planning to build a stick built building on a 6" concrete pad. The concrete pad would be on 4" of pea stone. The material below that would be native gravel. I live in Massachusetts near Cape Cod. Do you think the pad will provide adequate support?

 

[EddieWalker]

A six inch pad is overkill in my area, but might be worht the extra expense in your area. It sure couldn't hurt.

I like half inch rebar on 2 ft centers, but in extreme conditions, 1 ft centers is better, and 5/8 is even stonger.

From what I understand, your footings should be below the frost line. Since I don't have a frost line here in East Texas, we just dig down a shovel depth and douple up the rebar.

Your dimensions are pretty tight. 12 ft is wide enough for your tractor, but that's about it. Are you sure you want to go to the trouble to build the building, and only make it that small? 16ft would be better, with 20 ft being allot better.

The price to build bigger is only a small addition to the overall project. Big enough never is, and too big really does't exist.

As for the gambrel roof design. I'm not a big fan. The extra space is nice, but when I looked at building one, I realized how much extra work was involved for minimum return.

A rule of thumb to remember is that it costs about 20% more to build a second story.

Eddie

 

[beenthere]

Welcome to the forum.
Look at the great pictures and thread by dholly for a shed slab that should work real well for you. There are others who have also shown their slabs, that I know as 'floating slabs' for areas that have frost and don't want to go to below that frost for footings.

See
this thread

 

[Hagar]

That's basically what I did in central Mass. Only the outside perimeter is about 10-12" thick and then 4-6" pad for the rest. I used screen suspended on concrete bricks to keep it up. Its been 10 years and no cracks.

 

[8561]

I'm getting ready to put in a floating slab right now for an addition to my garage. A friend of mine has an entire detached garage built on a floating slab. Contact a concrete floor contractor and talk to him about doing it.

 

[dholly]

clemnd86 - I see no reason why you can't put a shed of that size and design on a slab assuming it meets local code. Have you discussed your foundation design with the local building inspector? Even if you can do away with full footers below the frost line, many municpalities will not allow a monolithic slab of equal thickness. My town requires that, among other things, the outer edge of the slab be significantly thicker than the middle. This design feature may help minimize side-to-side movement while still allowing up-and-down frost heave. The foundation preparation pics I posted recently in THIS THREAD for my own 24'x34' garage project may be of interest. Good luck.

 

[JJT]

I'm further north than you, so what worked for me should work for you. About 15 years ago I built a 16x16 shed with a pressure treated floor, 5 years ago I poured a 6" slab, (no thickened edges) and didn't get around to moving the shed for another 3 years. No cracks in 3 years and no movement in 5. I used 4000psi concrete and welded wire with a few pcs of #4 rebar. I scraped off several inches of top soil and poured the slab, (no stone under it).

Unless your local zoning requies it, I would skip the footers, but would probably thicken the edges.

 

[drm]

clemnd86,

I would definitely thicken the edge 12" deep by 6" min wide at the bottom. Run 2 #5 (5/8" dia.) rebar in the thickened edge, One 2" down from top & the other 3" up from bottom. Lap the rebar 30" & tie and provide 90 deg bent corner bars lapped 30" with the longitudinal bars & tied.

The interior could be as thin as 4" if your supporting soil is firm and has little to no clay. Use #4 (1/2” dia) rebar at 12” OC each way supported on concrete bricks & tied at each crossing. Locate 2” down from the top of the slab. 5” would be a better thickness and 6” would be great. The water to cement ratio is more important that the strength. It should be no more than 0.40. Add an air entraining admixture to help resist winter damage and a plasticicer to help workability of the concrete mixture. Strength should be at least 3,500 psi with 4,000 psi being the most common (but this is regional).

Compact the subbase (soil) and remove any soft spots. Place 4” of a well draining gravel and 6 mil vapor barrier under the slab. Locate the shed so that water runoff does not run under it or if not possible provide the gravel 12” wide around the outside to give the water a path away from the shed. Also provide a pipe or trench of gravel at the lowest point to keep the water moving away.

A good quality concrete contractor will do all of this without question. The "low bidder" will whine or just not do it. You get waht you pay for and your shed will be on this for some time. Of course be sure your local zoning/building dept. will permit this type of construction.

Good Luck…Derek