# Picking a foundation for shed/simple workshop



## Corey in NE (Jan 31, 2010)

I'm building a shed/simple workshop (12'x16' or 16'x16') this spring, (I am in New England). The sight is slightly sloped and does not have a problem with standing water. We have no zoning rules here so that is not an issue. I am struggling to pick a foundation. I'm deciding between these basic options:

1. Concrete blocks at the corners and in the middle of the spans. 
2. Concrete piers sunk ~4' in the ground using sonotubes and maybe bigfoot forms
3. Concrete pad

Option #1 has a lot of appeal due to its cost and simplicity. (I have an old outbuilding that was built on stacked stone and seems to have withstood the test of time.) Option #2 seems more rugged than #1, but is the extra effort worth it? Option #3 seems by far the most robust, but again is the extra effort worth it? 

The building will be used mostly for storage and occassionally simple carpentry projects. 

Thanks for any advice.


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## Scuba_Dave (Jan 16, 2009)

My pool cabana has a pad on the pre-existing part
I opted to pour a perimeter "foundation" 12" deep
Then I'm filling the area & putting down pavers

Main reason was to be able to pull the pavers up to access the pipes to & from the filter & pump
And I did not want critters using the space under a wood floor as a home


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## vsheetz (Sep 28, 2008)

I would suggest to put in on a concrete pad - and make the building as large as is reasonable while you are doing it. This makes for the best use experience, longest lasting, and most value add to the property.


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## Bondo (Dec 8, 2007)

Ayuh,... I'd do a floating slab,... 12" deep x 12" wide footers, to a 4" slab...

That way, frost shouldn't be an issue,+ you'll have a "Nice" floor surface to work or store on...


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## Corey in NE (Jan 31, 2010)

Does a floating slab mean it is not connected to the footers? I've read about putting in slabs and it shows how to do the footers but it always seems to be one piece.


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## jlhaslip (Dec 31, 2009)

A floating slab usually has a thicken edge which acts as a footing to carry the building's weight.


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## cellophane (Sep 29, 2009)

http://home.howstuffworks.com/home-improvement/repair/house4.htm

A nice diagram there on how a floating slab works.


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## RegeSullivan (Dec 8, 2006)

If you do a floating pad be sure to put a vapor barrier under the slab. If you don't you may have problems with rusting tools. I would prefer a wood frame floor for a shop if you will be using it daily. That said, the slab would be less maintenance and less of a problem with varmints.

Rege


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## COLDIRON (Mar 15, 2009)

I have built many sheds and storage buildings for myself and relatives.

Just level off the ground and put in a 4" slab on grade , before the concrete starts to cure put hold down bolts around the perimeter, and be sure to use pressure treated lumber for your bottom plates.

Don't worry about frost if you are not connecting the shed to an existing building it will float by itself, " Keep it Simple"


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## Corey in NE (Jan 31, 2010)

Thanks for all of the good advice. It seem the consensus is to go with a concrete pad with footers. We have plenty of critters here so that would be a big concern if I built on blocks. 

Now since the site is on a slope (unfortunately I don't know the grade), what is the best approach? Do I have to excavate the uphill side to make it level with the downhill side or can I do a sort of wedge to save myself some exacating?


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## vsheetz (Sep 28, 2008)

Corey in NE said:


> Thanks for all of the good advice. It seem the consensus is to go with a concrete pad with footers. We have plenty of critters here so that would be a big concern if I built on blocks.
> 
> Now since the site is on a slope (unfortunately I don't know the grade), what is the best approach? Do I have to excavate the uphill side to make it level with the downhill side or can I do a sort of wedge to save myself some exacating?


I would investigate to do a stem wall. You need to understand the grade to see how high the stem wall needs to be.

Here is a relavent thread:
http://www.diychatroom.com/f19/concrete-footings-detached-garage-61300/


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## jlhaslip (Dec 31, 2009)

You could add some fill to the low end and compact the bejeeses out of it.
If you do that, consider that the weight from the slab will require the dirt to be installed in 8" 'lifts' and compaction applied at each level.
Use good clean fill, not the topsoil or what you excavate for the slab. Gravel or road crush would be good.

I used 22 loads of pit run to level off my 24 x 26 shop and a 5 ft vibratory packer. I had to come up 6 feet at the back end and have a 4 ft wide walkway around the shop. The gravel also needs to slope away at its 'angle of repose, which means however it wants... The force of the slab will travel down at a 45 degree angle, and the gravel needs to placed and compacted so that the force is travelling along that slope line.
Do you need/want a picture?


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## bmancanfly (Mar 3, 2009)

No zoning codes! Sounds like heaven. Do you mind if I ask where you are? I would love to be able to build a cottage without dealing with the zoning man.


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## minidriver8 (Feb 2, 2010)

ever consider a pole barn type of construction? cheap and easy to build. can either have concrete slab on a level site or on a slope you can frame the floor with wood joist and decking which keeps the moisture at bay. wood floor is 1/4 the cost of concrete/finishing.


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## Corey in NE (Jan 31, 2010)

This covers a few people's comments: 

"No zoning codes! Sounds like heaven. Do you mind if I ask where you are? I would love to be able to build a cottage without dealing with the zoning man."

I live in a small town (< 1000 people) in Orange County, Vermont. The only real town zoning/permitting we have is when people put in driveways since those connect to roads.

"ever consider a pole barn type of construction? cheap and easy to build. can either have concrete slab on a level site or on a slope you can frame the floor with wood joist and decking which keeps the moisture at bay. wood floor is 1/4 the cost of concrete/finishing."

This was my original plan and I even bought a book on it and actually read it. I got bogged down a bit in the load calculations (partially me over thinking it, partially the book's poor explanation). One thing I didn't understand was if a concrete slab is used, how is that different than traditional construction? The framing and such seemed to be the same.

"You could add some fill to the low end and compact the bejeeses out of it. If you do that, consider that the weight from the slab will require the dirt to be installed in 8" 'lifts' and compaction applied at each level.
Do you need/want a picture?"

I think I can picture what you are saying and it makes sense. 

I did a rough check of my yard's slope (there's snow on the ground so it was very rough), and it seems to be well below 5 degrees. For the size building I am thinking of, this puts the extra excavating on the uphill side at less than 1 ft.


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## minidriver8 (Feb 2, 2010)

the pole barn exists as a post and beam type of construction with a floating slab usually placed an minimum of 8 inches of gravel. as the ground heaves, the slab moves independently from the structure which is resting 4' below the surface where frost is less of an issue. well....i'm guessing up in vermont, your frost depth is around 4 feet or deeper. here in pittsburgh pa, we sink poles to 4' below grade. it does two things here, one is frost and the second is added stability for wind.

not sure what you mean by load calcs.....roof? i'd go with engineered trusses for the roof and they rest on one 2x12's on either side of the top of the poles at the desired soffit height. the posts are 8' o.c. so the roof loads are minimal depending upon the size of the building. the only problem i could see with the pole building in vermont is the type of soil you're going to run into below grade. if you have huge rocks hiding under the surface, you're going to be in for a nightmare when it comes to drilling the post holes.

traditional construction and a footing that is dug by a machine is expensive but eliminates the problems of rocky soil.

ultimately the building's life and useability depends upon a good foundation resting on virgin soil that limits it amount of freeze-thaw. without a deep enough footing, you'd essencially be wasting your time and the best thing would be to put it right at grade on gravel. but if you need the building to stay put and not have differential movement then you'll need a good, deep footing whether it's using poles or a traditionally built block foundation at required frost prevention depth.

just an aside here in pittsburgh in the 70's we experienced frost at 54" below grade. our codes at the time required a minimum 30" deep footing, now they require 36" deep footings.


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## minidriver8 (Feb 2, 2010)

true, it is a simple shed that he wants. the concrete slab alone is going to cost 1100 bucks or more all by itself. the entire pole barn on a slope with no grading, no gravel, an elevated wooden floor above any moisture could be built by one person for not much more than the cost of the slab. if the ground is conducive to digging(as mentioned above). if not, he could level the ground throw down 6-8 of gravel and build a platform out of treated ties then dimensional lumber deck and go up from there "if he doesn't mind the shed moving". these are just options for a DIYer to decide between cost and function and amount of time they want to spend on a project. the simple way is to find a company who builds these sheds and will deliver it for 5 grand. they show up with a rolloff hook a chain around the shed hook the chain to a tree and drive away, the shed falls off the rolloff onto a pre leveled gravel base. the only do it yourselfing in this option is to pick up the phonebook and order one, and this is a completely acceptable option.


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## Corey in NE (Jan 31, 2010)

_


minidriver8 said:



i'm guessing up in vermont, your frost depth is around 4 feet or deeper. here in pittsburgh pa, we sink poles to 4' below grade. it does two things here, one is frost and the second is added stability for wind.

Click to expand...

_


minidriver8 said:


> _not sure what you mean by load calcs.....roof? i'd go with engineered trusses for the roof and they rest on one 2x12's on either side of the top of the poles at the desired soffit height. the posts are 8' o.c. so the roof loads are minimal depending upon the size of the building. the only problem i could see with the pole building in vermont is the type of soil you're going to run into below grade. if you have huge rocks hiding under the surface, you're going to be in for a nightmare when it comes to drilling the post holes. _




4' sounds right for up here. There is a serious ledge where I live but I am not sure how deep it is. I have dug 2' holes in the past and not encountered a problem. When I was first considering a pole building I spoke with a co-worker who has done construction in the past. He was the one who suggested using big foot forms and sonotubes to make piers instead of trying to sink the poles in the ground themselves (see 1st post).

The load calculations I got hung up on were for the decking suspended between the poles. Basically, trying to calculate the number of poles per side of the building. I was probably over complicating when I was trying to figure it out. 

Part of the point of doing this project (besides needing a small out building) is for the experience. However, the more I think about it, if I do a concrete pad, I will seriously look into hiring someone to do that part for me.


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## Daniel Holzman (Mar 10, 2009)

So back to the shed.... You are not building a house, barn etc., so foundation types suitable for a house or barn are simply not applicable here. You are almost certainly not too concerned about frost heave, since the worst that happens is the floor gets a little out of level, or the concrete cracks. So what, its a shed.

I have a 20 ft x 10 foot shed in my backyard built in 1959, still in excellent condition. The foundation consists of a row of fieldstone approximately 18 inches deep and 18 inches wide. There is a sill beam around the perimeter, a 6x6, not even pressure treated. The floor is dirt on the left half of the building, and wood 1x6 on the right side (where we store a few tools). I added some gravel on the left side so that the firewood I store in there would not get wet.

The walls are conventional stud walls, the roof is standard asphalt. This is NOT a habitable building, but it works great for firewood storage, tools, and the occasiona odd project like cutting shingles etc. Did I mention, this building could probably be constructed today for a few thousand dollars, which includes the electrical service?

If you go with a slab that is 4 inches thick, for every 80 square feet of floor area, you are going to purchase a cubic yard of concrete. If you have it delivered, and finished by a professional, it will cost you something like $300 per yard in place (maybe less in rural Vermont). That means the floor alone for a 200 square foot building is going to run close to a grand. For a 10 x 20 foot building, you have approximately 60 running feet of footers. If you go with 4 foot deep footers, 8 inches wide, you are going to use 2.67 cubic feet of concrete per running foot of footer, or about 150 cubic feet (over 5 yards) of concrete just for the footers. That is close to $1500 in place where I live.

You can save all the money for concrete by placing a thin floating slab (4 inches), and use a single line of concrete block around the edge, or skip the concrete floor completely and just place 3 or 4 inches of crushed stone. IT ALL DEPENDS ON WHAT YOU ARE GOING TO DO WITH THE BUILDING. I agree with Sultini, anything beyond the 4 inch slab with a block wall is overkill, unless you are planning to eventually convert this to habitable space. Then it is a totally different ballgame.


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## hacksepe (Sep 11, 2009)

For a concrete slab, I'd recommend you use some steel reinforcing. Sheets of metal grid with about 8 inch square holes is sold at big box stores. Place these close to the top of your surface by supporting them with bricks or stones before pouring. This will ensure your slab does not crack. My shed cost $3000 of which $400 was the cost of getting a concrete truck (and pizza for a few friends). Well worth the extra effort and minimal cost.


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## jomama45 (Nov 13, 2008)

This isn't just a tiny shed, maybe more like a small garage.

Either way, I'd put in the 4" slab on grade with a thickened edge, maybe a total of 8" thick around the outside perimeter. Leave the outside 8-12" thicker, than taper up to the 4". There is no way I could ever install this for someone (and sleep at night) w/o runnig 1 or 2 1/2" rebar continuous around the outside perimeter.


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## Daniel Holzman (Mar 10, 2009)

Reinforcing. This is possibly one of the most misunderstood topics on this forum. Reinforcing normally refers to the use of steel bars (rebars) or steel mesh (usually referred to as welded wire fabric or wwf) to increase the tensile strength of concrete. The steel is not normally used to increase the compressive strength of concrete.

In order to increase the tensile strength of concrete, the steel must be placed on the TENSION side of the slab. In order to avoid corrosion of the steel, the minimum cover typically specified is 1-1/2 inches, or 2 inches if the concrete is bedded in soil. This means that for a 4 inch thick slab, the steel must be placed approximately at the center of the slab, which happens to be the neutral axis. The neutral axis is under effectively zero stress, therefore steel placed at the neutral axis has NO EFFECT on the structural strength of the concrete, hence is useless in improving the tension capacity of the slab.

So if that is the case, why bother putting steel in at all? Well, a lot of slabs are in fact built with no steel, and if they are properly built and cured, they do fine. The main purpose of the steel in a four inch thick slab is to reduce cracking due to temperature changes in the slab, which can induce cracking due to thermal expansion of the concrete. A fair number of experts think that steel is not very effective in this regard, because most concrete slabs are never exposed to significant temperature variations (i.e. slabs for a shed which are underneath a building, and exposed to soil on the bottom). However, steel reinforcing is useful for slabs that are exposed to temperature fluctuations such as parking lots.

For slabs at least 6 inches in thickness, it is possible to place the steel reinforcing on the tension side, and increase the strength of the slab. This is a structural use of steel, not simply a temperature steel use. For concrete walls, structural steel is commonly used to increase the strength of the wall. This works well for walls that are 8 inches or thicker, which is common for houses and commercial projects. However, note that many homes are built with unreinforced walls, which is acceptable if the loading on the wall does not put significant tension on the wall.

Conclusion: For the shed project under discussion, inclusion of steel reinforcing is unnecessary, and is unlikely to improve the performance of the end product. If the OPS is concerned about cracking of the slab, a far more important issue is proper preparation of the subgrade, proper placement of the concrete, good curing practices for the concrete, and inclusion of an adequate number of control joints in the slab. If the OPS decides that they wish to include a thickened perimeter due to concerns about frost heave, they may wish to consider installation of bars in the perimeter only. If it were my shed, I would put in a crushed stone floor and a concrete block perimeter foundation, and use the money I saved for a nice vacation.


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## COLDIRON (Mar 15, 2009)

Dan let me say I admire you , over the last 40 years or so working with engineers of all disciplines I have never met one Engineer with knowledge like yours or the common sense that you have , If I would have I wouldn't be retired today I would have stayed.
Couldn't take it anymore.


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## jomama45 (Nov 13, 2008)

jomama45 said:


> This isn't just a tiny shed, maybe more like a small garage.
> 
> Either way, I'd put in the 4" slab on grade with a thickened edge, maybe a total of 8" thick around the outside perimeter. Leave the outside 8-12" thicker, than taper up to the 4". There is no way I could ever install this for someone (and sleep at night) w/o runnig 1 or 2 1/2" rebar continuous around the outside perimeter.





Daniel Holzman said:


> For slabs at least 6 inches in thickness, it is possible to place the steel reinforcing on the tension side, and increase the strength of the slab.
> 
> 
> *That's partially why I suggested an 8" total depth where the rod would be placed.*
> ...


 
Dan, I respectfully disagree with you on this one. I've replaced & trouble shot more than a few failed garage/shed slabs in my career. I can't even begin to tell you how many garages I've seen with inferior, failing slabs under them, no re-enforcement & no additional strength at the perimeter, which happens to be the only part of the slab with a permanent load on it.

I can't see saving a "few bucks" by not installing a thickened edge along with a few rebar. It's less than a yard of concrete & $50 for 2 continuous rebars. Not quite overkill IMO. Just because his locale doesn't require inspections doesn't give him good reasons to cut corners. Maybe your region doesn't get the frost that Vermont, or my region does for that matter, but a thickened edge w/rebar is required here for any outbuilding over 150sq. feet. It appears to be similar in other parts of Vermont as well:

http://www.ci.lakeville.mn.us/departments/departmentspdf/garage.pdf


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## Scuba_Dave (Jan 16, 2009)

A 16'x16 "shed" is pretty big & bigger then a lot of 1 car garages around here

If I had a floor poured that big & it cracked I'd be upset
Especially if it just took some metal to re-inforce
I stored my MC in my 10x12' shed until my garge was built
Many people store a ride on lawnmower in their shed

I have had lawn fertilizer & cement stacked up - 320lbs worth in one spot
And with 16x16 if it is accessible from the street it may be used as a garage one day
More then one back yard shed has been used to restore an old car

For the intended use nothing extreme is really needed:


> The building will be used mostly for storage and occassionally simple carpentry projects


Unless a lathe or other heavy machinery is going in


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## Daniel Holzman (Mar 10, 2009)

Cracking of concrete, and how to avoid it.

Concrete cracks after placement for a variety of reasons, including:

1. *Normal shrinkage*. During curing, concrete forms by chemically bonding water to silica and other materials in the cement. The concrete shrinks during curing, and shrinkage cracks develop. This is normal, and cannot be avoided unless specialized, expensive additives and special curing procedures (not normally used for home slabs) are used. *Solution*: Install control joints at recommended depth and spacing. This will NOT avoid cracking, but will direct the cracking into control joints where the cracks are not a problem. *Solution*: Use proper curing procedure for concrete. Concrete should be kept moist during curing, and out of direct sun. Also concrete should not be allowed to freeze during curing. The standard methods to insure this are to place moist burlap bags over the concrete, and if placing concrete in the winter, use heat if possible, or use special additives to make the concrete more freeze resistant during curing. Proper finishing of concrete is very important to avoid cracking. Most concrete will be delivered with air entrainment, this helps to minimize shrinkage cracking. *Solution*: Use welded wire fabric at approximately the neutral axis of the concrete to improve crack resistance. Unless the wwf is carefully placed and supported, this technique is ineffective. Even if placed carefully, wwf is less effective than the previously discussed techniques for crack control. *Solution*: Use fiber reinforced concrete. Concrete can be ordered with steel or plastic fibers designed to improve tension strength and reduce cracking. This is claimed to be as effective as steel reinforcing, and has the advantage of not requiring placement of steel. If you are really concerned about shrinkage cracks, and have done the first two steps, you can pay the extra money for fiber concrete and you may get improved results.

2. *Foundation issues*. Concrete placed on inferior soil such as soft clay, weak silt, loose sand, organic soil or garbage is prone to differential settlement. Differential settlement means that one part of the slab settles due to gravity a different amount than other parts of the slab, and this will inevitably lead to cracking. *Solution*: Make sure the foundation soil is suitable for the thickness of slab and imposed load on the slab. Remove inferior soil and backfill with structural fill such as medium to coarse sand, crushed stone, or mixed sand and stone. Avoid fine material in the fill such as silt or clay. Avoid all organic material in the fill. Make sure fill material is adequately compacted using a plate whacker or similar tool.

3. *Frost heave*. Frost heave is caused by a combination of three things, cold temperature, moisture, and frost susceptible soil. If your soil is free draining, meaning it is sand and gravel, you are not going to get frost heave, so you can have another beer and stop worrying. If your soil is silty or contains fine sand, it is prone to frost heave, so put that beer away and start thinking about solutions. If you insulate the slab, it may be possible to minimize frost heave, even in poor soils. If you direct all drainage water away from the slab, and you are sufficiently far above the water table, you can minimize frost heave. If you don't care to spend the time and money to deal with the frost issues, you can build your floor out of crushed stone and regrade the stone each year to level. The foundation will be subject to heave, and you can accept that your structure will be out of plumb and level. You can also choose to improve the soil underneath the perimeter foundation only by removing frost susceptible soil under the perimeter and replacing with crushed stone down to frost level, ledge, or granular soil, whichever comes first. The last approach would be my choice, simple, inexpensive.


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## jomama45 (Nov 13, 2008)

Daniel Holzman said:


> You can also choose to improve the soil underneath the perimeter foundation only by removing frost susceptible soil under the perimeter and replacing with crushed stone down to frost level, ledge, or granular soil, whichever comes first. The last approach would be my choice, simple, inexpensive.


 
IMO, this is still far more work and expense than simply installing a thickened edge with a little rebar.

Probably would cost an additional $125-150 for a 16'x16' shed.


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## stuart45 (Jun 20, 2009)

Scuba_Dave said:


> A 16'x16 "shed" is pretty big & bigger then a lot of 1 car garages around here


It's bigger than a lot of the houses around here.


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## COLDIRON (Mar 15, 2009)

He said 12x 16 or 16 x16 it's still not a garage and a car should not be driven in there lets remember he said shed.

Another thing have any of you people seen a crack in concrete?? I have and it's not the end of the world even a couple cracks who cares it's a SHED. Op hope you gathered enough info as to what you will be placing for your foundation.


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## Corey in NE (Jan 31, 2010)

It looks like I opened a can of worms here. I'll try to summarize everything:

One of my primary reasons for the posting was that I was paranoid about what could happen to the structure because of the cold. From everyone's input and my own observations of some old out buildings on my property, it seems that my fear is unfounded: You can put a building on a concrete slab, stacked stone or concrete block and as long as there are no weird soil conditions (like poor drainage) it should be fine.

It seems to come down to a matter of cost, aesthics and projected use. I'm not a car guy and my planned location isn't really conducive for making the building a garage eventually. I also can see no reason that the building will ever be lived in. I do want it to be something that looks good, not feel jerry rigged and will last. 

The benefit of blocks is the cost and simplicity. There is (relatively) no excavation involved. Since the site is on a slight slope, it is easy to stack the blocks on the downhill side higher. Also the blocks make "offsetting" the building easy so that if I put in swinging double doors on the front they will clear the ground without additional landscaping abd putting in a ramp seems straightforward. The drawbacks seem to be it feels less rugged and permanent, having to guard against critters getting under the building, and anchoring the building (I've seen some warnings about wind causing problems).

The benefit of concrete is its ruggedness, no risk of critters, it's easy to anchor the building and if for some reason I do want to put in heavy equipment, I wouldn't need to reinforce the floor. The drawbacks are material cost and the need for excavation. If I don't excavate, I would be trucking in and compacting a significant amount of fill. 

Except for the question of concrete reinforcement, does this pretty much cover it?


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## Scuba_Dave (Jan 16, 2009)

Corey in NE said:


> Except for the question of concrete reinforcement, does this pretty much cover it?


Yup, that covers it
All comes down to budget & what you want to use the structure for


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