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I've read through just about everything here on the shop builds and the time has come to add onto an existing shop to house my forge.  The addition would be 12 ft wide by 20 ft long.  I'm adding onto an existing 1960's single wide trailer that already serves as a flintknapping shop.  It has electric, working bathroom, and an oil furnace for heat.  

My thoughts were, why recreate the wheel?  If I build a stand alone forge I've got to run electric to it and heat it (well heating is nice but not essential) and hey, a working toilet is nice to have nearbye.  With an addition, I can just extend heating ducts from the trailer into the addition to keep it at a decent temp in there all winter.  I keep the shop at 45 degrees when not in use, so that's plenty to prevent tool sweat when I start the forge up.

Here's what I need help with.  If I pour a concrete slab, I'm thinking a 4 inch slab will be thick enough if I have 6-8 inch depth on the load bearing section.  Think that's thick enough?  The single wide has a stand alone roof build over the old roof, so connecting off of that roof is not a problem as the support beams for that room go down below the frost line and do not bear any load on the original trailer frame.

Any ideas on how to avoid the cost of a concrete slab?  I'm afraid critters will just burrow under a brick floor for warmth and over time the floor would suffer.  Same with a dirt floor.  

Everything else I've got pretty well worked out.  I know the IFI family must have some guys out there with contractor experience that might be willing to give an opinion.  I'd sure appreciate a point in the right direction.

 

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Load bearing?  Like building stud walls on the outside edge of the slab?  If so, you need a footer below the frost line or else your walls / roof and everything else will warp as the frost heaves.

Cheapest/easiest would probably be a pole barn style addition.  Then slab the floor.

I poured a 24' x 32' x 4" slab for my pole barn shop.  It was $1600 for the concrete (with fiber in the mix for strength) and a 5gal bucket of sealer.  For your size it shouldn't be more than $800 unless they hit with a short load fee.

 

Also.  My house is a 14' x 60' mobile home with a 12' x 60' addition on one side and a 8' x 60' porch on the other.  The addition is 6"x6" larch poles with a crawlspace under regular style floor joists, and stud walls build between the poles and on top of the floor.   The roof is built over the entire structure, using the poles floor rafter support.

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RobS - My thinking is a module type slab like what is done for garages (which are not below the frost line).  The module type slabs have an 18 inch footer that would be on the load bearing walls.  In my case, the roof would extend out from the existing roof line so the load would be carried by wall that the roof rafters terminate on (I think).  The side walls would not be bearing any load other than the weight of wall its self, correct? 

Steve Sells, I've decided to call a contractor to come up and give me an estimate, so when he's here I'll pick his brain on the foundation part and code.  I can do the concrete work, but I need it to be right and I believe code would allow me to have the module foundation on something that isn't a living space.  I might get away with only a 4 inch thick slab this way, but the last thing I need is for code to tell me it's not right and have to tear things apart.  I do live in the sticks and could probably do this without a permit if I did it myself since this is nothing but an outbuilding, but code exists for a reason and sometimes those reasons make a difference 5 years down the road when you notice something is wrong.  I like to do things right or not do them at all, but I'm also as cheap as the next guy and don't want to sink a fortune into a 12 x 20 building.

Another question I had is if anyone has insulated under their concrete floors with 2 inch polystyrene sheets?  My worry is that it's not going to make that much of a difference, but I've only worked on slabs that were not insulated.

 

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Good point Steve.  I rake my roof so I tend to forget how much the weight of snow can be.  My problem is I know enough to be dangerous, so that's why I'm calling someone in that knows more about it.  I guess I see so many quick and dirty forging shops built on here for cheap and its hard to see the bottom line of doing things the right way.   It will be better in the long run though.  Having heat and a properly built shop will pay its dividends when I can work all winter long on forging orders. 

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We put down a plastic sheet for a vapor barrier under the shop floor.  I don't have first hand experience, but all the guys there said it does stop the floor from sweating.  I did not do foam insulation, but again they all said it does help keep the building noticeably warmer.  Again no first hand experience, just going on what these guys said.  

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