# New shop framing



## MalcolmLaurel (Dec 15, 2013)

I'm looking for advice on the new shop I'm planning to build this spring. Currently my shop is a 20×20 2 car garage, but space is shared with metalworking, car repairs (not that there's any room to get a car in right now!), flying stuff, and woodworking. Right now the woodworking stuff is maybe ¼ of the total space. My plan is to extend the back of the garage another 12', possibly a bit more, retaining the back wall (with a door in it, of course) to make a separate room for the wood shop (grease and sawdust don't mix!).

The big unknown right now is the floor. The garage has a slab floor, of course, but I can't afford to pour a slab for the new shop, so my current plan is to put in three sonotubes with a footer beam to support the back wall of the new shop, running the floor joists across to the existing garage back wall. That seems straightforward enough, and I don't mind having a step up from the garage to the wood shop. This has the added advantage of allowing me to insulate the floor.

What I don't know, right now (and why I'm soliciting advice) is how far above the ground I have to put the floor. Is 6" enough? Too high and I lose ceiling height, too close and I worry about ground moisture and debris rotting the floor. I'll probably go with scissor trusses for the roof or just move the rafter ties up to mid span on the sloped roof, which will be an extension of the current garage roof. I figure there should be a vapor barrier under the joists (I may go pressure treated for good measure), and screening to prevent leaves and debris from accumulating underneath.


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## Iguana (Jun 22, 2011)

I'd suggest that a slab would be your best choice, given the structural requirements of supporting machinery. For a 12' span, a 2×12 SPF#2 on 16" centers is rated to carry a total load of 100 lb/ft2. Deduct from that the dead load of the joists and subfloor, and you get something like 90 lb/ft2. That is right at the limit for carrying something like a cabinet saw.

Have you worked out the cost of concrete slab vs. wood floor? Assuming you don't have a lot of ground work to do, and do the prep, forming and finishing yourself, I think you might find they're very similar in cost.

Something else to consider - if your soil conditions are stable, prepare a gravel pad (4" deep) and lay 4×4 or 6×6 PT lumber at 16" centers directly on top of that. Your subfloor gets fastened to the PT lumber, with vapor barrier between. Deals with the structural issues, but that much PT lumber isn't cheap.

What you are doing will need a building permit. You should talk to your local permitting authority about what you want to do and they will be able to offer some suggestions as well as ensure you have something structurally sound.


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## MalcolmLaurel (Dec 15, 2013)

I don't need to support heavy machinery… I'll have a RAS built into a bench along the back wall, no table saw or cabinet saw or anything like that. Structurally it's very similar to the 12×30 screen porch addition I put on my house some years back, only this time it needs to be closer to the ground to give me a reasonable ceiling height. I don't think I can just support the floor on a gravel bed without going below the frost line, thus the sonotubes, but I had considered gravel underneath with the idea that it would allow PT joists to be closer to the ground. If I did a slab, as I understand it (not my area of expertise) I'd need a footing down to frost line around the perimeter, which is a substantial amount of work, and the cost (when I looked at it a couple of years ago) was more than I can afford. Of course I'll need a permit… as an engineer I'm not worried about designing a structurally sound wooden structure, but I'm not as clear on what's acceptable for a wooden floor in close proximity to the soil.


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## Whiskers (Nov 6, 2012)

The footing is not as hard work as you would think, and is a easy do it yourself project. You can rent a ditch witch to dig it out and get it done on a weekend. Most rental places like weekend warriors cause most of their rentals are to professionals during the week, which means they have all this unrented equipment all weekend. Since they are not open on Sunday a homeowner basically gets 2 days of rent for the price of one. At that point concrete is cheaper to get from a company who specializes in that, So your work will be done. Just make sure you locate any plumbing or septic tank lines if you have any before you start. Around here, the easiest way to find septic lines is in August. Just stop watering the grass for a couple weeks and than walk out and see where it is still green and needs mowing.


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## Racer2007 (Jan 13, 2011)

check your Local codes and don't forget the Permit.


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## dhazelton (Feb 11, 2012)

If you use material rated for ground contact I don't see why you couldn't do it exactly as you described. Only thing which could be an issue down the road is fasteners corroding and you won't have access to address them. Just make sure you have adequate drainage away from the building so moisture doesn't pool under there and keep things well ventilated. Essentially you're making an enclosed porch addition is all.


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## MalcolmLaurel (Dec 15, 2013)

I do intend to run my plans by the local building inspector first to avoid any unpleasant surprises.

Just to see, I asked a local contractor for a ballpark price on just a slab and footers… and nearly fainted. $9485 for a 12×20! Guess that option's out. I don't want to do that level of concrete work myself, so assuming the building inspector has no objections, it's going to be a board floor.


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## crank49 (Apr 7, 2010)

I just framed up my new shop during the recent warm spell here in Tennessee. I think it was -8 degrees the day we framed up the floor joists.;^)
Anyway, My site was sloped so the bottom of my joists are 6" off the ground on the closest side and about 24" on the highest side. My shop is 16' x 24' with the joists running the short way. There is a beam down the middle so my joists are 8 feet long and I put them on 12 inch centers. I used 2×10s, pressure treated for the joists and the floor is a special treated 3/4" T&G OSB with a 50 year warranty.

I ran Tyvek across the tops of the floor joists with draped pockets to hold the insulation between the joists. This way the insulation is protected from varmints and moisture from below. Tyvek will allow the insulation to breath, but blocks moisture from rising from the ground. I installed the insulation batts with the vapor barrier up, to the floor side. I proposed this system to an insulation specialist who I have known to be highly respected in this area for 40 years. He said it was the best way to do this. After everything was framed up we installed metal under-pinning around the building and installed 4 crawl space vents to keep moisture from being trapped under the floor and causing mold.


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## MalcolmLaurel (Dec 15, 2013)

Thanks Michael, sounds similar to what I'm planning. Is yours on a perimeter foundation?


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## crank49 (Apr 7, 2010)

No. The yard where I put the shop has about 12" of dirt on top of limestone bedrock. Sloped bed rock. 
That's why I had to go to a wood floor. I would have had a concrete floor if I could have dug out a level spot. 
I had to dig a hole every 8 ft. (12 holes total) and set a 6×6 pressure treated post in each hole and pour concrete around it. I drilled into the rock and set rebar in anchor cement (Pour-Rock) in the corners.
And about 3 of the holes I was able to get 18 inches deep by busting out some rock.
Also, I cut a 1 1/2" by 11" rabbet into the top of each post so the 2×12 band sits in a full lap joint that is lagged together.
I'm not finished of course. It's framed, sheeted, roofed and trimmed out. The outside looks finished, but the inside is just open studs and trusses. Got to wire it next, then insulate and finally I'll cover the inside walls with 7/16 OSB or 3/4" plywood. Depends on what I can get on sale.
Here's a picture before we got the double doors on and put the Hardie plank siding on the end.


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## MalcolmLaurel (Dec 15, 2013)

Oh, OK, when you talked about venting the crawl space I thought it was enclosed by a foundation. What did you use for skirting between the building and the ground?


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## mojapitt (Dec 31, 2011)

Have you checked into a steel building? As I prepare for a new shop, it's starting to look like my best bet. Just a thought.


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## crank49 (Apr 7, 2010)

That skirting is painted, galvalume steel, pretty much the same stuff used as siding or roofing for steel buildings.


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## BillWhite (Jul 23, 2007)

Be SURE to use at least 12 mil plastic on the ground. Anything less than that will rot pretty quickly. Weight it down in spots with some sand to keep it from flopping around.
Bill


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## MalcolmLaurel (Dec 15, 2013)

Monte, a steel building wouldn't work for me. I'm adding on to an existing wood structure, and steel buildings tend to sweat as they heat and cool (my shop is only heated when I'm in it). Also part of the project is moving the electric service entrance from the back wall of the garage to the back wall of the addition, with half of the work being done close to the existing wires before I move them… wouldn't want to be working with steel there.

Bill, I'm not so sure about putting plastic on the ground… under the insulation, yes, but I would think putting it directly on the ground would lead to moisture buildup on top of the plastic.


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## dhazelton (Feb 11, 2012)

If you have space to crawl under you could just shoot high density foam onto the flooring. The closed cell foam effectively is a vapor barrier at 1 inch and no moisture will migrate through it.


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## MalcolmLaurel (Dec 15, 2013)

No space to crawl… I need to put it as close to the ground as possible to get ceiling height.


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## REO (Sep 20, 2012)

6 mill poly is enough and code. you do not want to go direct contact with joists on the ground if you are on frost footers but you don,t need much clearance. a 50 psf floor will suffice even with a heavy cabinet saw(600 lbs) you arent stacking bricks to maximum on each square foot. an area 6 feet x 2 feet is covered with the saw so no additional weight could be added to the joists in that area anyway. shorten the spans, add more frost footers and match the joists with the beam dimensions. use joist hangers instead of running the joists over the top of the beams. the hardware will last for many years if you want an extra layer of protection buy a couple cans of cold galvanizing and coat all the parts a second time. for the roof you cannot support the new trusses or rafters anywhere along the span of the existing truss alone! they are not designed for the point load. You will need to incorporate a support that carries the load of the new roof at the wall. you can still tie into the old because the old roof will not see any additional load in this way.


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## crank49 (Apr 7, 2010)

If you put the Tyvek in there, like I did, you don't need the plastic.
Plastic under the insulation puts the vapor barrier on the cold side of the insulation and will sweat and hold moisture in the insulation. Not good.

I thought about this a long time and that's why I went to an expert for advice.

Starting from the bottom:
Dirt, then at least some air space vented to outside, then Tyvek, then insulation, then vapor barrier, and finally floor.


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## Elksniffer (Feb 5, 2009)

I think I would look very hard at a monolithic slab construction. Very simple for you to form with 2 X 10 or 2 X 12 on three sides, you only have to dig the perimeter down a little more than a foot, and it will match your existing floor level. It is small enough that you can do the screeing and finishing yourself with maybe a friends help. Material wise you are looking at around 5 yds of concrete and 2 0r 3 yds of gravel. Around here that would be a total cost of less than $700. The monolithic slab design prevents water (or drains it away) from getting under the slab and causing the frost heaving. A 2×4 scree board and a finishing trowel would be enough tools to get the job done.


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## MalcolmLaurel (Dec 15, 2013)

I don't think you can hook a monolithic slab to another building that has a conventional foundation. Also the water table is pretty shallow here, so I don't think I'd have good results without going down below frost line.


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## crank49 (Apr 7, 2010)

Malcolm said, "I don't think you can hook a monolithic slab to another building that has a conventional foundation"

Wouldn't a framed house siting on a regular foundation with an attached garage with a slab floor be an example of that? I'm not trying to be a wiseass, I'm just curious because I did not know there was a problem with that.

In fact, I'm remodeling a house where I am adding a bedroom on a slab floor to a house with regular framed construction. I'm having to do that because of the slope of the grade and shallow possibilities of digging a footer where the bedrock is less than 12" below grade.


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## MalcolmLaurel (Dec 15, 2013)

You can have a slab floor with a conventional footer under the walls, which is different from a monolithic slab with no footer. I assume that's how houses with attached garages are built, but this isn't my area of expertise.

I would think if you have bedrock 12" below grade, you'd just run the footer and/or the slab down to the bedrock. That would be just as good as (or better than) a foundation below frost line.


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## MalcolmLaurel (Dec 15, 2013)

Everything's a compromise when you start getting into it. I started the layout today. With a structural ridge beam I can have decent ceiling height except along the side walls, but it makes a door on the side problematic as the studs are only 69" high. That's with the floor edge beam 6" above grade. I could put the door in the back wall, but I planned a full length bench there, with the RAS in the middle. Perhaps I can make a hinged section of the bench to access the door… I won't be using it much; there will be a door into the existing garage. I'm also planning largish windows at each end of the bench that I can bring long stock in through if necessary.


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## MalcolmLaurel (Dec 15, 2013)

I talked to the building inspector, he tells me untreated wood must be 18" above grade but pressure treated can be as close as I want, so 6" it is. Since this topic is mostly about the foundation decision I broke further discussion off into a new topic.


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## bbrewer (Dec 28, 2012)

Malcolm if you do the structural ridge you could do a side door by adding a dormer or perpendicular peak. It would also break up the exterior view of the roof line. All you would need to do is double up a couple of the rafters and cripple the ones in between with a header. I think the structural ridge and dormer also gives a good look on the inside.


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