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Forum topic by MalcolmLaurel posted 174 days ago 972 views 0 times favorited 25 replies Add to Favorites Watch
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MalcolmLaurel

210 posts in 227 days


174 days ago

Topic tags/keywords: framing shop floor foundation

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.

-- Malcolm Laurel - http://MalcolmLaurel.com https://www.etsy.com/shop/MalcolmLaurel


25 replies so far

View Mark Kornell's profile

Mark Kornell

472 posts in 1134 days


#1 posted 174 days ago

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.

-- Mark Kornell, Kornell Wood Design

View MalcolmLaurel's profile

MalcolmLaurel

210 posts in 227 days


#2 posted 174 days ago

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.

-- Malcolm Laurel - http://MalcolmLaurel.com https://www.etsy.com/shop/MalcolmLaurel

View Whiskers's profile

Whiskers

389 posts in 631 days


#3 posted 174 days ago

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.

View Richard's profile

Richard

799 posts in 1294 days


#4 posted 173 days ago

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

View dhazelton's profile

dhazelton

1157 posts in 900 days


#5 posted 173 days ago

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.

View MalcolmLaurel's profile

MalcolmLaurel

210 posts in 227 days


#6 posted 173 days ago

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.

-- Malcolm Laurel - http://MalcolmLaurel.com https://www.etsy.com/shop/MalcolmLaurel

View crank49's profile

crank49

3343 posts in 1575 days


#7 posted 173 days ago

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.

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

View MalcolmLaurel's profile

MalcolmLaurel

210 posts in 227 days


#8 posted 173 days ago

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

-- Malcolm Laurel - http://MalcolmLaurel.com https://www.etsy.com/shop/MalcolmLaurel

View crank49's profile

crank49

3343 posts in 1575 days


#9 posted 173 days ago

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.

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

View MalcolmLaurel's profile

MalcolmLaurel

210 posts in 227 days


#10 posted 172 days ago

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?

-- Malcolm Laurel - http://MalcolmLaurel.com https://www.etsy.com/shop/MalcolmLaurel

View Monte Pittman's profile

Monte Pittman

13360 posts in 942 days


#11 posted 172 days ago

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.

-- Mother Nature created it, I just assemble it. - It's not ability that we often lack, but the patience to use our ability

View crank49's profile

crank49

3343 posts in 1575 days


#12 posted 172 days ago

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

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

View Bill White's profile

Bill White

3352 posts in 2564 days


#13 posted 172 days ago

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

-- bill@magraphics.us

View MalcolmLaurel's profile

MalcolmLaurel

210 posts in 227 days


#14 posted 172 days ago

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.

-- Malcolm Laurel - http://MalcolmLaurel.com https://www.etsy.com/shop/MalcolmLaurel

View dhazelton's profile

dhazelton

1157 posts in 900 days


#15 posted 172 days ago

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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