Floating slab for $10 sq ft. Reasonable for Minneapolis

[reader2580]

I just got a quote for a floating slab in a northern suburb of Minneapolis for $10.50 per square foot. Does anyone know if this is reasonable for the Minneapolis area? For a 1,440 sq ft slab I am looking at $15,000. It seems high, but I really don't know.

Due to the costs spiraling out of control on my original plans for a 40x60 stick built building with 16 foot sidewalls I am going back to the drawing board for a smaller 24x60 building using 12 foot sidewalls and scissor trusses to have a 14x14 door in the center. It has been suggested to do a floating slab instead of frost footings if I can. The other building required frost footings.

 

[e36jon]

I am guessing folks will want a bit more info before they can answer your question:

1. Slab thickness? (Or total yards of concrete.)
2. Grade of concrete? (What PSI are you specifying?)
3. Reinforcement? (Type and pattern of rebar / wire / fiber. Who's placing it?)
4. Any out of the ordinary geometry that would influence delivering the concrete? (Do you HAVE to have a pumper? Etc.)
5. What, in detail, are you expecting that contractor to do for his fee? (Coming in and placing concrete when everything else is done is different than having to do the vapor barrier, put the rebar in, etc...)

Having said all that, I expect to get spanked as folks on here REALLY know the scoop, but they'll appreciate the above at least.

Cheers,

Jon

 

[matt_i]

Dunno.

For a 25x40x6" deep slab (plus a little rectangle that connects total of 1100sf)
I paid $1500 cash to the finishers for the job + saw cutting.
22cuyd of concrete for somewhere areound $2600-ish
$1000 for the pump truck.

All the prep, compaction, stone base, rebar, chairs, wire-tying, forms/blockouts vapor barrier and curing sealer was done by me...so there is definitely money ******* in materials. Lets say it was another $1500.

I was in around $6/sf, 5 years ago. So if you can get all of the other work turnkey for $10.5/sqft you're probably doing well.

I will add I think its a terrible gamble to build without a frost protected foundation and one that I would personally never take. If your soil gets mushy or heaves you scrap 100% of your investment in the whole thing and it will then cost 200% of original to re-do it correctly, somewhere equivalent to a fire burning it down minus the loss of the contents.

So how do you get an inexpensive frost protected foundation? In your other thread about same subject I proposed a trenched/earth-formed foundation down to frost depth which is one that a motivated person with a plan could complete inside of a week, working mostly alone (rental equipment, need spotter for laser level to save time, etc), with flat ground.

 

[andyvh1959]

My 24x28 4" thick locked edge slab with wire reinforcing, 6ml vapor barrier and 2" foam insulation cost me $4500 in Green Bay this past October (4500 psi). That figures to $6.67 per sq ft. I also spent $1500 to another contractor to prep the site and haul in four large trailer loads of recycled asphalt fill, so that gets me to $8.93 per sq ft.

If your contractor is doing all the site prep and fill required, the cost is close. But if your slab quote doesn't include a vapor barrier and 2" foam insulation I'd counter with a lower offer or find another contractor.

Up here in east central WI we're right in line with you in Minneapolis zone wise. A locked edge style slab is very common here for a detached garage. I've done three of them over 30+ years and not had any issues other than perhaps normal cracking. Largest one was a 28.5 x 48 locked edge slab, that was also pinned (rebar) into the existing 22.5 x 24 slab. I used the old garage and built onto it. No issues with that one either. Fill and site prep are key to being certain you have a stable base under the new slab. Though, I have not done a floating lock edge slab under a garage the size you propose.

 

[reader2580]

The price quoted was the contractor doing absolutely everything including some minor site prep. The contractor was out at my site on Monday to give me price on excavating for frost footings. This guy did some minor earthwork for me in the past, but not concrete.

He neglected to mention depth of concrete so I assume 3.5" or 4".

I would prefer a frost footing, but a floating slab is probably a $5,000 savings over a frost footing and a poured slab. I have a 720 square foot detached garage on a floating slab that has no issues after about 35 years.

 

[reader2580]

So how do you get an inexpensive frost protected foundation? In your other thread about same subject I proposed a trenched/earth-formed foundation down to frost depth which is one that a motivated person with a plan could complete inside of a week, working mostly alone (rental equipment, need spotter for laser level to save time, etc), with flat ground.

The minimum size for a footing locally is 16" wide by 8" deep. Where you able to find a trencher that could trench that wide and 4 feet deep? I assume it would need to be a large ride on unit like a Ditch Witch.

How do you not end up filing the entire 16" wide trench with concrete? 16" wide by 48" deep would be a lot of extra concrete over an ICF wall with a 6" core.

I am not visualizing how a stem wall 8" out of the ground would be formed. Do you have photos or a website that shows this? I would think it would be difficult to properly place a form on top of the trench while the concrete truck is waiting to finish the pour.

 

[TurnipTruck]

Four years ago I paid $9/sq ft for a 40x60 frost protected floating slab with a fancy drain trench and sawcut in Alaska. My gravel pad was already compacted and flat and ready to go, tho.

I later added the edge insulation.

 

[matt_i]

The minimum size for a footing locally is 16" wide by 8" deep. Where you able to find a trencher that could trench that wide and 4 feet deep? I assume it would need to be a large ride on unit like a Ditch Witch.

How do you not end up filing the entire 16" wide trench with concrete? 16" wide by 48" deep would be a lot of extra concrete over an ICF wall with a 6" core.

I am not visualizing how a stem wall 8" out of the ground would be formed. Do you have photos or a website that shows this? I would think it would be difficult to properly place a form on top of the trench while the concrete truck is waiting to finish the pour.

Mini excavator. You waste some concrete if you require 16" width (here only 8" width is required)....so back to your 40' x 60' = 200' of wall x 4ft deep x 1.333ft wide...~40yd of concrete. You're probably in the $5000 range but back to my original admonition: I wouldn't build a building that could be 100% trashed for unwillingness to spend $5000 in materials and a weekend of miniex time. But what you "waste" in ~20 cuyd of concrete you pick up in cost and time of not having to form anything at all.

I would just trowel that level with the grass, and then place your slab on top of that. Various vertical rebar pins to keep it tied together. If you want to go 8" higher you could set 1 row of block or just live with the 4" to 6" elevation (slab thickness) on top of the "grass"....which I'm saying is the basic level of the ground and also the foundation poured above in step 1.

Potentially one could do a 3rd pour to avoid the block and form an 8" tall x 6-8" deep wall with framing lumber, 2x10s ripped, on top of the slab, or can you live with 7-1/4" height to use 2x8s that aren't ripped. I would use cross-ties (pencil rods, etc) inside that style of form to avoid having to try to anchor it on the inside.

 

[reader2580]

The city inspector called me today. He will allow a thickened edge floating slab for my 1,440 square foot building even though the typical limit is 1,000 square feet.

The inspector would only allow the trench type footing using the earth as forms with a smooth wall. He said I would never get my fine sand smooth enough and it would probably only work in clay.

I talked to the concrete guy again and he said that a normal slab is $5 per square foot, but that the floating slab is $10 per square foot due to all the extra concrete and rebar along the edges. He is going to get me a fine tuned quote for the floating slab.

 

[reader2580]

Mini excavator. You waste some concrete if you require 16" width (here only 8" width is required)....so back to your 40' x 60' = 200' of wall x 4ft deep x 1.333ft wide...~40yd of concrete. You're probably in the $5000 range but back to my original admonition: I wouldn't build a building that could be 100% trashed for unwillingness to spend $5000 in materials and a weekend of miniex time. But what you "waste" in ~20 cuyd of concrete you pick up in cost and time of not having to form anything at all.

The city inspector pretty much ruled out doing a footing like this. He requires the walls of the trench to be smooth to do this type of footing. He said this typically only works with clay soil, and with my fine sand that the walls would almost certainly slowly fall in and not be smooth enough.

He didn't say I couldn't do it, but that I would be unlikely to pass the required inspection before concrete is poured. He basically said in the long run with my soil conditions that I would probably spend just as much money with a trench as doing a poured frost footing.

 

[matt_i]

He requires the walls of the trench to be smooth to do this type of footing.

Its probably not worth fighting but its a clear power trip if the person can't cite a local code or an IRC reference that's adopted locally.

The one hole I dug into coarse sand ~5-ft deep held its dimension very well but I didn't have it opened for very long either.

What is the difference between a flat and floating slab? I didn't know the difference and I couldn't find much via goooogle either.

 

[reader2580]

Its probably not worth fighting but its a clear power trip if the person can't cite a local code or an IRC reference that's adopted locally.

The one hole I dug into coarse sand ~5-ft deep held its dimension very well but I didn't have it opened for very long either.

What is the difference between a flat and floating slab? I didn't know the difference and I couldn't find much via goooogle either.

I didn't ask for a code reference when the inspector said a trench footing probably wouldn't work. He didn't say I couldn't try it, but that it probably would not pass inspection in my fine sand.

A normal slab is the same thickness all the way across like 4". It can't be used to support the building. A floating slab is much thicker around the edges with extra rebar to support the building.

 

[Showkey]

My son got an estimate for a concrete driveway replacement 12’x50’ 4” in the center, 6” at the edge, rebar edge, wire mesh in the center, came in at $10/sqft. That included tear out and prep. Tear out was estimated at less than a hour as the excavator and dump truck was already on site for the prep.

( In central Wisconsin)

 

[K13]

Make sure you get something official about the floating slab and not just a verbal ok from the inspector. If he happens to not be around later on or another inspector checks it down the road saying you were given the ok is going to be worth nothing if they change their mind.

Sent from my SM-G950W using The Garage Journal mobile app

 

[reader2580]

I figure it is better not to get into an adversarial relationship with a building or electrical inspector. By my calculations I think an ICF frost wall would cost about the same as a trench footing considering double the concrete for a trench footing at $145 per yard.

 

[matt_i]

Imo, the roof loads concentrated in the bearing walls for a 40' wide x 16ft tall vs a 24' wide x 12 ft tall building are almost a factor of 2 different.

If you built a 24' x 24' "standard garage" would they still require a 16" wide pad footing? [It seems like overkill, and that's never a bad thing, but it also doesn't equate with low cost ]

Seems like an engineer who had designed locally would know.

 

[ddurrett896]

Dunno.
For a 25x40x6" deep slab (plus a little rectangle that connects total of 1100sf)
I paid $1500 cash to the finishers for the job + saw cutting.
22cuyd of concrete for somewhere areound $2600-ish
$1000 for the pump truck.

All the prep, compaction, stone base, rebar, chairs, wire-tying, forms/blockouts vapor barrier and curing sealer was done by me...so there is definitely money ******* in materials. Lets say it was another $1500.

I was in around $6/sf, 5 years ago. So if you can get all of the other work turnkey for $10.5/sqft you're probably doing well.

I'm close to this did mine last year.

I did footer + block + Vapor Barrier and hired a crew to pour/finish the slab and fill the block on 1,000 sqft slab.

20 yards was like $3,000
Crew was $1,500

Around $5/sqft

 

[WisJim]

I had a 24 by 40 floating slab done in April. 4" slab, 16" by 12" thickened edge, rebar throughout (don't recall exact specs of rebar), and it was $5.50 a square foot for the slab. With site work, 2" foam under the slab, and a 8" high curb all around it came to $9000. It didn't require any fill. I'm in Wisconsin about an hour east of St Paul.

 

[reader2580]

Imo, the roof loads concentrated in the bearing walls for a 40' wide x 16ft tall vs a 24' wide x 12 ft tall building are almost a factor of 2 different.

If you built a 24' x 24' "standard garage" would they still require a 16" wide pad footing? [It seems like overkill, and that's never a bad thing, but it also doesn't equate with low cost ]

I was told that 8"x16" was the minimum by my engineer, but that may have been for my original building. I looked at the Minnesota Residential Code and it seems to require only a 12" wide footing based on my soil type and 1 story construction. I can't find where the code specifies thickness.

I will ask my engineer what the requirements for footing are.

 

[reader2580]

I had a 24 by 40 floating slab done in April. 4" slab, 16" by 12" thickened edge, rebar throughout (don't recall exact specs of rebar), and it was $5.50 a square foot for the slab. With site work, 2" foam under the slab, and a 8" high curb all around it came to $9000. It didn't require any fill. I'm in Wisconsin about an hour east of St Paul.

That seems like a deal although your costs may be a little less due to being more rural. Your all-in price per sq ft with insulation is less than just the slab with no insulation in my case.

 

[WisJim]

That seems like a deal although your costs may be a little less due to being more rural. Your all-in price per sq ft with insulation is less than just the slab with no insulation in my case.

The company that I ended up with was near the average of the prices I was quoted, and I went with them based on recommendations from friends who had recent work done-and they communicated better than some of the others. And my site work was fairly simple as there was only about 2 foot difference in elevation and the soil is sandy with no rock or problems.

 

[reader2580]

The company that I ended up with was near the average of the prices I was quoted, and I went with them based on recommendations from friends who had recent work done-and they communicated better than some of the others. And my site work was fairly simple as there was only about 2 foot difference in elevation and the soil is sandy with no rock or problems.

Your soil conditions and elevation changes are basically exactly the same as my conditions. My elevation changes less than 2 feet and my soil is all fine sand.