Slab Cost - same question, different day...

[Scooter218]

I realize this subject has been touched on before, but I'm hoping to gain some clarity. So I've got a 30x40 garage plan done and have most of the material costs reasonably estimated (building it myself). The slab seems to be the uncertain area and the part I'm the least familiar with...

Typical in the Duluth, MN area is a 5" slab with 10x12 perimeter with rebar throughout and four-foot apron. I'm electing to use vapor barrier with 2" foam under and 1" foam around the perimeter. Add to that, 3 course of block installed/anchored to set the framing on. A couple of floor drains, too, but no floor heating, etc. I'm doing the dirt fill myself, but will need likely 6-12" of gravel fill for the bed (as part of the slab quote).

I've had quotes (mostly high) all over the map, up to $13,600! Based on what I can find and from talking to other people, I was figuring about $5-6/sqft for the slab and another $600-700 for the block/install. Is there some gouging going on here? I can live with paying for quality work, especially for the foundation, but I'm not sure who to believe on this one. Any thoughts would be appreciated.

Thanks -

 

[Radix2]

what is a 10x12 perimeter ? A thickened slab edge? I would say if your are not going to do a full foundation down to the frost line, then your should follow the "Frost protected shallow foundation" design standards - this basically involves placing foam so many feet out from your building perimeter to prevent frost getting under it and heaving.

As far as cost - you have to deal with the contractors in your area, so someone else's numbers don't do much good. Best way to figure it out is just to get enough bids to convince yourself that it is what it is...? Maybe some of those are assuming a full foundation?

 

[Scooter218]

Correct, that 10x12 infers a thickened edge. This is typical for a garage slab in the area; the contractors are not assuming a full footing in this case.

I've received a number of quotes, but they don't give me a warm and fuzzy compared to what coworkers/friends have told me for similar work (although not super recent work - just a couple of years old). Seems like I shouldn't really need to get 15 quotes just to justify a realistic range.

I do feel like I'm missing some assumption that's not specifically stated in the quote (or something is horribly marked-up). There's nothing out-of-the-ordinary about my situation or site. That's why I'm asking...hoping some northern climate folks might chime in as to why the disparity. Plenty of contractors in the area, not like it's a supply-and-demand thing.

 

[Jack D]

If your doing the grading why not call a concrete supplier and get a quote for enough concrete and the same PSI ratings that you are getting to cover your job. That seems to me, besides any rebar or wire that this is the biggest expense. Once you have 'your materials cost' you can better understand the labor. Their cost might be lower per yard because of their prior usage or contracts but it might give you a warm fuzzy on the quotes.

I just completed a foundation on a much smaller garage, 17x26 and I have a full foundation at 42" below grade 3000 PSI with 12" above grade and 5" 3500 PSI fiber/wire mesh floor and mine was 12K. It's pretty expense here building and my quotes were pretty close about 5 to 8% price range from high to low. BTW, the guy that came the furthest was the most expensive.

 

[Thumper68]

Hey welcome from a neighbor. I will call and get you the number for the guy I used to build my uncles garage. He did very good work on both the slab and the driveway and I will be using him for more work in the future.

Expect a pm with the info later tonight or tomorrow.

 

[Thumper68]

Sorry no PM, the guy is no longer doing concrete.

 

[Scooter218]

Good advice...I'll get some numbers directly from the concrete supplier. Since my site is rather low to start, I'll have to build up the base to get above the level of the gravel drive. I think the bottom of the footings will sit about where the current grade is, so there will be some back fill to do. Excavation will be minimal, iso t should be a pretty straightforward pour. That's why I'm surprised with some of these higher bids.

 

[Scooter218]

Sorry no PM, the guy is no longer doing concrete.

Thanks anyway, much appreciated. I've got feelers out with more folks, so we'll see if some better numbers come back. I feel like this job should be no more than say $8500 complete, at the high end.

 

[ddawg16]

what is a 10x12 perimeter ?

A more common term is 'footer'. I'm assuming you mean 10" wide and 12" deep. For a garage....and having to deal with a frost line, I would think you would want a much deeper footing....say at least 2'?

Concrete wise, you are looking at 'around' 30 yards of concrete...depending on your footing.

In Los Angeles...concrete is running about $1000/10 yards (full truck). So figure at least $3k for concrete. Most likely more like $4k.

Next, look at labor. The smart way is to use a backhoe to dig the footer and grade. Add $1000.

If it's undisturbed soil...you won't need gravel...but if it's disturbed, then either use a compactor or add gravel...or both. $500-1000

Sand...you have to have at least 2" of sand. $200 + labor to spread.

Materials - Rebar, forms, anchor bolts, etc. Figure $600-800

Pour day. At 'least' 2, maybe 4 guys plus tools. $1000 or more.

At this point you are close to $7-8K just for raw costs. Contractors don't work for free..

If you get a quote for less than $10K....you are most likely going to get hosed on quality.

But while you are at it.....I'd suggest looking at using stem walls. I'm at a loss as to why people pour flat slabs and build on top of them. The most common wood rot starts on the bottom plate.

Click on the garage build link in my signature and you will see what I'm talking about.

 

[mustangfan]

Are you heating this shop? Because if you are already planning on putting the insulation down, then you should look at running PEX tubing in the concrete. EVEN if you don't think you will heat the floor, if you ever WANT to, it will be there. Another $500 will pay for the tubing in the concrete now. If you ever THINK you might want a heated floor, do it NOW.
I live in MN and I will tell you once you put in a heated floor in a shop, you will not go back to the old way. Try standing on COLD concrete for a day compared to the WARM floor and your body will tell you the difference!!

 

[NUTTSGT]

Some of those contractors may not want to do the job if you're doing the dirtwork and they don't know your capability. They jack the quote so you won't call them back.

 

[Showkey]

A more common term is 'footer'. I'm assuming you mean 10" wide and 12" deep. For a garage....and having to deal with a frost line, I would think you would want a much deeper footing....say at least 2'?
.

Think he is floating a slab.........in Duluth he might need to be 4' to 6' down to be below the frost line

Three years ago water lines were freezing at 6' in some northern cities. It was a cold one....

 

[Thumper68]

Slab on grade is very common in unattached garages around here, mostly due to cost. In order to get below the frost line you are looking at a 52" deep trench, then you need another 12" for the footer.

Slab on grade if done right, thickened edge (Mine is 18" deep by 14" wide with 3 double rows of 5/8ths" rebar with a 6" slab, wire mesh and rebar construction) will hold up just as well for half the cost.

 

[joes169]

17 years ago, when I started my business, I could get $5 a sq. foot easily for a simple grade-beam slab like yours. Add in the fact that 2" foam is over $1.00 per sq. foot, and obviously a huge amount of inflation (especially in concrete), and I think you're being unrealistic at $5-6 a sq. foot. Also, the block at $6-700 is extremely low in my professional opinion. Without seeing the sight and the plan, I'd expect closer to $10-12 a sq. foot complete.

 

[wssix99]

Excavation will be minimal, iso t should be a pretty straightforward pour. That's why I'm surprised with some of these higher bids.

This does not work in your favor. Mobilizing the heavy equipment to your site, and paying for a full day of those pieces of equipment (and the operators) is probably what is getting you.

This alone is thousands of dollars and if you had a lot of excavation, you probably would see a minimal cost increase as a result.

I'd ask your contractors about this and ask them to itemize the excavation and equipment cost for you separately so you can see the effect. You may find out that:
- Your costs between contractors will vary wildly depending on what their equipment strategy is, what types of equipment they are bringing/can bring to the job, what skills are needed to operate the equipment, how far they need to truck the stuff, etc.
- Taking in to account the cost of you renting equipment to do the soils work, it may be cheaper to just have these guys do it while they are out with their already-mobilized equipment.
- If you have a neighbor that needs work done, etc. - you may be able to save a ton if you can pool your jobs together and give your contractor a situation where they can more fully utilize their equipment and operators for the day.
- Having your contractor rent a smaller/more modest piece of equipment from a local rental company may be less expensive then them mobilizing a large/overkill piece that they may already own.

 

[Scooter218]

Thanks all for the input. When I say the excavation is minimal, I mean that I've already cleared the site and have begun the soil fill to bring it up to grade (compacted). I've requested the quotes only include the remaining gravel fill for slab on grade (common here, as Thumper68 mentioned), so I'm not sure where a bunch of heavy equipment would be necessary. I have indeed figured the insulation to be at least a buck a foot, so I'm still surprised how we get to over 10k on this, even with the addition of the block. As suggested, I will definitely get specific itemization from here on out. Perhaps they are gouging on the remaining fill or they indeed don't really want to do the job. Since the building season in this region is relatively short, I also realize that many contractors don't need or want to take on a small job such as this as opposed to something bigger, so they may inflate the bid and see what happens. I ask these questions not for specific numbers, but for some justification as to why I should pay someone a big pile of money.

 

[sundevil64]

That sounds about right for my neck of the woods. Here concrete is going for $110 cu yard!

 

[wssix99]

Since the building season in this region is relatively short, I also realize that many contractors don't need or want to take on a small job such as this as opposed to something bigger, so they may inflate the bid and see what happens.

I don't think it's so much they don't "want" the small jobs, its that they keep equipment and staff to handle larger jobs. So, when a small job comes along, that project has to take on the burden of all that overhead, larger equipment than is really needed, higher skilled operators than is needed, 8 hours of labor time when they may only work 3-4, etc.

The worst part of having an oversize contractor for a small job is that the big jobs are always higher priority. When something needs to give - your project is the one that gets delayed...

 

[matt_i]

Not saying its right for you but for a 25x40, I excavated, formed, placed washed stone, compacted, rebar'd, and vapor barrier'd all myself. I scheduled the pump truck, the ready-mix and a guy + his crew to place and finish the floor, a 21yd pour.

So I paid for all the materials, $1500 cash to the finisher crew, which I thought was a bit high up front, but they were worth it in the end. That also included cutting the control joints the next morning. It was relatively easy work for them because all they had to do was show up and work a full day, cut the joints and they were gone. No multi-day job, no subbing out to other trades or other equipment, no scheduling around the weather. I had 2 guys interested, but one was on vacation and couldn't get back to me in time. I got their names from the local ready mix yard. My belief is make it easy for the contractor, and their price will come down.

The biggest gain I had was from buying a rotating laser level, it made so many jobs go easily when doing the dirt work.

 

[Scooter218]

Good to know, Matt. I'll be doing the construction myself, but have never worked on a slab before. I'm game to try just about anything, but I'd want to be pretty confident I'd have everything right before giving it a go. Had you done concrete work prior to doing your own slab? Frankly, I'm more inclined to have more control over the how and when this part of the project goes down, so I'll look into this in more detail and contact the local Redi-Mix for some names. Thanks.

 

[matt_i]

To answer your question, I've done some "quickrete" jobs but the shop was my first project involving ready mix trucks delivering concrete.

The skill I don't have is to float the slab, trowel and finish it to look good and be mostly level, and know the timing of what has to be done when as the concrete sets. The rest of it was almost all grunt work other than using the excavator which I rented from a local equipment rental place, and I used my tractor to fill the trenches back up and help spread stone. The rotating level takes the guesswork out of establishing planes, and then its just tape measure from there. I bent the rebar using a tube that's part of my gooseneck trailer, but a 2" receiver hitch could do the exact same. Jumping jack rental to compact the raw dirt, and a plate compactor to harden up the limestone. I used Grip Rite Pro-Lok chairs for the rebar and wire ties from HD or Lowes.

 

[Scooter218]

Thanks - excellent info.

 

[TractorJeff]

Hire a Hispanic crew!
Laugh, Criticize if you want BUT I know a couple of guys down here they are picture heavy of previous work and have reference's available who do great work and are cheaper!

 

[c4clewis]

Try being down here in Louisiana. Post-tension foundation for a 25x50 shop/garage was $25k.

Coming from the desert with slab-on-grade, I almost had a heart-attack.

 

[itsrotary]

I'm in Duluth too and just did my 24 x 30 garage addition slab last November. Typical pricing from an established flatwork company was running around $10 sq/ft at that time. Concrete was $140/yd delivered. I'm actually about 20 miles north of town.

I had the dirt work done and prepped to grade. 6" thick floor, 5/8" rebar 2' on center doubled in the perimeter footing, vapor barrier, 2" foam under slab and footing, finished, sealed, with control joints cut in 2 days later. 4 guys for the forming and pour, 2 stuck around to finish and strip forms. About 6 hrs start to finish. $7500, worth every penny.

They are busy and in demand, and I'm sure there's cheaper, but IMO the best guys around are in Carleton.

 

[SouthernGemini]

Try being down here in Louisiana. Post-tension foundation for a 25x50 shop/garage was $25k.

Coming from the desert with slab-on-grade, I almost had a heart-attack.

I live in St Tammany Parish in southern LA and dropped of my architectural plans to my engineer last week for the foundation work estimate (footing dimensions, etc) on my 30x30 two story. From what I have read on here, my footings will be a bit thicker b/c my sidewalls are 25 feet high with the peak of the roof around 36 feet. The downstairs is the garage. My engineer said increase of footings will be 'not by much'. Anyway, if your quote was $25k, I hope to be on muscle relaxers when I hear my quote. Sheesh!

 

[Thumper68]

Did you find someone to do the work?

 

[SouthernGemini]

Thumper, who is your question directed to?